This guide is not yet as complete or corrected as I would like.
I will add the missing photos as I am able to take them.
Suggestions and bug reports are welcome.

Gulfstream III / DC

G III / SN # 300 thru 356, # 358 thru 401 and # 875

     This guide addresses only the G III DC model.   The G III with the AC electrical system is much more like a G IV from an electrical  standpoint.  Other than electrical, the G III's are pretty much the same.  Of all the corporate aircraft I have flown, ( 22 Type Ratings )  the G II-B and G III are are my favorites.  The G III is about two and a half feet longer than the G II-B.  This amounts to about two million dollars per foot.  The G III is quieter in the cockpit due to the improvements to the windshield design over the G II's  The Falcon 2000 is hard to beat, but the Gulfstream is like a 67 Corvette  427 with dual quads and a big cam.  It goes like lightning, and creates it's own thunder.

Gulfstream III
Gulfstream III / DC  Flight Deck

    Not all of the DC G III's have EFIS.  The big CRT's are nice to display FMS information, but they don't make or break the flight.  The Gulfstream III is really the same as a G II-B that is a little bit longer and has  a cockpit with a modernized look  It does have  a different windshield that results in a reduction in cockpit noise.  Behind the fancy new switches and displays, the two airplanes are about the same, except in the case of the "AC" G III's.  They have a different electrical system, the same one that wound up in the G IV.  Some like it better, some don't.  The "AC" airplanes will have their own study guide.  I present it this way because one who is studying for an oral on one airplane needs to concentrate on that one machine, without having to sort out the differences until a bit later.

S t u d y    G u i d e

    For the latest and most accurate information, refer to the  current  AFM.  Many flight manuals on older airplanes, if subjected to inspection, will be found to be out of date.  Guess who gets the shaft if there is a problem, and you were operating with out of date information.  Look in the nearest mirror and see!
Systems  &  Limitations  Overview

Gulfstream III

Wing Span
77' 10"
Tailplane Span
Gear Track / Center Axle
13' 8"
Overall Length
 83' 2'
35' 2"
24' 6"
Turn Radius / Wheel to opposite Tip
47' 5"
Gear axle to opposite Elevator
Gear axle to Nose
Nosewheel Steering Angle
82 deg L & R



Max Ramp Taxi  Weight
70,200 lbs
Max Takeoff Weight
69,700 lbs 
Max Landing Weight
58,500 lbs 
Max Zero Fuel Weight  # 300 - 426
# 427 & Sub OR earlier with ASC 70
42,000 lbs 
44,000 lbs 

    The above weights are maximum certificated limits.  The actual maximum weights for a particular flight may vary due to the performance limitations.  If the aircraft can not meet the required "Takeoff Field Length" and "Climb" limitations, (engine out climb performance), the maximum takeoff and/or landing weights are reduced such that the requirements are met.  See the performance charts in the AFM for details.


Vmo / Mmo
340 kts / 0.85 Mach 
206 kts
Vfe      10 Deg 
            20 Deg 
            39 Deg
250 kts / 0.60 Mach
220 kts 
170 kts
Emergency Extension
225 kts / 0.70 Mach
250 kts / 0.70 Mach
175 kts
89 kts
100 kts
Mach Trim Inop
20,000 MSL / 0.75 Mach
Windshield Wipers
200 kts

Altitudes & Misc.

Max Alt T.O. & LDG
  15,000 ft 
Max Enroute Altitude 
Max Altitude  Flap & Gear
45,000 ft
20,000 ft
Min Temp T.O. & LDG 
Max Slush 
Max Water
-40 Deg C
0.75 inch
0.50 inch
Max Temperature 
Abv 10,000 ft
ISA + 35 C 
ISA +30 C
Min Temp SL - 3,500
-40 C
 3,500 - 5000 Linear
-40 C to -35 C
5,000 - 10,000
-35 C
10,000 - 35,000 Linear
-35 C to -70 C
Above 35,000
-70 C
Max Demonstrated X-Wind
21 kts
Max Runway Slope
Max Tailwind Component T.O. & LDG
10 kts
Load Factor Limit 
    Flaps Up 
    Flaps Extended 
2.50 G
2.00 G

Turn Clearance Limits

Gulfstream III
44.0 Feet
47.5 Feet
45.0 Feet
Taxiway for 180 deg Turn
52.0 Feet

Spey Mk 511-8  Engine Limitations

 570 C
106.6 %
100,1 %
585 C
5 Minutes
Max Continuous
106.6 %
98.1 %
No Limit
Rec Max Climb
94.5 %
490 C
No Limit
Rec Max Cruise
94.0 %
470 C
No Limit
Minimum / Full Bleeds
64.0 %
540 C
No Limit
Max Reverse
91.7 % 
490 C
30 Seconds
Max Overspeed
103.1 %
20 Seconds
Max Overtemp
610/615 C **
20 Seconds
Max Acceleration
595 C
2 Minutes
Ground Idle
52.0 - 55.5 %
540 C

Note:  Above 30,000 ft, it is possible to exceed the engine RPM limits without exceeding the maximum recomended temperatures.  You will find that it is the N2 that you must watch at high altitudes, as it is the one that will be the limiting factor.

   Engine Oil System Limitations

Min /  @ Max Continuous
40 psi
Min / Idle
15 psi
Min To Complete Flight @ Max Cont. 
35 psi
Min To Complete Flight @ 92% N2
30 psi
Min To Complete Flight @ 84% N2
25 psi
Min To Complete Flight @ 52% N2
13 psi
Max Oil Temp
+100 C
Max Transient Oil Temp / 15 Min
120 C
Min Oil Temp for Start 
Min Oil Temp Above Idle
-40 C
-30 C

Flight Controls

     The flight controls on the G III are cable driven with hydraulic boost..  They are powered by both the "Combined" and "Flight" hydraulic systems.  Each of the systems provides 1500 psi to the flight control servo actuators.  When the landing gear or flaps are extended, the combined system pressure increases to 3,000 psi.  If one system fails, the remaining system pressure increases to 3000 psi.  Failure of one hydraulic system therefore, does not result in a loss of control effectiveness.


