Performance and structural requirements.

This is explanations of some of the terms used in relation to aircraft performance. Such as speeds, weight and balance calculation and loadsheet.

Abbreviations.


SPEEDS.
  • IAS = Indicated Airspeed = airspeed indicator reading.
  • CAS = Calibrated Airspeed = IAS corrected for static position error.
  • EAS = Equivalent Airspeed = CAS corrected for compressibility error.
  • TAS = True Airspeed = EAS corrected for OAT and pressure altitude.
  • GS = Ground Speed = TAS corrected for wind component.
  • M = Mach number = relationship between TAS and speed of sound.
THRUST SETTING.
  • EPR = Engine Pressure Ratio.
  • FF = Fuel Flow.
  • MCT = Max Continuous Thrust.
  • RPM= Revolutions Per Minute.
  • OAT = Outside Air Temperature.
  • RAT = Ram Air Temperature = TAT.
  • SAT = Static Air Temperature = OAT.
  • TAT = Total Air Temperature = RAT.
MISCELLANEOUS.
  • BA = Braking Action.
  • CG = Center of Gravity.
  • µ = friction coefficient.
  • FL = Flight Level.
  • GWC = Gross Weight Chart.
  • RC = Rate of Climb.
  • RTO = Rejected Takeoff.
  • RWY = runway.
  • SL = Sea Level.
  • T/O = TKOF = takeoff.
  • TODC = Takeoff Data Computer.


Characteristic speeds.


Min. Control Speed on Ground VMCG is defined as the minimum speed at which directional control on ground can be recovered after lateral deviation of max 30 ft and maintained under the following conditions:
  • Sudden engine failure on the most critical engine.
  • Takeoff thrust on the remaining engine(s).
  • Most critical takeoff configuration, normally smallest takeoff flap, unless a VMCG is established for each takeoff flap configuration.
  • Most unfavorable TOW an CG position.
  • Control maintained by rudder only.
Note - VMCG determines the lowest V1 to be used.

Min. Control Speed Airborne VMCA is defined as the speed at which directional control can be recovered and maintained in flight under the following conditions:
  • Sudden engine failure on the most critical engine.
  • Takeoff thrust on the remaining engine(s).
  • Flaps in the smallest position.
  • Landing gear up.
  • Zero yaw or an angle of bank not in excess of 5.
Note - This speed is used in determining VR and V2.
The 5 bank is used by the manufacturer in certification of VMCA. The effect in speed of this bank differ from one aircraft type to another. For wings level flight the VMCA will be in the order of 15 kt for MD-80/90/DC-9. VMCA problem do normally only occur at low takeoff weights. At high weights VMCA is overruled by the stalling speeds, and directional control can be maintained with wing level, practically without any sideslipping. Consequently, the wings level technique is used because more emphasis is placed upon aircraft climb performance at high weights. At low eights the performance capability normally is in excess of that required, so favorable bank angle may be used for heading control if necessary.

V1 = Decision speed is the speed at which, for the purpose of determining the required takeoff runway length, engine failure is assumed to be recognized and the rejected takeoff initiated. The reaction time used in the calculation is about 1 second. Thus for calculation of the required runway length, the engine failure is assumed to occur 1 s before V1. The total time for pilot actions , until full brakes are applied, is the time demonstrated during certification plus two seconds (typically 3-4 seconds).

At V1 it should be possible to either:

  • Reject the takeoff and stop at the end of the runway.
  • Continue the takeoff and reach 35 ft at V2 speed at the end of the runway.
Note - In certification, stopping from V1 is based on the most efficient wheel braking on a dry runway without credit for reversing. The performance in an actual accelerate-stop case may differ from the demonstrated performance due to a number of reasons such as worn or lost brakes, excessive pilot reaction time, etc. The benefit of reversers is very small on top of max braking on a dry runway. Thus, stopping from V1 at a runway limited weight is a very critical task.

Reduced V1 (15 ft screen height) is possible to use. In connection with reduced braking action, as on a wet runway, a rejected takeoff close to V1 becomes even more critical than described above. For such cases, a reduced V1 can be used to transfer safety margin from the continued takeoff case to the rejected case to better balance the margins between the two options.

In accordance with CAA regulations, V1 on wet and contaminated runway is based on obtaining 15 ft at the end of takeoff distance instead of the normal 35 ft. Takeoff weight calculations must be designed to consider V1 reductions. Otherwise the contamination may degrade acceleration, after the engine failure, so the aircraft will not lift off before the end of the runway.

VR = Rotation speed is the speed at which, for the purpose of determining required takeoff runway length, rotation of the aircraft is initiated.

VR must be determined not to be less than:

  • V1.
  • 1.05 times the minimum control speed, airborne.
  • A speed which permits the attainment of V2 prior to reaching 35 ft height at the end of the takeoff distance.
  • An upper limit for VR is set to max tire speed.

V2 = Takeoff safety speed is the speed used to determining the performance during the initial climbout. In these calculations, V2 should be reached prior to attaining a height of 35 ft above the runway surface.

