AIRCRAFT PERFORMANCE

PERFORMANCE: FAR PART 25


Civil airplanes are certificated under part 23 or part 25 of the Federal Aviation Regulations. Normal, Utility, and Aerobatic category aircraft are certified under FAR part 23. These aircraft have a maximum gross takeoff weight of less than 12,500 lbs, and are normally propeller driven. FAR part 23 airplanes require only one pilot. Transport Category airplanes are certificated under FAR part 25. They usually have a maximum gross takeoff weight of more than 12,500 lbs, require two pilots, and in some cases, a Flight Engineer. The Certificate of Airworthiness will tell us under which category a specific airplane has been certified.

FAR parts 23 and 25 specify criteria that airplanes must meet to obtain certification. These regulations contain many standards relating to materials, construction, performance, flight characteristics, and more.

In a normal category airplane such as a light twin, the regulations do not require that the aircraft be able to climb, or even maintain altitude if an engine failure occurs during the departure. In fact, if the failure occurs between rotation and Vyse, you may find yourself unable to stop on the runway, AND unable to continue. The result will likely be a controlled crash.

Transport Category aircraft, if operated within their limitations, never depart at a gross weight such that the failure of one engine will leave the pilot without the performance capability to either stop on the remaining runway, or continue the takeoff with the remaining engine(s). Similar weight limitations exist for landing. If operated at legal weights, the part 25 aircraft can always execute a go around or missed approach with an inoperative Powerplant.

Transport Category aircraft have two sets of weight limitations. "Certificate Limitations" such as Maximum Takeoff or Landing weight, and "Performance Limitations" such as Maximum Takeoff Weight permitted by climb requirements, or Runway Limited Takeoff Weight. Compliance with BOTH limits is mandatory. The certificate limits remain constant, but the performance limits vary as the altitude, temperature, runway and wind change. A hot day, high elevation or short runway can limit your Max Takeoff Weight to a figure well below the Max Certificated Takeoff Weight. Remember, in FAR part 25 (Transport Category) aircraft, performance limits are OPERATING LIMITATIONS, not just "Good Information".

There are some terms and definitions one must know if the FAR part 25 performance data presented in most Flight Manuals is to be understood. We will define them now.
 

Definitions
What you don't know can hurt you!

  V mcg The minimum speed at which directional control can be maintained on the ground with the critical engine windmilling and the remaining engine(s) at takeoff thrust. (Max rated sea level power.) Only aerodynamic controls are used during Vmcg determination, as nose wheel steering and differential braking may have no effect on a wet or icy runway. V mca The minimum speed at which directional control can be maintained in the air with the critical engine windmilling and the remaining engines(s) at takeoff thrust. The rudder input is considered to be maximum while aileron input is limited to whatever it takes to establish 5º bank angle into the operating engine or engines. V be Maximum brake energy speed. The maximum speed, (IAS) from which the aircraft may be stopped with the brakes without exceeding the brakes' capacity to dissipate the heat generated. Exceeding the brake energy limit usually results in flat tires due to melted fuse plugs in the wheels, and/or brake fires. Refer to the Maximum brake energy speed and minimum turnaround time charts in the AFM. V 1 Takeoff decision speed. (Formerly, critical engine failure speed.) If an engine fails before V1 is reached, the takeoff is aborted in any case. Aborted takeoffs at or after V1 are only advisable in the case of catastrophic failure. An engine failure or systems malfunction does not constitute a catastrophic failure on a Part 25 Aircraft. There is no performance data available nor is there performance criteria established for any takeoff aborted after V1 is reached. You are much safer continuing the takeoff than attempting an abort after V1. This is not opinion, but documented fact. V r Rotation speed. The speed at which rotation to takeoff attitude is initiated. Gross weight and takeoff flap setting are the variables used to determine Vr. Some aircraft charts also consider altitude, but the changes in Vr due to altitude are usually small. V 2 Takeoff safety speed. V2 is the speed flown in the case of an engine failure during takeoff or initial climb. V2 is reached by 35 ft AGL, and maintained to 400 ft AGL such that the second segment climb performance criteria will be met. V2 is never less than 1.1 x Vmca, or 1.2 x stall speed in the configuration used for takeoff. V2 is not Vxse or Vyse, these speeds can be 30 to 60 knots higher. V2 will assure that the aircraft performs as stated in the flight manual. If an engine is lost at a speed greater than V2, you will be above the minimum required takeoff flight path. V2 speed is set as somewhat of a compromise between what would be optimum for takeoff, and what would result in the most optimum climb performance. V2 varies with weight and flap setting. Less flaps means better climb performance, but usually will require a longer runway. Vfs The speed used for climb during the 4th or final segment of a departure with one powerplant failed. This speed will be close to best rate of climb speed with one engine failed and the wing flaps and leading edge devices retracted. V ref V ref is the minimum safe approach speed in landing configuration. It is equal to 1.3 times the power off stall speed in landing configuration. V ac The speed flown during the initial phase of a missed approach or go around with one engine inoperative and the flaps in the approach position.
 