  The hydraulically boosted ailerons provide roll control.  Either or both hydraulic systems will power the ailerons.  Roll control is enhanced  by the flight spoilers.  When one of the ailerons is deflected up, the two outboard speedbrake panels on the same wing will extend, reducing lift and increasing drag on that side.  The speedbrake panels on the side of the down aileron remain stowed.   A bungee connects the spoiler mixer and aileron on each side so that the ailerons and spoilers can be moved regardless of the status of the the other.   Roll trim is provided by a trim tab located on the left aileron.  The aileron trim tab is set with a manual trim wheel in the cockpit.  No electric stuff here!


    The Gulfstream III is equipped with a moveable stabilizer, elevator, and elevator trim tab system.  The elevator is hydraulically boosted.  Elevator trim adjustments are made with the manual trim wheel in the cockpit, or electrically from the control yoke.  The stabilizer is automatically adjusted when the flap setting is changed in order to compensate for changes in trim caused by flap extension and retraction.    The stabilizer is moved via a gearbox in the tail.  The flap gearbox is connected to the gearbox in the tail.  The stabilizer position is indicated by a gauge within the flap position indicator.  If the stabilizer does not position itself properly with each change in flap setting, return to the previous setting and go  to the checklist.  The aircraft may be landed safely with the stabilizer out of trim, however, much higher than normal control forces may be required, as the trim will not be as effective.


  The rudder, like the rest of the flight controls on the Gulfstream, is hydraulic.  Rudder trim is provided by redefining the neutral position with the rudder trim wheel in the cockpit.  The maximum rudder travel is 22 deg each side of center.  The rudder may be trimmed 10 units, or 7.5 deg either side of center.  The yaw damper is a full time system, powered by the "Flight" hydraulic system.  It has 3 deg of authority each side of wherever the trim and pilot input would place the rudder.  If the "Flight" hydraulic system fails, the yaw damper is inop.
    The rudder on the G III is equipped with a single and a dual load limiter system.  These systems limit the forces applied to the rudder in order to avoid structural failure of the tail at high speeds.  This single rudder load limiter system limits hydraulic pressure to a maximum of 2650 psi to the rudder actuator to avoid excessive loads on the aircraft at high speeds.  The dual load limiter system further limits the pressure to 2250 psi.  If only one of these two systems is operable, and you have an MEL for the aircraft, you can fly.   Each  rudder load limiter system has it's own indicator light, an amber one for the single rudder load limiter system, and a green one for the dual rudder load limiter.  Before start, the amber light should be on.  Only after both engines are running, the amber light will  go out, indicating that the single rudder load limiter system is working.   When the combined and flight hydraulic systems are operating, the dual rudder load limiter may be checked.  This is done by slowly depressing one rudder pedal to the floor, and looking for a green light to illuminate at full rudder deflection.  Now press the other rudder pedal to the floor, and the green light should illuminate again, indicating that the rudder has reached it's mechanical stop, and the dual rudder load limiter is in fact limiting the pressure.  If you get the green light in one direction but not the other, check the rudder trim and verify that it is centered.  If not, center the rudder trim and repeat the test.  It will most likely work the same on both sides now.


    The flap system is hydraulic.  Flaps may be extended to 10 deg, 20 deg, and 39 deg with the normal system.  The alternate flap control system can be used to extend or retract the flaps to any setting between 0 deg and 39 deg.  The flaps may be operated by "Combined", "Utility", or "Aux" hydraulic systems.  When the flaps are moved, the stabilizer is repositioned via a gearbox depicted below..

Flap / Stab Gearbox
Flap / Stab Gauge

      Because the system changes the angle of incidence of the horizontal stab, it minimizes trim changes during flap extension and retraction.  Flap asymmetry protection is provided via electrical signals from the outboard flap jackscrews.  The flaps system will stop if there is 1/4 inch or more difference in the linear travel of the flap actuators.  A torque limiter prevents damage to the system in the event of a jammed mechanism or an attempted extension at excessive airspeed.  You can, however, damage the flaps by accelerating to an excessive airspeed after the flaps are already extended.  Because they are mechanically extended by hydraulically driven jackscrews, they do not retract prior to structural failure.  A flap asymmetry will not reset until it is fixed.  If flap movement is arrested, it is either loss of hydraulics, flap asymmetry, or torque limiting.  If it was torque limiting, slow the airplane down to within the limiting speed for the flap setting you tried to get.  Retract the flaps to the previous setting, then re-extend at the proper airspeed.  If this restores flap operation, it was in fact the torque limiting system just doing it's job.  The airplane flies just fine if you stay clean until around 200 knots.  Extending the flaps at less than the limiting speeds will minimize the unwanted pitch activity during flap extension.
    In the event of unwanted flap movement, the flaps may be stopped, and the flap selector disabled by a switch on the copilot's lower arm ledge.  With this switch activated, the flaps are moved by the emergency flap control.  Any setting between zero and full may be selected.  You still have flap asymmetry protection in the emergency mode.

Max Flap Extension Speeds

10 deg
250 kts
20 deg
220 kts
39 deg
170 kts

Flight Spoilers

    All G II's and G III's have six spoiler panels on the wing.  They are referred to as 4 and 6 panel systems.  This addresses the fact that on the "4 panel" airplanes, the inner two panels only extend when ground spoilers are deployed.  The outer 4 panels extend when speedbrakes are selected, and all extend in the ground spoiler mode . The "6 panel" airplanes use all of the spoiler panels in flight, but only extend them to 26 deg as opposed to the 43 deg deployment on the 4 panel system.  In a turn, the spoilers on the lower wing will extend to 55 degrees in conjunction with full aileron deflection.  Gulfstream III,s # 300 thru 384, and # 386 & 389 have the four panel system.  These airplanes are equipped with the Ground spoiler deact handle.  Numbers # 385 and higher, except 386 and 389 have the 6 panel system, and thus do not have the ground spoiler deact handle.   The Flight Spoilers / Speedbrakes are powered by both the "Combined" and "Flight" hydraulic systems.  Speedbrakes  may be extended with up to 20 deg of flaps. Spoiler / Speedbrakes may not be deployed in flight with the flaps more than 20 deg, or with the Landing Gear extended.  The Flight Spoilers also assist in roll control, as they operate selectively with the ailerons.  A bunge allows aileron movement in the event the spoiler is stuck.

