AIRPLANE PERFORMANCE
Aircraft Performance means the capabilities of an aircraft under various
stages of its flight. These aspects of flight are considered under two distinctive
divisions:
1. Mandatory Performance
2. Operational Performance
MANDATORY PERFORMANCE
Mandatory Performance requirements must be met to obtain Certificate of
Airworthiness. Aircraft in public transport category has to be conforming to
certain international standards. It specifies the required performance of a
transport aircraft if an engine failure occurs during any stage of flight. The
Airplane Flight Manual (AFM) shows the various performance limits with one
engine inoperative for each type of aircraft. It also caters for all engines
operating takeoff field performance and landing climb performance. These
limitations and stipulations of AFM have to be complied with throughout the
life of the aircraft.
OPERATIONAL PERFORMANCE
It provides the all engines operating performance data for climb, cruise,
descent and holding. This data is provided for normal operation of the
aircraft. If aircraft does not perform according to data provided, it does not
mean that aircraft is not airworthy. Conspicuous differences should be
investigated and corrective action has to be taken.
REGULATIONS
FEDERAL AVIATION REGULATIONS (FAR)
PART 1 DEFINITIONS AND ABBRIVIATIONS
PART 25
AIRWORTHINESS STANDARDS – TRANSPORT CATEGORY AIRCRAFT
 DESIGN CRITERIA – MANUFACTURERS RESPONSIBILITY FOR
DEMONSTRATION OF COMPLIANCE
 ADVISORY CIRCULARS : CLASSIFICATION &
AMPLIFICATION
PART 121
CERTIFICATION AND OPERATIONS: DOMESTIC, FLAG AND
SUPPLEMENTAL CARRIERS AND COMMERCIAL OPERATORS OF
LARGE AIRCRAFT
 OPERATING CRITERIA – OPERATORS RESPONSIBILITY FOR
DEMONSTRATION OF COMPLIANCE
PERFORMANCE RELATED TERMS
ISA
International Standard Atmosphere assumes sea level temperature as 15C,
pressure as 1013.2 hPa, density as 1.225 kg/m3 with a lapse rate of
temperature of 1.98C per 1000 ft till 36089 ft beyond which it is assumed as
constant at –56.5C.
For example, at 5000 ft above mean sea level, in standard atmosphere
conditions, temperature will be 5.1C, pressure will be 843.1 hPa and density
will be 1.055 kg/m3.
OAT
Outside Air Temperature is the free air static ambient temperature.
SAT
Static Air Temperature is outside air (ambient) temperature as computed by
the air data computer from TAT and presented on the static air temperature
indicator. It is almost equal to OAT.
RAM RISE
It is an increase in temperature due to compressibility of the air at higher
speed. Higher the speed, higher the ram rise.
TAT
Total Air Temperature as shown by the TAT gauge. It equals OAT plus Ram rise.
The higher the speed, the higher would be the Ram rise and so the TAT.
RECOVERY FACTOR
It is the efficiency factor of TAT probe. TAT probe factor is 1 for B737.
QNH
The barometric pressure at the aerodrome level reduced to mean sea level
as per ISA conditions. When QNH is set on the subscale the altimeter reads
height above mean sea level.
NAUTICAL MILE
It is defined as the arc of the meridian subtending an angle of one minute at
the centre of the earth. This distance is maximum near the poles and least
near the Equator but it is assumed as equal to 6080 feet.
ICING CONDITIONS
Icing conditions exist when OAT on the ground and for takeoff, or TAT in flight
is 10C (50F) or below, and visible moisture in any form is present (such as
clouds, fog, with visibility of one mile or less, rain, snow, sleet and ice crystals).
Icing conditions also exist when the OAT on the ground and for takeoff is 10C
(50F) or below when operating on ramps, taxiways or runways where surface
snow, ice, standing water or slush may be ingested by the engines or freeze
on engines, or nacelles.
ELEVATION
Elevation is the height above mean sea level of a place.
PRESSURE ALTITUDE
Pressure Altitude is the altitude in the standard atmosphere corresponding to
the outside pressure. In the aircraft it is obtained by setting 1013.2 hP in the
subscale of the pressure altimeter. At standard atmosphere conditions,
Pressure Altitude is equal to Elevation.