Balanced Field Length

The distance required to accelerate to V1, and stop; or accelerate to V1, lose an engine, and reach a speed of V2 at an altitude of 35 ft AGL, or at least 115% of the distance required to reach V2 and 35 ft AGL with all engines operating, whichever is greater. If the accelerate stop distance and accelerate go distances differ, the longest distance will be used. The term Takeoff Field Length would be used to describe the required takeoff distance in this case.

    High speed aborts have their share of horror stories.  If you have to make one in the first place, your day is not going as well as it could.  The major cause of accidents during high speed aborts is, in my opinion, lack of proper decision making and hesitation on the part of the crew.  They can be done safely, if you use the V speeds as they were intended.  Initiate the abort at a speed no higher than V1.  Don't hesitate when you initiate the abort.  Throttles to idle, speedbrakes, lift dump, ground spoilers, or whatever they call the drag devices on your airplane should be deployed immediately.  Apply maximum reverse thrust, and apply the brakes aggressively if runway length is a consideration.  Be decisive, and don't change your mind in the middle of the procedure.
    I had the pleasure of performing this procedure once when I was departing Monterey California headed for Hawaii in a Westwind II.  A few knots below V1, the right engine went "BANG".  We were full of fuel and within a few pounds of maximum takeoff weight.  Without hesitation, Throttles Idle, Lift Dump, Deploy, Reverse, Deploy, Brakes as necessary, and within a few moments we were stopped.  I pulled off the runway and performed the shut down procedure on both engines.  I looked into the tailcone of the right engine and it looked as if Don King's hairdresser had been there.  They wound up totaling the engine, but there was no damage to any other part of the airplane.  The end result was a bit of inconvenience and a few moments of ass clenching excitement for the passengers.  The safe outcome was never in doubt.  This is most often true when you do not try to brake the laws of physics.


Gradient

A gradient is a means of answering the question "How steep is it?" Gradients can be expressed in terms of "percent". During a drive through the mountains we will likely see a sign that says " 6% Grade next 5 miles Trucks use low gears". A gradient is the ratio between a horizontal distance and a vertical distance. Hills with a 6% grade will change elevation six feet for every 100 feet of horizontal distance traveled.
 

CLIMB SEGMENTS


 

First Segment

This segment begins liftoff and ends when gear retraction is complete and you are at least 35 feet AGL and have attained a speed of V2.  The first segment is generally not the one that is limiting.


Airspeed
Accel to V2
Engines
1 Failed at V1
Thrust
Takeoff
Gear
Retracting
Flaps
Takeoff Setting
Minimum Climb Gradient
Positive Rate

Second Segment:

This segment begins at the end of the first segment and ends at a minimum height of 400' above the runway. This is the always the most restrictive climb segment for all two engine jets currently certificated.  It may not be the most limiting for some older airplanes who's engines have a substantial difference between Take-Off, and Maximum Continuous power.  One example of this is the Jetstar -6.  The engines have 3,000 lbs thrust for takeoff, and 2,575 lbs at maximum continuous.  The power reduction at the end of the third segment results in a substantial reduction of thrust.  In this case, the 4th or final segment climb can be the one that is limiting.  If your aircraft has a chart like "Maximum Takeoff Weight Permitted by Climb Requirements", you need not know which of the segments is actually limiting.


Airspeed
V2
Engines
1 Failed
Thrust
Takeoff
Gear
Retracted
Flaps
Takeoff Setting
Minimum Climb Gradient
2.4%

Third Segment:

This segment extends from the end of the second segment to the completion of flap retraction and acceleration to Venr.  Meeting the 3rd segment requirements means that you can start at V2 and flaps takeoff, and accelerate to Venr or Vfs, retracting the flaps during the appropriate part of the acceleration, all without any loss of altitude.

Airspeed
Accelerating to Vfs
Engines
1 Failed
Thrust
Takeoff
Flaps
Retracting to 0 deg
Minimum Climb Gradient
0.0%

Fourth Segment:

This segment begins at the end of the third segment (flaps retracted), and ends at a height of 1500 feet AGL. This segment completes the takeoff path.

Airspeed
Vfs
Engines
1 Failed
Thrust
Max Continuous
Flaps
Retracted
Minimum Climb Gradient
1.2%

Balked Landing Climb:

This segment begins when an all engine go around is initiated and the engines have reached go around thrust.
 