Ground  Spoilers

    The ground spoiler system uses the same spoiler panels as the flight spoilers.  It merely extends them to 55 degrees, instead of the 26 or 43 degrees, depending on the system, available in flight.  The ground spoilers may not be armed without a successful nutcracker test after landing gear extension.  There is an easy way to figure out if you have a four or six panel airplane.  The six panel airplanes do not have a ground spoiler deact handle.  By serial number, the six panel airplanes are:   G III # 385, 387, 388, and 390 and subsequent as well as any G III incorporating ASC 50.  Notice that with the 4 panel system, the only time that the two spoiler panels closest to the fuselage extend is when the ground spoilers are deployed.    With the 6 panel airplanes, the inboard 2 panels extend with speedbrake application, but not in conjunction with aileron deflection.  The six panel system uses the inboard panels for speedbrake and ground spoiler deployment only.

Four and Six Panel Spoiler / Speedbrake Systems

4 Panel System
6 Panel System
Max Aileron, 
0 Speedbrake
2 Low Wing Spoilers 43 deg
2 Low Wing Spoilers 43deg
0 Aileron
Max Speedbrake
4 Flight Spoilers 43 deg
6 Spoilers 26 deg
Max Aileron
Max Speedbrake
2 Low Wing Spoilers 55 deg 
Up wing 43 deg
Low Wing Spoilers 55 deg 
Others 26 deg
Auto Ground Spoilers
All Panels 55 deg
All panels 55 deg

Nosewheel Steering

    Ground steering is provided by a hydraulically actuated steerable nosewheel.  The nosewheel may be steered 82 degrees each side of center. The nose steering is powered by the "Combined" or the "Utility" hydraulic systems.  In the event those systems are inop, differential braking may be used for directional control on the ground.   In order to use differential braking to steer, you must turn the nosewheel steering system off.  This is done with a red guarded switch located just forward of the tiller.  The nose steering system is disabled unless the nut crackers are in the ground position.  Loss of Main DC Bus power has the same result as turning the nosewheel steering system off.

    If on landing, the "No Ground Spoiler" light is illuminated after touchdown, the pilot not flying shall announce it, deploy the flight spoilers, and remind the pilot flying that the nosewheel steering system will be inop.  This is because one or both of the nutcrackers remained in the flight mode after touchdown.  One nut cracker switch is sufficient to tell the airplane that it is in the air, but both are required to enable the ground spoiler and associated ground systems.


      The normal braking system provides braking with all of the main gear wheels.  The main wheel rotation is arrested during gear retraction.  The combined hydraulic system provides 250 pounds of hydraulic pressure to the brakes during the gear retraction cycle in order to stop main wheel rotation.  An anti-skid system provides skid protection during braking with the normal system.  This includes protection if the brakes are applied prior to touchdown.  Anti-skid does, however, require Main DC Bus power.  The parking / emergency brake is powered by an accumulator.  Use of the emergency brake system usually blows the tires, as it is almost impossible to modulate, and locks the main wheels.


   The SPZ-800 autopilot was installed on all of the G III's, as well as G II # 220 and  # 239 and later.  The SPZ 800 is an integrated Auto-Pilot / Flight Director, or Flight Guidance System.  The Sperry auto pilots are pretty good units if you look at the SP 50's and the SPZ 500 or 550 series and later.  I have, however, always wondered why Sperry insists on placing the glideslope indicator on the wrong side of it's ADI and HSI units.  A word of warning:  Do not develop too much confidence in any gadgets in the airplane.  Remember, these machines usually do what you tell them, not necessarily what you want them to do.   I sat and watched one fellow descend to 400 feet AGL 20 miles outside the Outer Marker on an ILS approach.  His excuse was that the flight director made him do it.  It was he, however, who programmed it.  Things like that don't just happen to stupid people.  A good pilot having a bad day can suffer the same fate.  This guy was lucky that his actions resulted in nothing more than a "Pink Slip"
    "Why did it do that?" is something most pilots have said from time to time.  Most auto pilot errors are due to flawed programing by the crew, but not all of them.  Use the bells and whistles, but keep one eye on the raw data just to make sure.  Paranoia is only sound thinking when the world is truly conspiring against you.

 No Photo Yet
SPZ-800 Autopilot

Do not operate the SPZ-800 Autopilot:
Above Vmo/Mmo
Below 1.2 Vs
Below 42,000 lbs and CG aft of 43% MAC
Auto-Pilot self test prohibited in flight
Mach Trim test prohibited in flight
No F/D VOR approaches without F/D computer mod.
Yaw Damp Inop - Minimum 200 kts or below 28,000 ft
No Yaw Damper 1 Eng Inop abv 250 kts or 35,000 ft
Mach Trim Compensators Inop 
     Dispatch - 0.75 Mach & 20,000 ft
     Inflight Fail - 0.75 Mach
AOA - Both required for flight


    All of the Gulfstream II and III aircraft are powered by two Spey Mk 511-8 engines.  These engines produce 11,400 pounds of thrust each.  The Spey Mk 511-8 is a twin spool axial flow Jet Engine.  The LP compressor, (N1 or Fan)  is a 5 stage compressor.  Some of the air from this LP compressor is routed around the engine core, and provides thrust.  The rest of the LP air is ingested into the HP or N2 compressor.  The N2 section has 12 compressor stages.  Behind the compressor sections is the combustion chamber, then the HP turbine, and finally the LP turbine.  The HP and LP turbines drive their respective compressor sections.