DENSITY ALTITUDE
Density Altitude is the altitude in the standard atmosphere corresponding to
outside density. Density Altitude and Pressure Altitude will be the same when
standard atmosphere is prevailing. When temperature is more than standard,
density altitude will be more than pressure altitude and vice versa.
WIND VELOCITY
The direction from which the wind is blowing and its speed is called wind
velocity. Wind velocity. Wind velocity reported at an airfield is generally the
wind measured at 10 meter height at control tower.
INDICATED AIRSPEED (IAS)
Airspeed as indicated by the ASI uncorrected for position error.
CALIBRATED AIRSPEED (CAS)
Indicated airspeed corrected for static source position error and instrument
error. CAS can be same, more or less than IAS depending upon the position
error and instrument error.
EQUIVALENT AIRSPEED (EAS)
Calibrated airspeed corrected for compressibility. EAS will always be less than
CAS, since compressibility correction is negative. The higher the speed the
more will be the difference.
TRUE AIRSPEED (TAS)
EAS corrected for atmospheric density, since ASI is calibrated at standard sea
level density. The higher the flight level, the lower the density and hence
higher the TAS.
CONFIGURATION
A particular combination of the position of the moveable elements such as
wing flaps, landing gear, etc. which affect the dynamic characteristics of the
aircraft.
LOAD FACTOR (n)
The ratio of lift generated by the wing to the weight of the aircraft.
VSmin
Calibrated stall speed or the minimum steady flight speed, at which the
airplane is controllable in specified configuration at zero thrust or idle thrust (if
having no appreciable effect on stall speed) and C of G in the most
unfavourable position (forward). Stall speed varies with weight, flap setting
(configuration), bank angle and C of G. Stall speeds are based on the
minimum speed in the stall maneuver with an entry rate speed reduction of 1
knot/sec. Full up elevator deflection is used, and the achieved load factor is
less than 1 G at the minimum speed.
VS1G
The 1-G stall speed is determined from the maximum lift coefficient (CLmax)
corrected for load factor (n) during the stall maneuver in level flight.
For information, on B737-800 at sea level, VS1G for 70000 kg with Flaps 5 takeoff
is 129 KCAS and for 65000 kg with Flaps 40 landing is 111.5 KCAS. As VS1G
speed is generally 6-7% higher than VSMIN, the minimum regulatory factor for V2
in terms of stall speed (earlier 1.2 VSMIN) has been amended 1.13VS1G. Similarly
minimum regulatory factor for VREF in terms of stall speed (earlier 1.3 VSMIN) has
been amended 1.23VS1G.
VS1
It is the stalling speed at specified flaps setting.
VS0
It is the stalling speed at the most extended landing flaps setting.
MACH NUMBER
It is the ratio of TAS to the local speed of sound. Since speed of sound varies
directly with temperature. At a constant Mach number when temperature
increases TAS will also increase and vice versa.
MACHMETER
It is the instrument which indicates Mach Number. Since high speed buffet
resulting from flow separation associated with shock waves occurs when the
TAS is close to speed of sound, we need a Machmeter to define high speed
limits on a jet aircraft.
BUFFET ONSET CHARACTERISTICS
Buffet onset occurs when the airflow starts to separate from the wing. This
characteristic is a function of angle of attack and Mach number/speed.
HIGH SPPED BUFFET
The maximum speed at which buffeting starts. It is a function of weight and
altitude. Higher the altitude and weight, high speed buffet occurs at a lower
maximum speed.
LOW SPEED BUFFET
The lowest speed at which buffet onset occurs. Higher the altitude and
weight, the earlier will be the buffet onset.
VA
Design Maneuvering Speed - It is the maximum speed at which application of
full available rudder or elevator will not overstress the airplane. In the flaps up
configuration, full aileron can be applied at any speed.
VB
Design speed for maximum gust intensity. It is used to establish the turbulent
penetration speed (Rough air speed).
VC
Design Cruising Speed - The maximum design cruising speed shall be
sufficiently greater than VB to provide for an inadvertent speed increase likely
to occur as result of severe atmospheric turbulence.
VC  VB + 43 knots
VD/MD
Design Dive Speed - It is used to determine the VMO/MMO, which ensures that
VD shall not be exceeded if the airplane is upset from flight at normal
operating speed.
VDF/MDF
It is the maximum demonstrated flight diving speed.