Airspeed
Vref
2 Engines
Go Around Thrust
Flaps
Full
Minimum Climb Gradient
3.2%

Approach Climb:

This segment begins when the gear is retracted, flaps are in the approach position, one engine windmilling and the remaining engine(s) at go around thrust.
 

Airspeed
Vac
Engines
1 Failed
Flaps
Approach
Minimum Climb Gradient
2.1%
 
Climb Limit The climb limit is the maximum weight at a particular altitude and temperature, at which the aircraft can meet the FAR part 25 climb gradient requirements for takeoff (First, second, third and fourth) or for landing (Approach and balked landing climb). Climb limited weights are lower if engine anti-ice is used because of the reduction in available engine power. Climb limits do not consider runway length, gradient or surface condition, only atmosphere, configuration, and use of systems requiring bleed air. (Engine or airframe anti-ice and pressurization.) The Approach climb and the landing climb limits are established to insure that the aircraft has the capability to successfully execute a "Go around" from as low as 50 ft AGL with all engines operating, and/or a missed approach with an inoperative engine if initiated soon enough. Fly a part 25 certificated airplane within it's operating limitations, and you are not likely to encounter a situation where the aircraft's lack of performance capability will kill you in the event of an engine loss. Runway Limit The runway limit is the maximum weight at which the aircraft may takeoff from, in the case of the takeoff weight limit, or land on, the case of the landing weight limit, on a given runway at a specified temperature, altitude, and runway condition. Runway limits consider any factor that will influence the ability of the aircraft to accelerate, and/or stop. Altitude, temperature, configuration, wind, runway gradient, runway clutter, and inoperative aircraft systems are all considered. Runway limit is expressed in terms of weight, because unless you are building your own airport, your concern is not "How much runway do I need?", but "How heavy can I operate using the runway available?"
 
 
 
Certificate Limit The maximum weight at which an aircraft may be operated with respect to a specific situation, (Takeoff, landing, zero fuel weight). These weights are NEVER exceeded, even if all performance requirements are met or exceeded. Brake Energy Limit The maximum speed from which the aircraft may be stopped in the case of a rejected takeoff, (expressed in KIAS), or the maximum weight at which the aircraft may be landed and stopped with brakes without exceeding the capability of the brakes to dissipate the resulting heat without damage. Reverse thrust may NOT be considered when computing ANY Part 25 takeoff or landing data. Brake energy may limit the maximum landing weight of the aircraft, or limit the maximum value of V1, thereby limiting the maximum takeoff weight for a given runway in some cases. On most aircraft, brake energy will only be limiting when high V speeds and minimum flap settings are used to meet climb segment requirements. Brake energy limits which apply to landing usually occur only under high density altitude and high landing weight. BRAKE ENERGY If the least understood airplane subject areas were listed, brake energy would surely be among them. This need not be so, as brake energy is not complicated. It merely involves some simple physics. (Yes, there is such a thing as simple physics.)

As we learned in our old high school science classes "Energy can't be created or destroyed, only changed." Mankind has built many machines that "change" energy from one form to another to suit our needs. Take for instance the automobile. Chemical energy within the gasoline is changed to heat, causing expansion of gasses within the engine. The energy in these expanding gasses is converted to mechanical energy used to turn the engine's crankshaft. Through the use of gears, this energy is finally used to make the car go. When the car is in motion it has what is called "kinetic energy". When you need to stop the car, you must convert this kinetic energy into another type of energy so the car will stop. We use devices called "brakes" to do this. The brakes convert the kinetic energy into heat. This heat will be absorbed by the atmosphere as the brakes cool.

The brakes can only handle a certain amount of heat. If they are forced to exceed this limit they may be damaged, and the intense heat generated may damage other nearby equipment.

If the brakes convert too much kinetic energy into heat in the process of stopping the airplane, they will heat the wheels and tires. If the wheel & tire assembly gets to hot, they could catch fire, or worse yet the tire could explode. The tires used by large aircraft are usually inflated to pressures in the 100 - 200 PSI range. This can make for a bang large enough to seriously damage an airplane and possibly really bugger up your day..

In order to avoid this, "fuse plugs" are installed in the wheels. The cores in these fuse plugs will melt and release the pressure within the tires prior to the tire failing or catching fire due to the heat. It is far better to have a flat tire during taxi or parking than to risk an explosion. The best way to deal with all this is to check the Aircraft Flight Manual for brake energy limitations and do not exceed them in the first place.