Percent vs RPM

N1 / LP  100%
8,393 rpm
N2 / HP 100%
12,484 rpm

    Accessories mounted on the engine include oil pumps, high pressure fuel pump, a hydraulic pump as well as alternator and generator for each engine.  These are driven by the HP or N2 section.  The engine oil is cooled by fuel via a fuel / oil heat exchanger.  It heats the fuel and cools the oil.  Oil pressure indications are provided by an AC powered gauge and a DC powered low pressure light (Idiot Light) for each engine.  There is an engine oil replenishing system installed on Gulfstream Jets.  Engine oil may be serviced only on the ground.
    Variable inlet guide vanes, and farther back, a surge bleed valve on the 7th compressor stage are installed to stabilize the engine idle, prevent compressor stalls, and optimize the engine acceleration characteristics.  A malfunction of the surge bleed system will result in unstable idle, poor acceleration, and compressor stall in many instances.

Engine Gauges
Engine Start Panel


    Vibration is monitored in the engine with the TVI or Turbine Vibration Indicator system.  This system allows the crew to monitor the engine vibration.  This can alert the crew of impending malfunction or failure of the rotating engine components.  Some vibration is normal, however a change in vibration level may indicate impending doom for the engine.  See the chart below.

TVI  Indication
Pilot  Action
Increase of 1.0 or more during steady state operation Slowly close thrust lever & observe vibration during deceleration.  Make an note of the maximum value.  If less than 3.0 (3.5 between 67% and 74% N2), restore power and monitor engine instruments.
A sudden increase of 3.0 (3.5 between 67% and 74% N2) Perform engine shutdown if situation permits.  Monitor engine warning systems.
During start, Increase of 3.0 or more. Abort the start.
During shutdown, Increase of 3.0 or more. Record incident and have engine inspected.

Fuel Heat

    An automatic Fuel Heating system is installed on these engines.  This is to prevent any ice formation in the engine fuel system.  The fuel is heated by P3 bleed air, and monitored by fuel temperature gauges in the cockpit.  These gauges are powered by the essential DC bus.  Some fuel heating does result from the fact that the hydraulic systems have heat exchangers in the hopper tanks, however the intent of these is to cool the hydraulic fluid.  The fact that the fuel is heated a small amount is not important.

Top Temp Control

    A temperature control and limiting system is installed on the Spey 511 Mk-8 engine.  This system, if on, will prevent a TGT overtemp.  The temp control amplifier is powered by the 115 Volt AC instrument Inverter Bus.  The Top Temperature Control System is to be ON except in the event the system fails.

G III / DC - Ground use Only

    The great thing about the APU on the Gulfstream is that it will allow you to start the engines and go someplace, instead of just sitting on the ramp.  The APU on the G III / DC model is for ground operation only.  It can provide air for engine start, or air conditioning, as well as AC electrical power.  Some of this AC is converted to DC with the Emergency TR to power the DC system on the ground, or in flight in the event of a problem with one or both DC generators..  The DC can then power the inverters, providing constant frequency AC.  So, in a round about way, the APU can power the entire airplane on the ground.  If you have the 300 Amp TR, you have plenty of  DC for the task, unless you get stupid with an excessive number of boost pumps and landing lights.  The 200 Amp TR is more than enough if you monitor the amp gauge, and temporarily disconnect one battery during the initial charging after APU start.  You may not need to do this very often, but be aware.

 No Photo Yet
GTPC 36-100G APU Control Panel

    To start the APU, make sure the Main and Essential DC Busses are powered.  It can be started when only the Ess DC Bus is powered, but problems may result if you attempt to use the APU air.  Turn on the APU Master Switch.  Wait for the Low Oil Pressure light to illuminate, indicating that the APU door is open.  Press and release the start button, and wait.  When the APU RPM is stable at 100%, turn on the Alternator.  Check that both AC busses are powered, then select the Emergency to Main & ESS.  Check that the DC Volt Meter reads at least 26 Volts.  The DC Emergency Feeder light should be illuminated, indicating that the emergency TR is powering the DC Bus/Busses as desired.
    The APU Air must be turned on if you wish to do anything really festive, like start the engines or use the heating / air conditioning system.  Do not run the engines much above idle with the APU Air Valve Open, as this is not good for the APU.
    To shut down the APU, turn off the Air and Alternator, let the temperature stabilize, then press and release the "Overspeed" test switch.

Note:  Performing a "Nut Cracker" test with the APU running will shut it down, as the APU will think that the airplane has just become airborne.  I learned this from experience.  The bright side is that I was the only occupant of the aircraft at the time and therefore did not have to explain my mistake.  The APU is a ground only item on the G-III / DC aircraft.

Fire / Overheat  Protection

Engines / APU

    The engines and have continuous wire fire detection loops in the engine compartments.  When heated to their alarm value, they will cause the engine fire warning system to activate.  The respective left or right ENGINE FIRE lights will illuminate.  On # 173 & Sub and aircraft with ASC 152,  fire bell will sound.  Press the "Fire Bell Mute" button to cancel the bell.  The fire may be extinguished by pulling the respective fire handle, and discharging one or both of the engine fire bottles.  Their are two engine fire bottles, each containing 4.5 pounds of  Bromotrifluromethane and about 600 psi of nitrogen to disburse it.  Either or both bottles may be discharged into the selected engine.

G III Engine Fire Warning

    The APU fire warning is provided by a thermal switch.  The APU fire bottle is like the engine fire bottles, only smaller.  It contains 2.5 pounds of the extinguishing agent, and the 600 psi nitrogen to distribute it.  The APU fire bottle may be fired by lifting the guard and moving the switch.  Any fire bottle that is discharged must be removed from the airplane, serviced, and reinstalled.  The engine and APU fire detection systems are not part of the Master Warning System.