VF
Design flap speeds. The flap placard speeds are determined to meet design
criteria in accordance with rules. Wing flaps and their supporting structure
and operating mechanism must be designed for the critical loads occurring
during transition from one flap position and airspeed to another.
VF shall not be less than 1.6 VS1 in takeoff configuration at MTOW, 1.8 VS1 in
approach configuration at MLW and 1.8 VS0 in landing configuration at MLW.
VMO/MMO
Maximum operating limit speed is a speed which shall not be deliberately
exceeded in any regime of flight. It should not be greater than the design
cruising speed VC and sufficiently below VD/MD or VDF/MDF.
VLO
Landing gear Operating Speed - This shall be established not to exceed a
speed at which it is safe to extend or retract the landing gear as limited by
design or by flight characteristics.
VLE
Landing gear Extended Speed - It shall be established not to exceed a speed
at which it has been shown that the airplane can be safely flown with the
landing gear secured in the fully extended position and for which the
structure has been proved in accordance with rules.
OPTIMUM ALTITUDE
Optimum altitude is the altitude at which the best fuel mileage is achieved.
Optimum altitude is irrespective of temperature and varies with weight only.
When weight reduces optimum altitude increases. Therefore a step climb
enables aircraft to fly at or close to optimum altitude.
ALTITUDE CAPABILITY
The altitude capability is the Maximum Cruise Thrust or Maneuver Capability
altitude for a given Mach no. Maneuver Capability altitude is irrespective of
temperature and varies with weight for a given bank angle. However
Maximum Cruise Thrust altitude, to achieve a target Mach no., varies with
weight and temperature.
COST INDEX
It is the ratio of variable cost per hour in dollar ($) to the cost of 1 lb of fuel in
cents (¢). Where fuel cost is minimum, higher cost index can be used and vice
versa. Cost Index of zero generally refers to Maximum Range Cruise speed
and infinity refers to VMO/MMO. Since Cost Index is a function of fuel price,
different values of Cost Index can be selected depending on route distance
and reasonable difference of fuel prices between departure and destination
stations.
MAX RANGE CRUISE
This cruise technique gives best fuel mileage for a given weight and altitude.
It means when airplane is flown at a constant altitude, thrust decreases with
reduction of weight. When fuel cost is overriding factor, this technique is
recommended.
LONG RANGE CRUISE
This cruise technique gives 99% of MRC fuel mileage. This penalty of 1% in fuel
mileage gives considerable increase in speed. This is generally used in
alternate planning (diversion).
CONSTANT SPEED CRUISE
When a constant TAS or Mach is to be maintained at a given altitude, thrust
decreases with reduction of weight.
CONSTANT THRUST CRUISE
This cruise technique is based on constant fuel flow to maintain constant
thrust, which increases speed at constant altitude or increases altitude at
constant speed.
ECONOMY CRUISE
This method is used to minimize the total trip cost (generally, the sum of Fuel
Cost and Time Cost). Economy cruise speed is a function of gross weight,
altitude, cost index and wind.
BEST ENDURANCE CRUISE
To obtain the greatest endurance for all holding purposes, it is necessary to fly
at the minimum fuel flow. But holding speed selected is generally the highest
value of three requirements: Minimum fuel flow, Minimum maneuvering speed
and 1.3 g maneuver to initial buffet.
WIND ALTITUDE TRADE
This enables trading altitude for wind advantage. Favourable wind is a factor
which may justify operations off-optimum altitude. When ground speed is
more at lower altitude than optimum altitude, fuel used at lower level could
be lower than the optimum altitude.
CENTRE OF GRAVITY
The point through which the total weight of a body is acting. For the same
weight, centre of gravity may vary depending on the load distribution.
MEAN AERODYNAMIC CHORD
It is the chord of a section of an imaginary airfoil on the wing which would
have force vectors throughout the flight range identical to the actual wing.
The entire lift generated by the wing is assumed to take place along the
MAC. The aircraft C of G movement is measured in terms of MAC.
INDEX UNIT
When loading an airplane, summation of moments (Weight  Balance Arm) is
necessary to determine the net centre of gravity for the airplane, payload
and fuel. Moments would normally be expressed in kilogram-inch units
resulting in very large numbers. For the sake of convenience, an indexing
system is used to normalize moments to more manageable numbers by
dividing the moment by a constant (C).