Brake energy and its limitations may be expressed in a number of ways. Maximum takeoff or landing weight may be restricted as a result of brake energy. In this case, the brake energy limit is expressed in pounds. You might also see brake energy expressed in knots indicated airspeed. This addresses the question: How fast can I be going at a given weight, altitude and temperature and still stop without exceeding the brake energy limit? (V1 can NEVER be higher than this figure, as you must be able to stop from V1).

Lets examine the relationship between speed and energy as to better understand what the brake energy charts tell us.

E=½MV2

Kinetic energy is equal to half the mass times the square of the velocity of the aircraft. From this we can see that the Energy is proportional to the square of the speed.

Lets say that a particular aircraft with a speed of 100 KTS has "one unit" of brake energy. (We are defining our own units here to keep it simple.) The energy for a given speed will be as follows:
 
 

141 Kts
2.00 Units
130 Kts
1.69 Units
120 Kts
1.44 Units
110 Kts
1.21 Units
100 Kts
1.00 Units
90 Kts
0.81 Units
80 Kts
0.64 Units
70 Kts
0.49 Units
60 Kts
0.36 Units
50 Kts
0.25 Units
40 Kts
0.16 Units
30 Kts
0.09 Units
20 Kts
0.04 Units

    If you use 100 kts and one unit as a baseline, select your initial braking speed on the left, and see the percentage of brake wear as compared to applying the brakes at 100 knots.  Do not get stupid about this and go off the end of the runway.
    As you can see, a stop from 50 kts generates only 1/4 th of the heat (and brake wear) as a stop from 100 kts. Stopping from 120 kts on the other hand generates 1.41 times the heat and brake wear as the 100 kt stop. We are talking about ground speed here. Think about this next time you are considering a downwind takeoff or landing. A 10% increase in speed gives you a 21% increase in kinetic energy. You who like to land at Vref plus a bunch remember this.

While on the subject of brakes, I would like to dispel an old myth. You do not help your brakes by pumping and releasing them as far as heat is concerned. There is not enough time to dissipate any meaningful amount of heat during the landing roll. Apply the needed braking and hold it until you are at taxi speed. Stop straight ahead. Heavy braking in turns is hard on tires and landing gear.

TAKEOFF AND LANDING

The following is a brief guide to solving typical performance problems with respect to part 25 aircraft. The AFM for the particular machine must be thoroughly reviewed, as there may be additional limitations that are not discussed in this document.

Takeoff

Obtain the necessary information about the departure runway, length, temperature, wind, elevation, gradient, and condition. Draw a diagram similar to the one depicted below, and compute the limitations necessary to fill in the boxes. The figures used are fictitious examples, and do not apply to any actual aircraft.
 

Flaps 15º
Flaps 0º
Climb Limit
10,500 Lbs
11,250 Lbs
Runway Limit
10,300 Lbs
  9,600 Lbs

Max Takeoff Weight Limit º

10,300 lbs at Flaps 15 º

The LOWEST weight in a particular column is the maximum takeoff weight for that flap setting. Determine the takeoff weight limit for each available flap setting. The flap setting that allows you the HIGHEST takeoff weight limit is the preferred configuration for that particular takeoff.

After working several performance problems using various combinations of temperature, runway length, and elevation, you will see that short runways, and lower density altitudes favor the use of more flaps. Balanced field length is generally more limiting than climb performance in those situations. High elevations, hot days, and long runways make the minimum or flaps up takeoffs advantageous due to the improved climb performance. If you have more than enough runway, but are short on climb, you may "Trade" some of that extra runway for climb performance by using less flaps, and higher V speeds. Climb performance improves as V2 approaches Venr or V yse

LANDING ( Usually done at end of flight )


In order to meet the FAR Part 25 landing performance requirements, the aircraft must be able to:

Land and come to a complete stop from 50 Ft AGL and Vref. The total distance required is the sum of the distance from Vref and 50 ft AGL to touchdown, and the distance from touchdown to a complete stop. If operating under FAR part 135 or 121, the actual landing distance must be divided by 0.6 to determine the minimum field length, as the aircraft must be able to land within 60% of the available runway.

Stop without exceeding its brake energy limit. If brake energy may be a limiting factor for landing, a chart will be provided in the AFM allowing the pilot to determine what the brake energy limited weight is.

Climb (Minimum 3.2% gradient) in landing configuration with all engines operating. (Balked landing Climb)

Climb (Minimum 2.1% gradient) with landing gear up, flaps at the approach setting, and one engine inoperative. (Approach Climb)

If these requirements can not be met at the maximum certificated landing weight, the maximum landing weight is reduced such that the above requirements can be met. These performance limits are "Operating Limitations". They define the maximum landing weight for the particular situation at hand. Again, these are "Operating Limitations", not just advisory information.