Note:  If you get a fire warning, and perform the appropriate procedure, test the fire detection system after you believe that the fire has been extinguished.  If the system won't test, the fire detection loop may have been damaged.  In this case, you don't really know if the fire was actually extinguished.


    Overheat warning is provided for the generators, alternators, TR's, radio rack, and inverters.  Some of the warning lights serve two items, such as a TR and an Inverter.  These require some diagnosis.  Refer to the checklist.  The E and B inverters and the B inverter's TR have an auto shutoff feature, as they are located in the radio rack inside the airplane.  When the B Inverter shuts down due to an overtemp, it also pops the breaker for the bus it was powering.  If one bus caused both it's own inverter and the B Inverter to overheat, the problem is most likely on the bus.


    Overheat warning is provided for the Flight and Combined, systems, and the Aux hydraulic pump.  These systems do not incorporate any automatic features to deal with the problem.  Consult the checklist for the appropriate procedure.


    The wings, tail compartment, bleed air manifolds, aft compartment, and bootstrap unit are equipped with overheat warnings.   On the ground, the bootstrap unit will shutdown automatically in the event it overheats.  In flight, the bootstrap unit will only give you a warning, and must be shut down by the crew.  Selecting either Emergency Pressurization or Ram Air will shut down the bootstrap unit.  Emergency pressurization air is available from the LP compressor on the right engine.  The remainder of the warnings must be handled by the crew.

Fuel System

    The G III Fuel System is simple.  Fuel is stored in the wings.  Each wing tank has a "hopper" tank that is kept full with ejector pumps and gravity.  Four DC electric fuel pumps provide fuel pressure to the engine driven fuel pumps.  Warning lights will illuminate when the hopper tank has 675 pounds of fuel or less.  This amounts to between 20 and 25 minutes at 200 knots IAS at 5000 MSL, or about 50 miles to dry tanks.
    Two of the fuel pumps are designated as "Main", and two as "Alternate".  They are in fact identical.  They are located in the main wheel wells.  Normally, all four pumps are on.  If one pump on a given side is turned off, the other pump can supply ample pressure.  In the event that the operating pump fails, and the failed pump's switch is in the on position, the other pump on that side will come on.  The main fuel pumps are powered by the ESS DC bus.  The alternate fuel pumps get there power from the MAIN DC bus.
    You may supply fuel to either or both engines from either tank by opening the crossfeed valve and switching the fuel pumps off on the side from which you do not wish to feed.  An intertank valve is also installed, allowing fuel to gravity flow between the tanks.

Left Tank
Right Tank
14,150 Lbs
14,150 Lbs
28,300 Lbs
2,095 Gal
 2,095 Gal
4,192 Gal
Fuel Pump Switches

    The airplane burns about 5,00 lbs the first hour, 3,500 lbs the second hour, 3,300 lbs the third hour, then 3,000 lbs per hour thereafter.  Low altitude holding requires 4,000 lbs / hour fuel flow.  This gives you 7 hours range with an hours worth of holding fuel, or and hour and 15 minutes reserve according to the regulations.  This will take you  more than 3,000 nautical miles at normal cruise.  Use of  long range cruise will result over an hours worth of additional flight time.  I know of flights of 8 hours duration that landed with legal fuel reserves.  The G III is quite an airplane.  Thank God it has a bathroom.

Hydraulic System

    The Gulfstream III hydraulic system uses skydol.   It has two engine driven pumps, an electric aux pump, and a utility pump that is driven by the "Flight System".   This "Utility Pump" is a hydraulic motor, (driven by the flight system) connected to a hydraulic pump that can pressurize the "Combined System" except the flight controls in the event that the combined system pump, or the left engine were to fail.  There is no fluid transfer between the flight and combined systems.  If the failure of the combined system includes loss of the fluid, the utility pump will not have any effect, as there is nothing for it to pump!  The combined and flight hydraulic systems have heat exchangers located in their respective fuel hopper tanks in order to cool the hydraulic fluid.  These have nothing to do with the "Fuel Heat" system which is located in each engine's nacelle.

Combined System
    The "Combined System" is powered by a hydraulic pump on the left engine, or in the case of  Combined System pump, or engine failure, the Utility Pump. The utility pump is driven by the Flight System.  The electric "Aux Pump" can power 5 items only.  See the chart below.

Flight System
    The "Flight" System" is powered by an engine driven hydraulic pump located on the right engine.  It's normal operating  pressure is 1500 PSI.  It will operate at 3000 PSI if the combined system pressure drops below 800 psi.  This is to increase the pressure to the flight controls, and to enable the flight system to power the Utility Pump.

Utility Pump
    The Utility pump is a hydraulic pump that pressurizes the Combined System when it's own pump is not putting out sufficient pressure for whatever reason.  Operation of the utility pump requires at least 2,200 psi flight system pressure.   The Utility pump can drive all of the combined system items except the ailerons, elevator, rudder and flight spoilers.  In the event it is necessary to use the utility pump, those items are operated by the flight system.  If the flight system does not work, the utility pump won't work either, as the utility pump is driven by the flight system in the first place!

Aux System
    As you can see from the table below, the aux system, in normal operations, is used on the ground.  It allows you to extend and retract the landing gear doors during the preflight, closes the cabin door, and provides pressure so you can set the parking brake.  It also operates the Flaps and Brakes on the ground or in flight.

Hydraulic System Control & Indication

Combined System
Ailerons   Ailerons
 Elevator Elevator
 Rudder   Rudder / Yaw Damper
 Flight Spoiler / Speedbrake   Flight Spoiler / Speedbrake
 **Ground Spoilers   **Ground Spoilers
---Check Valve---
  Right Thrust Reverser
<Utility Pump<
Utility System Motor
Ground Spoilers <<<--- Right Thrust Reverser
Nose Steering  <<<--- Flight Sys Reservoir Press.
Thrust Reversers <<<---
Stall Barrier <<<---
Windshield Wipers <<<---
Combined Sys Reservoir Press. <<<---
<-Aux Pump
Landing Gear <<<---Note: Gear on ground only!
Pedal Brakes <<<---
Flaps <<<---
Parking Brake <<<---
Cabin Door (Close only) <<<---

** Six Panel System only.  On the four panel system, the ground spoilers can be powered only by the Utility system, or by the flight system pressure that is downstream of the one way check valve.  That check valve is installed to prevent the utility pump from powering the combined system flight control servoactuators.