BASIC WEIGHT OR TARE WEIGHT
This consists of the weight of the aircraft equipped with basic inventory like
chairs, racks, fixed pantry, oil and fuel in the lines (Without crew and galley
items).
OPERATIONAL EMPTY WEIGHT
OEW or Basic Operating Weight (BOW) or Aircraft Prepared for Service Weight
(APS Wt.) equals the basic weight plus crew & crew baggage plus cabin and
catering items.
OPERATING WEIGHT
Operational Empty Weight plus Takeoff fuel.
MAXIMUM TAXI WEIGHT
MTW is the maximum weight for ground maneuver as limited by aircraft
strength/applicable regulations. (It includes weight of taxi and run-up fuel.)
MAXIMUM TAKEOFF WEIGHT
MTOW is the maximum weight at brake release as limited by aircraft
strength/applicable regulations.
MAXIMUM LANDING WEIGHT
MLW is the maximum weight
strength/applicable regulations.
for
landing
as
limited
by
aircraft
MAXIMUM ZERO FUEL WEIGHT
MZFW is the maximum weight allowed before usable fuel (fuel available for
aircraft propulsion) must be loaded in the aircraft as limited by
strength/applicable regulations.
This is a structural limit of the wing roots which should not be exceeded.
PAYLOAD (TRAFFIC LOAD)
The load that can be carried in an aircraft in the form of passengers and
dead load (baggage, cargo and mail).
MAXIMUM PAYLOAD
The maximum payload is the MZFW minus APS Weight. However, the
maximum allowable traffic load is also a function of MLW and RTOW.
AIRPORT
RUNWAY
A rectangular area of defined dimensions on a land aerodrome prepared for
landing and takeoff run of an aircraft.
STOPWAY
An area beyond the runway, having the same width as the runway centrally
located about the extended centre line of the runway. Stopway must be
able to support the airplane during an aborted takeoff without causing
structural damage to the airplane. It must be designated by the airport
authorities for use in decelerating the airplane during a rejected takeoff.
CLEARWAY
An area beyond the runway not less than 500 ft wide (250ft on either side of
the extended center line of the runway) and under the control of airport
authorities. The clearway is expressed in terms of a clearway plane, extending
from the end of the runway with an upward slope not exceeding 1.25%
above which no object nor any terrain protrudes. However threshold lights
may protrude above the plane if their height above the end of the runway is
26 inches and if they are located to each side of the runway. Clearway
cannot exceed 50% of runway length.
TAKE OFF DISTANCE AVAILABLE (TODA)
The total area available for an aircraft to complete its takeoff run and
achieve V2 before 35 ft height. It will include the runway and clearway. TODA
shall not be more than 150% of TORA.
TAKE OFF RUN AVAILABLE (TORA)
TORA is the length of the runway declared as available and suitable for
accelerating the aircraft to VLOFF plus half the distance to reach 35 ft during
a takeoff. TORA equals runway length.
ACCELERATE STOP DISTANCE AVAILABLE (ASDA)
The total ground distance available for an aircraft to accelerate to V1,
throttle back and stop with normal application of brakes. It includes the
runway length (TORA) plus stopway.
RUNWAY SLOPE
It is the gradient of the runway surface from the beginning to the end of
runway. Effective slope may be calculated by different methods, but
generally average slope is considered for airplane performance. An uphill
slope is a disadvantage for takeoff and an advantage for landing and viceversa. Aircraft is generally certified up to  2%. Effect of slope is not factored in
the calculation of landing distance.
APRON
It is a designated area of the airport where the aircraft parking bays are
located. At some airports, aero-bridge facilities are provided. These areas are
used for embarking and disembarking of passengers, loading and offloading
of cargo. Refuelling and aircraft preparation for service are also done in this
area. In short, apron is meant for arrivals and departures.
THRESHOLD
It is the beginning of that portion of the runway available for landing.
Generally the beginning of the runway is the threshold.
DISPLACED THRESHOLD
Sometimes the threshold is advanced or displaced ahead of the beginning of
the runway due to presence of obstacle in the approach path for landing.
Threshold must be at or after the line where clearance plane intersects the
runway.
PAVEMENT CLASSIFICATION NUMBER (PCN)
ICAO introduced the ACN/ PCN system as a measure to classify pavement
bearing strength for an aircraft with all-up weight for more than 5700 kg. PCN
is a number expressing the bearing strength of a pavement for unrestricted
operations. PCN will be determined and reported by airport authority. PCN
consists of type of pavement, subgrade strength, tire pressure and calculation
method.