Thus V2 is used in determining:

  • The required takeoff distance, prior to the end of which it should be attained.
  • The climb requirements limitaions.
  • The obstacle clearance limitaions.
V2 shall not be less than:
  • 1.1 times VMCA.
  • 1.2 times VS.
In practise, V2 should be used as climbout speed in case of engine failure until the aircraft has attained a safe height above obstacles in the takeoff direction. Flying at speeds below V2 will result in considerable loss in climb performance and may crate problems regarding stall.

VFl up and VSl in are minimum speeds for retraction of FLAPS and SLATS after takeoff.

VClean is the speed to be used in the final segment of a takeoff with engine failure, i.e with "CLEAN" aircraft (after flap/slat retraction).

VP = Pattern speed is "maneuvering speeds" and gives better margin to stall. Used in low level holdings, procedure and circuits.

VP clean "PATTERN CLEAN", normally the lowest speed with all engines running.

VMO Max operating IAS, limited by structural requirements.

MMO Max operating Mach, limited by high speed aerodynamics.

Rough Air Speed is the speed which should be used in connection with severe turbulence.

VHOLD Speed to be used during holdings. Below FL 250 the same as VP clean. FL 250-290 VP clean + 10 kt. Above FL 290 VP clean + 20 kt.

Vref is reference speed used to derive other speeds for different phases of flight by adding of fixed increments. The amount to be added are stated in the respective aircraft operating manual. When Vref is used without additional flap suffix, it is based on the full landing configuration. Occasionally it is necessary to use Vref based on other flap setting. In such cases the configuration is always indicated as a suffix (Vref 20).

Landing runway length requirements are based on Vref at 50 ft over the runway threshold.

VTH For aircraft not using Vref, landing runway length requirements are based on VTH at 50 ft over the runway threshold.


Weight and balance.


In connection with issuance of loadsheet and flighplan, some specific load control terms are used. This table shows the explanation in alphabetic order.

WEIGHT terms.

BW = Basic Weight. Basic empty weight, including:
  • aircraft structure.
  • systems.
  • engines.
  • unremovable equipment.
  • unusable liquids (fuel, oil and others).
  • standard loose equipment.
DOW = Dry Operating Weight. Operational empty weight. Basic weight plus operational items such as crew and pantry (equipment, food, beverages).

LW = Landing Weight. Takeoff weight minus trip fuel.

MLW = Maximum Landing Weight. Weight limitation for landing, governed by structural and/or operational requirements.

MTOW = Maximum TakeOff Weight. Weight limitation for takeoff (brake release), governed by structural and/or operational requirements.

MZFW = Maximum Zero Fuel Weight. Structural weight limitation.

RW = Ramp Weight. Takeoff weight plus taxi fuel, i. e. weight of loaded aircraft before starting the engines.

TOW = Takeoff Weight. Gross weight of aircraft at brake release for takeoff, i.e. actual zero fuel weight plus takeoff fuel.

ZFW = Zero Fuel Weight. Dry operating weight plus total traffic load.

Fuel terms.

Ballast fuel. Non-usable fuel used for balancing purpose (only possible on some aircraft). The ballast fuel is separated from takeoff fuel (usable fuel) and loaded ia a separate tank. the fuel must not be consumed or jettisoned during flight.

Block fuel. Weight of total amount of fuel on board before starting taxi.

Burn-off fuel. Taxi fuel plus trip fuel.

Reserve fuel. Difference between takeoff fuel and trip fuel, consiting of:
  • Route reserve,
  • diversion,
  • holding, and
  • additional fuel.
Taxi fuel. Weight of fuel to cover APU consumption, engine start and ground maneuvers until start of takeoff. Standard weights are used which are, with a few exceptions, applicable at every airport.

TOF = TakeOff Fuel. Weight of total usable fuel onboard at the moment of takeoff (brake release).

Trip fuel. Weight of the precalculated fuel consumption from takeoff to touchdown at the next point of landing.

Load terms.

Allowed traffic load. The weight remaining after the subtraction of the operating weight from the allowed takeoff weight.

Deadload. Total weight of:
  • baggage
  • cargo
  • mail
Total traffic load (total payload). Total weight of:
  • passengers
  • baggage
  • cargo
  • mail

Underload. difference between allowed traffic load and load actually carried.

Balance terms.

BI = Basic Index. Center of gravity at basic weight (BW) expressed as an index value.

CG = Center of Gravity. Point about which an aircraft would be balanced if suspended.

DOI = Dry Operating Index. Center of gravity at dry operating weight expressed as an index value. Basic index (BI) corrected for the balance influence of the loads included in dry operating weight (DOW).

DLI = Dead Load Index. Dry operating index (DOI) corrected for the balance influence of the load in compartments.

LIZFW = Loaded Index at Zero fuel weight. Deadload index (DLI) corrected for the influence of passengers in cabin (fuel not included).

MAC = Mean Aerodynamic Chord. Imaginary reference line (chord) dividing the wing areas producing the same amount of lift. Location of CG of loaded aircraft is given as a percentage of the MAC.