Note:  Use of the Aux Pump to operate the Landing Gear on the ground is limited to the landing gear doors only.  The gear doors are opened on the ground for preflight, and to keep condensation from accumulating in the main gear doors.  If you leave the airplane in an environment where the temperature goes below freezing, any water that may be in the main gear doors can freeze, and may interfere with landing gear operation if it remains during gear retraction, or cause damage on the ground if a chunk of ice falls out and hits something.

Electrical System G III   DC 
SN # 300 thru 356, # 358 thru 401 and # 875

    The electrical system on the Gulfstream III  may look complicated at first glance, but it is not.  It consists of 2 batteries, two 300 amp brushless DC generators, and two variable frequency (wild AC) 115 Volt 20 KVA alternators plus an APU alternator of the same capacity.  Not counting the APU alternator, because it can't be used in flight,  the Gulfstream III has 2 DC generators and 2 Alternators.  Also they have 4 inverters to make constant frequency AC from DC, and 4 Transformer / Rectifiers to make DC from AC.  The bottom line is, as long as you have at least one of the four electrical generating devices operational, you will have at least the minimum electrical power needed to fly the airplane.  In the Gulfsteam, one out of four ain't bad!


    In general, the DC system powers most of the electrical components on the airplane.  The alternators provide power to heat the windshields, power some interior lighting,  and to power the TR's that produce the DC that  powers the inverters. It may sound complicated, but a peek at the electrical schematic will make it a bit less confusing.  The inverters supply the 400 hz AC that is needed by the gyros, avionics, and flight guidance system.  The "E" (instrument) inverter is powered by the ESS DC bus.  It is only needed when the Main Inv. Bus is not powered by the A or B inverter, or by the left generator tie bus through the use of the Inv Alt Pwr, or "Skip" switch.   The "A" and "C" inverters are powered by their respective TR's, but can also be powered, if desired,  by the DC system through the use of the INV ALT PWR switches.  These are also refereed to as "Skip" switches.
    The Emergency TR is powered by the left alternator bus.  This bus should be powered if any one of the three alternators is operating.  The emergency TR can power the ESS DC bus, and both the MAIN and ESS DC busses as desired.  When the airplane is on the ground without the engines running, the ESS TR powers the DC system.  This is normal.

Emergency Transformer-Rectifier

    The Emergency TR is installed in the tailcone of the airplane.  It is rated for 200 amps DC output.  If ASC 111 is incorporated,  the maximum output is 300 amps.  The emergency TR is used to power the aircraft's DC system during ground operations while using the APU alternator or external AC power.
    On airplanes incorporating ASC 155, the TR, when selected to ESS DC, powers the Essential DC Bus even if the battery switch is turned off, or fails.  If the battery switch is then selected to normal, the Emergency TR powers both the main and essential dc busses.

Constant Frequency AC System

    The constant frequency AC system is controlled by the switches below.  Each AC bus has it's own 3 position switch.  Normal, Off, and Standby or Emerg. are the switch positions.  With all three switches in NORM, the A inverter powers the Main and INST Inverter Busses, the C inverter powers the Secondary INV bus, and the B inverter powers nothing.  As long as the A or B inverter is powering the Main Inv. Bus, the Main and Inst AC busses can almost be considered one bus, as they are connected.  This remains so as long as either the A or B inverter is powering it.  If, however, the E inverter is powering the Emergency Instrument Bus, the relay between the two busses opens and the E inverter powers ONLY the Instrument Inverter Bus.  If the main or secondary inverter bus switch is selected to standby, the B inverter powers that particular AC Bus.  The B inverter can power the Main OR  the Secondary AC bus, never both at the same time.  If both the Main and Sec bus switches are in standby, the Main gets the power.

G III DC Model  AC Electrical System


   Variable frequency AC is provided by the two ships alternators, and on the ground only, the APU alternator.  Constant frequency AC is provided by converting the variable frequency AC to DC, with a Transformer / Rectifier or "TR".  The then "TR's" DC output powers  inverters that produces the constant frequency AC.  There are four inverters in this system.  The "E" inverter can power only the Instrument AC Bus.  The "C" Inverter powers only the Secondary AC Bus.  The B inverter can power the Main, or Main and Instrument AC Busses, OR  it can power the Secondary AC Bus, but not both.  The A inverter can power the Main AC, or the Main AC and the Instrument AC busses.  The E and B inverters are in the radio rack, and have thermal protection that will shut them down if they overheat.  The A and C inverters have overheat warning lights, but no auto shutdown protection.  If the TR's feeding the A or the C inverters fail, those inverters may be powered by the DC system through the use of the "Skip" switches.  The inverters overheat warning tells you that the respective inverter or the "TR" has overheated.  Switch inverters and let things cool for a minute, then if you wish to diagnose the problem, turn the inverter back on and use the "Skip" switch.  If the fault does not recur, it was most likely the "TR", and if it does, it was the inverter that was the problem.

Generator / Alternator / Inverter / Battery Switches

GO Inverter & Constant Frequency AC Bus Check

DC Generators

    The DC generators on the G III are really just huge slightly more sopisticated cousins of the alternators on your family car.  They really are alternators, producing AC power, but incorporate "Diodes" to convert this AC into DC electric current.  The end result is that the device supplies DC, and is therefore called a generator.  They are rated at 28.5 volts and 300 amps output.  If both are operating, you can draw 300 amps from each of them.  If one has failed, you are limited to 250 amps from the remaining generator.
    These generators have, in addition to the normal electrical system L GEN and R GEN failure lights, three warning lights that alert you to problems with the generator.