For example, PCN 60 F/B/W/T means:
Bearing strength
Type of pavement (F)
- 60
-
Flexible
Category of subgrade
below the runway surface (B)
-
Medium
Tire pressure (W)
psi)
Evaluation method (T)
- Medium
(up to 217
-
Technical
AIRCRAFT CLASSIFICATION NUMBER (ACN)
ACN is a number expressing the relative effect of an aircraft on a pavement
for a specified subgrade category. The ACN is generally calculated with
respect to the centre of gravity position, which yields the critical loading of
the critical gear. Normally ACN must be either equal to or less than PCN for
the particular type of aircraft.
LOAD CLASSIFICATION NUMBER (LCN)
LCN is a value indicating the load carrying capability of a runway or the
pavement loading characteristics of an aircraft relative to specific radius of
relative stiffness (a factor based on concrete slab thickness) or flexible
pavement thickness (surface course, base course and sub base course). LCN
has to be determined for a given aircraft and compared with the LCN of
runway.
There is no relationship between PCN and LCN. They are to be dealt
separately.
AIRPORT LAYOUT
Intentionally
Blank
TAKE OFF DISTANCE REQUIRED (TODR)
It is the greater of:
1. The distance to takeoff and climb to a height of 35 ft (15 ft on a wet
runway) with a failure of the critical engine at VEF; or
2. 115 percent of the distance to takeoff and climb to a height of 35 ft with
all engines operating.
TAKE OFF RUN REQUIRED (TORR)
It is the greater of:
1. The distance to takeoff and climb to a point equidistant between lift off
and the 35 ft height point with a failure of the critical engine at VEF (On a
wet runway, the takeoff run required is the distance to takeoff and climb
to 15 ft with a failure of the critical engine at VEF); or
2. 115 percent of the distance to takeoff and climb to a point equidistant
between lift off and 35 ft height point with all engines operating.
ACCELERATE STOP DISTANCE REQUIRED (ASDR)
It is the greater of:
1. The sum of the distances required to accelerate with all engines operating
and come to a complete stop assuming a critical engine failure at VEF; or
2. The sum of the distances required to accelerate with all engines operating
and come to a complete stop with no engine failure.
BALANCED FIELD LENGTH
It is the condition in which V1 is selected so as to make the TODR equal to
ASDR.
UNBALANCED FIELD LENGTH
It is the condition in which V1 is selected so as to make the TODR and ASDR
unequal.
VEF
Critical engine failure speed. VEF is the speed at which the critical engine is
assumed to fail. It shall not be less than the VMCG.
VMCG
ground minimum control speed, is the minimum control speed on the ground,
at which, when the critical engine suddenly becomes inoperative, it is
possible to recover control of the airplane with the use of primary
aerodynamic controls alone (without the use of nose wheel steering) to
enable the takeoff to be safely continued using normal piloting skill and
rudder control forces not exceeding 150 pounds.
V1
Action initiation speed. V1 means the maximum speed in the takeoff at which
the pilot must take the first action (e. g., apply brakes, reduce thrust, deploy
speed brakes) to stop the airplane within the accelerate-stop distance.
V1 also means the minimum speed in the takeoff, following a failure of the
critical engine at VEF at which the pilot can continue the takeoff and achieve
the required height above the takeoff surface within the takeoff distance.
V1(MCG)
The minimum takeoff decision speed, at which, when the critical engine
suddenly becomes inoperative at VEF with the remaining engines at takeoff
thrust, it is possible to control the airplane with primary aerodynamic controls
alone and continue the takeoff. This is the V1 speed which results when VEF is
set equal to VMCG.
VMCA
Air minimum control speed, is the airspeed, at which, when the critical engine
is suddenly made inoperative, it is possible to recover control of the airplane
with that engine still inoperative, and maintain straight flight either with zero
yaw or with an angle of bank of not more than 5 degrees towards the live
engine. VMCA may not exceed 1.2 VSmin or 1.13VS1G (Stall speeds determined at
the maximum sea level takeoff weight with maximum available takeoff
thrust). Rudder forces required to maintain control may not exceed 150
pounds.