FLD :BRG - Failed Bearing - These generators have both primary and auxiliary bearings.  This is basically one bearing inside another.   If the primary bearing is failing, and you have an MEL, you can operate for a maximum of 30 hours until repairs must be made.  You are limited to 250 amps during this period.

O.V. - Over Voltage.  Alerts crew that the generator trip was due to over voltage of 31 volts or more.  You may attempt to reset this one time only.  If it fails again, trip the generator and leave it off until repaired.

F.D. - Failed Diode - One of the diodes in the generator has failed, either open or shorted.  By accessing the voltage regulator and placing the emergency switch to OVERRIDE, you may be able to use the generator for 30 hours at a maximum of 250 amps.

DC System

   DC is supplied by either or both of the brushless DC generators, and / or by the Emergency "TR".  The Emergency "TR"  is powered by any one of the ships alternators through the left alternator bus.  The DC power is distributed by two busses, Essential and Main.  Loss of one DC generator is not a problem, as you merely revert to "Split Buss" operation to reduce the load on the remaining generator..  Place the TR switch to ESS, and Bat switch to Emergency.  Now, your remaining DC generator powers the main DC Bus.  The Emergency TR powers the Essential DC Bus, and keeps the batteries charged.  The Emergency TR is powered by the left AC bus.   When you select the Emergency TR, the left AC equipment bus becomes unpowered as indicated in the previous electrical schematic..  Operation of the Emergency TR is illustrated below.  With the failure of both DC generators, and at least one alternator on line, the Ess DC Bus can be powered by the Emergency TR.  You can also power the Main DC Bus, but be cautious not to overload the TR or the alternator.  The 300 amp TR is not installed on all airplanes, and the alternator is limited to 53 amps for continuous operation.

G III DC Electrical System

DC System Powered by Emergency TR
Normal during Ground Operation

  The Emergency TR can power the Ess DC Bus, or both the Ess and Main DC Bus, as selected by the crew.  The 200 Amp Emergency TR needs a bit more monitoring when the batteries are charging at a high rate.  Disconnecting one battery reduces the load on the TR by as much as 50 to 100 amps.

GO Battery Integrity / DC Systems Check

Gulfstream III / DC Electrical System

Note:  The DC G III's also include # 875 and exclude # 357
One look at the electrical panel and you will have no doubt
as to which electrical system is installed on your airplane.

Emergency Gen's & Inverter Alternate Power

One Generator & One or both Alternators Operating

    Split the bus!  Battery switch to Emergency, TR switch to Essential.  ESS DC Bus is powered by the TR, and the Main Bus is powered by the remaining generator.  Load on the operating generator is reduced.  The inverters are powered normally.  All busses are powered except the AC equipment bus which is lost whenever the emergency TR is in use.  If one alternator is inop, the AC monitor buses will be unpowered as well.

One Alternator & Both Generators Operating

    No action required; The alternator busses get power from the opposite alternator bus when their own alternator fails.  The AC monitor busses will not be powered.  Monitor the alternator load.  Windshield heat is a high draw item.  In non-icing conditions, it can be shed if speed altitude and humidity permit.  One of the inverters may be powered by the DC system through the use of the "Skip" switch if AC load reductin is desired.

One Generator Operating & Both Alternators Failed

    Monitor the load on the remaining generator.  Inst. Inv. Bus is powered by the E inverter.  You will have at least the Essential DC Bus and depending on the DC load, the Main DC Bus as well.

One Alternator Operating & Both Generators Failed

    Battery switch to Emergency, TR switch Essential.  ESS DC Bus is powered by the TR.  If the load on the generator permits, the main DC bus may be powered.  The inverter system should operate normally.

Electrical Capacities

Alternators / Engines 20 KVA
DC Generators
1 Gen Operation
28.5  Volt / 300 Amps
28.5 Volt / 250 Amps
Emergency TR / Standard 200 Amp
Emergency TR / Optional 300 Amp
APU Alternator 20 KVA 
Batteries 1 & 2 24 Volt / 36 Amp Hour

Ice and Rain Protection

    The Gulfstream II-B is certified for flight into known or forecast icing conditions.  We will discuss the various Ice and Rain protection devices system by system.  The vertical and horizontal stabilizers do not require ice protection.  This was determined by flight testing during the aircraft's certification process.  All of the Gulfstream's anti-ice systems are powered by the Essential DC system.


    The engine inlet hub fairing, inlet guide vanes, and the nose cowl are heated by engine bleed air in order to prevent ice formation.  The air is controlled by two engine anti-ice valves located on each engine.  These valves are opened and closed with the "ENG. ANTI-ICE" switches.  Each valve has a separate circuit.  To determine if one anti-ice valve has failed to open, compare the engine anti-ice duct pressures. The engine EPR pitot probes are heated electrically heated when engine anti-ice is selected on.  On aircraft with ASC 111, the EPR pitot probes are heated when the Pitot Heat switch is on.    If the anti-ice duct pressures are the same at equal rpm, they are OK.  If they differ by 15 psi or so, and one is below 45 psi, one valve is probably failed closed.  If the pressure is more than 60 psi, a valve has more than likely failed open.  These valves fail to the closed position when Essential DC power is lost.

Engine & Wing Anti-Ice


    The wing leading edges are heated with engine bleed air in order to prevent ice formation.  Wing heat is controlled with two "Wing Anti Ice" switches.  Each of theses switches controls it's respective wing anti-ice valve.  This does not mean that you may heat one wing and not the other.  The bleed air ducting downstream of the wing anti-ice valves is connected by a crossover manifold such that either Wing Anti Ice switch being turned on heats both wings.
    After activating the wing anti-ice system, the wing leading edge begins to heat.  When the respective wing temperature reaches 100 deg F, the that wing's green light above the Wing Anti Ice switches will illuminate, indicating normal operation.  The temperature is automatically regulated.  If the wing anti-ice controllers fail, and the temperature reaches 180 deg F, a red L or R WING HT light will illuminate.  You must then turn one or both wing heat switches off as necessary to extinguish the warning lights.  The entire wing anti-ice system is useable during emergency DC operation, as the CB's are on the Essential DC Bus.