VR
Takeoff rotation speed. VR is the speed at which rotation is initiated to attain
the takeoff safety or climbout speed, V2, at 35 ft above the takeoff surface. VR
must not be less than 1.05 times VMCA, nor less than V1.
VMU
Minimum unstick speed. VMU shall be the speed at which the airplane can be
made to lift off the ground and to continue the takeoff without displaying any
hazardous characteristics. VMU speeds shall be selected by the applicant for
the all engines operating and the one engine inoperative condition.
VLOFF
Airplane lift off speed. The lift off speed is closely associated to the VR speed
and is dictated by that speed. The all engines operating lift off speed must
not be less than 110% of VMU assuming maximum practicable rotation rate.
The one engine inoperative lift off speed must not be less than 105% of VMU.
On longitudinally limiting (stretched fuselage) airplanes VMU is high, VLOFF
becomes higher, necessitating a higher than normal V2 and hence VR.
V2
Takeoff safety speed. V2 is equal to the target speed to be attained at the 35
ft height assuming recognition of an engine failure at or after V1. It must be
equal to or greater than 113% of the VS1G in the takeoff configuration or 110%
of the air minimum control speed.
TAKEOFF FLIGHT PATH
The takeoff flight path begins 35 ft above the takeoff surface at the end of
the takeoff distance and extends to a point where the airplane is at least
1500 ft above the takeoff surface or has achieved the enroute configuration
and final climb speed, whichever is later. The takeoff path is divided into a
number of segments to meet various airworthiness requirements.
REFERENCE ZERO
A point on the runway or clearway plane at the end of the takeoff distance
and 35 ft below the flight path to which the height and distance coordinates
of other points in the takeoff flight path are referred.
FIRST SEGMENT
Extends from the end of the takeoff distance to the point where the landing
gear is assumed to be fully retracted, using takeoff thrust and takeoff flaps at
a constant V2 speed.
SECOND SEGMENT
Extends from the gear up point to a gross height of at least 400 ft, using
takeoff thrust and takeoff flaps at a constant V2 speed.
THIRD SEGMENT
The horizontal distance required to accelerate, at constant altitude using
takeoff thrust, to the final climb speed while retracting flaps in accordance
with the recommended speed schedule.
The minimum climb gradient capability should be 1.2% for a twin-engine
airplane, 1.5% for three-engine airplane and 1.8% for four-engine airplane.
MAXIMUM LEVEL-OFF HEIGHT
The maximum height at which the third segment can be completed before
the time limit on the use of takeoff thrust expires.
FINAL TAKEOFF SEGMENT
Extends from the end of the third segment to a gross height of at least 1500 ft
or where transition to enroute configuration is completed, whichever is later,
with flaps up at maximum continuous thrust.
The minimum climb gradient capability (relative to air) should be 1.2% for a
twin-engine airplane, 1.5% for three-engine airplane an 1.8% for four-engine
airplane.
GRADIENT
The ratio expressed in percentage of the change in geometric height divided
by the horizontal distance traveled in a given time. As an approximation,
Gradient (%)= ROC (fpm)  TAS (knots)
GROSS GRADIENT
The actual performance of the airplane under specified conditions.
NET GRADIENT
It is the gross gradient reduced by an amount as per regulations.
GROSS HEIGHT
The geometric height attained at any point in the takeoff flight path using
gross climb performance. Gross height is used for calculating actual pressure
altitude at which obstacle clearance procedures and wing flap retraction
are initiated, and level-off height scheduled.
NET HEIGHT
The geometric height attained at any point in the takeoff flight path using net
climb performance. Net height is used to determine the net flight path which
must clear any obstacle by at least 35 ft to comply with the regulations.
IMPROVED CLIMB
Normally manufacturers would like to use as low a V2 as is permissible for
presentation of their performance. A lower V2 would have cascading effect
on lowering VR and V1 in order to meet regulatory requirements (VR should be
such that V2 is achieved by 35 ft and V1mcg<V1<VR). This will help them show
that the aircraft is able to takeoff at MAX Weight with the least amount of
runway length. This is their sales pitch as this helps them show that the field
length required for operating their aircraft is less. However, optimum
performance is achieved at a higher V2 as shown below:-
The regulation requires a minimum climb gradient of 2.4% to be demonstrated
in the beginning of the second segment (one engine in-operative). When
using minimum V2 there is one MAX Weight at which this compliance is
achieved. We call this the climb limit RTOW. If we increase the V2
corresponding to maximum L/D, keeping the weight the same, then we can
achieve a higher climb gradient than 2.4% or by keeping the climb gradient
same as 2.4%, we can increase the climb limit weight. However, as you are
using higher takeoff speeds you should have excess runway available for
using the higher speeds along with higher tire speed limit and brake energy
limit weight.