    Rain removal is performed with electrically powered windshield wipers.  The left wiper motor is powered by the Ess DC Bus, and the right is powered by Main DC.  The windshields are also heated electrically.  They are heated with power form the right alternator bus.  Windshield heat is for anti-ice and defogging.  The windshield heat is not necessary for bird strike protection on the G III.  When the windshield heat switch is placed in the "ON" position, the windshields receive electrical power from the "Right Alternator Bus".  This bus can be powered by either of the ships alternators, or the APU alternator if it is operating.  The AC power on the Gulfstream is 3 phase.  Without going into lots of detail, three phase alternators behave like three alternators, providing 3 wires, and an electrical ground.
    The left front and right side windows share the same temperature controller.  The right front and left side windows do the same.  Each controller receives power from two of the three phases of AC power.  A green advisory light illuminates when a specific window is being heated.  The lights may be tested with the warning lights test button.  If a particular light tests OK but does not come on, it may indicate that the particular window is at the proper temperature and is not being heated at that moment.  If the light remains out for a long period of time at high altitude, the heat for that window has probably failed.

Pitot-Static & AOA

    The pitot tubes and the angle of attack probes are electrically heated.  The left form the ESS DC Bus, and the right from the Main DC Bus.  When pitot heat is on, the EPR pitot tubes are also heated.  The pitot heaters receive the same power in flight or on the ground, as long as they are turned on.  Do not use them on the ground for extended periods.  Pitot heat is to be turned on just prior to taking the active runway, and turned off after clearing the runway after landing.
    The AOA heaters, when on the ground, receive only 25% of the power they receive when the aircraft is in flight.  They can and should be turned on after the engine start check is complete, and left on until shutdown.

Pitot Heat Warning


Angle of Attack
1                           2




  The cabin is pressurized with HP bleed air from the N2 compressor section of each engine.  If the air-conditioning unit (boot strap) fails, emergency pressurization may be selected.  This provides air from the LP or N1 compressor section of the right engine.  The pressurization is regulated with one of two outflow valves.  The normal outflow valve is electrically operated.  It is AC powered in the "Normal" mode, and DC powered in the " manual mode.  The second outflow valve is a "Safety Valve", requiring no electrical power whatsoever.  This valve provides relief at maximum pressure differential, and vacuum relief, as well as a pressurization rate limiter.

    During normal operation, before takeoff, you set cruise altitude, barometric pressure, destination field elevation and cabin rate.  The system does the rest.  Change the barometric pressure setting if appropriate, and at top of descent, move the switch from "Flight" to "Landing"..  That's it!  If manual mode is required, move the outflow valve as necessary with the manual (DC electric) system.  Go easy, as the valve moves quite fast.  A general announcement to the passengers may keep you from looking  like a dummy!

Normal  & Manual Pressurization Control

Emergency Pressurization

    Emergency pressurization air comes from the LP bleed port on the right engine.  Selecting RAM CABIN AIR closes the air-conditioning valve, causing  the airplane to depressurize.  Once the cabin has depressurized, the ram air valve opens to ventilate the cabin.

Air Conditioning

      The same air that pressurizes the airplane also provides air conditioning and heating.  What a surprise we have here!  The engine bleed air is quite hot, up to 900 deg  F when it comes out of the HP port, and about 400 deg F once it passes through the precooler..  In order to cool it, there are heat exchangers in the engine pylons, and a "Boot Strap" unit to further cool the air.  The rest of the industry calls it an "Air Cycle Machine", "ACM" or "PAC", but Gulfstream insists on being different.    In simple terms, the hot air is cooled, compressed, cooled again, and allowed to expand, driving the cooling turbine, further reducing the temperature of the air.   This cold air is mixed with warm air to regulate the temperature.  (For more theoretical information, see the "Glossary" included in the study guide section of this web site.  See "Air Cycle Machine")  If for any reason, like maybe smoke evacuation, you wish to depressurize the airplane and ventilate it with ram air, you may do so by placing the "Cabin Air" switch to "RAM".  This closes the valve that allows bleed air to enter the cabin, and after the cabin pressure leaks down, allows ambient air to ventilate, but not pressurize the aircraft.
    Control of the temperature is managed by two selector switchs and two rheostats on the overhead panel.  The switch positions are "OFF", "AUTO", and "MANUAL" for the CKPT and CABIN, as each have their own switch.  There is a small switch between the temperature control rheostats.  This switch selects the temperature sensor that drives the "Cabin Temp" gauge.  With both switches in "AUTO", the system works as intended.  The cockpit thermostat regulates the temperature in the cockpit, and the cabin thermostat does the same for the cabin.  When one of the switches is placed in  "MANUAL" , that system takes it's orders from the other controller. This remains true regardless of the position of the other switch.   When a particular system is selected "OFF", that system drives it's temperature control valve to full cold.  The controller, however, may still be used to regulate the temperature of the other system should that other system be switched to "MANUAL".  Full cold  may be OK in Phoenix in the summer, but as for Bozeman Montana in the winter, or after about half an hour at cruise, you will freeze your boss, and he will be unable to sign your paycheck due to hypothermia..  If you must use the "OFF" position, there are two valves in the baggage compartment that will enable you to control the temperature manually.  They are located on the left, or aft edge of the external baggage compartment door.  Do not show the cabin attendant where these valves are.  They are for use only by the flight deck crew and only when both automatic systems have failed.

 No Photo Yet!
Temperature Control

Warning Lights

    GO  Entire Warning Light System

Note:  I will divide these up by system at a later date, but for now, here they are. If you are looking for a specific one, try CTRL  F, and type in what you want to find.

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