In other words whenever the field limit weight is more than the climb limit
weight we can use the over speed technique. This technique is also called
the improved climb technique.
We can also use the over speed or improved climb technique for increasing
the climb gradient to improve obstacle clearance thus increasing obstacle
limited weight. However, while doing improved climb for better obstacle limit
weight, the lift off point during takeoff roll gets closer to the obstacle. Thus,
there is a limit up to which improved climb can be done to increase the
obstacle limited weight.
REDUCED THRUST
In the life cycle of an engine, of say 5000 hours, if you are flying 2 hour sectors,
you will be executing 2500 takeoffs. Normally, takeoff thrust is used in every
takeoff for a period of approximately 1 minute when the EGT is at its
maximum. Therefore, in 2500 takeoffs, you are running the engine at takeoff
thrust for 2500 minutes OR approximately 40 hours. It is these 40 hours at
takeoff thrust out of 5000 hours which causes almost 75% of wear to the
engine as the EGT is at its highest during takeoff. Therefore, there is need to
reduce the wear of the engine during takeoff by using a lesser thrust than the
maximum, if possible.
The question then arises – How can I achieve this? Simple, whenever the
actual takeoff weight is less than the maximum weight at full thrust permitted
for the ambient conditions (P.A. and Temperature), then we can choose a
lower takeoff thrust than maximum takeoff thrust which would yet meet all the
regulatory requirements.
To calculate this takeoff thrust value, we check up from the airport
analysis chart, the temperature at which the actual weight is limiting and call
it the assumed temperature and use the thrust corresponding to this
temperature instead of the maximum takeoff thrust corresponding to the
actual atmospheric conditions (PA and OAT). If we do this, we are using a
lower takeoff thrust and so the engine will run at lower EGT than the
maximum.
However, we can reduce takeoff thrust by maximum of 25% of the
actual takeoff thrust for prevailing ambient conditions.
How safe is the assumed temperature technique?
Absolutely safe. Since actual conditions are colder, even with the lower thrust
the runway length required will be less, the thrust generated by the engine is
higher than what you would have got if actual OAT was the assumed
temperature.
There is another method used for using lower thrust for takeoff, called
“Derate”. In some aircraft, using FMC you can select a fixed derate. For
example, the B737-800 is certified for three takeoff thrust ratings 26,400 lbs of
thrust, 24000 lbs of thrust and 22,000 lbs thrust. If the aircraft is certified with
26.4 K thrust, pilot can select TO1 to set the maximum takeoff thrust as 24K or
TO2 for 22K. FMC will automatically, set the thrust to the appropriate levels.
The disadvantages of this method are that:
a) We have to provide airport analysis charts for 3 ratings.
b) Pilots can make mistakes by using the wrong airport analysis charts.
In Jet Airways, we have gone for fixed derates to avoid the problems
highlighted above. Our B737-800s have been derated to 24K. Similarly our 737400s are rated to 22K and 737-700s are rated to 22K maximum takeoff thrust.
We follow assumed temperature technique on these fixed derates.
APROACH CLIMB GRADIENT
To be demonstrated with approach flaps, landing gear up, one engine
inoperative, other engine at go-around thrust and airplane still meets the 2.1%
gross gradient capability for twin engine aircraft; 2.4% gross gradient
capability for three engine aircraft and 2.7% gross gradient capability for four
engine aircraft;
LANDING CLIMB GRADIENT
To be demonstrated with landing flaps, landing gear down with all engines
operating and the thrust is go-around thrust or the thrust available on the
engine 8 seconds after the thrust levers are moved from the minimum flight
idle position to the takeoff position, whichever is less. Aircraft should be
capable of demonstrating a gross gradient of 3.2% for twin/three/four engine
aircraft.
VMCL
The minimum control speed with the airplane configured for approach at
which the airplane is controllable with maximum of 5 degrees bank when the
critical engine suddenly becomes inoperative with remaining engine at goaround thrust. For three or four engine airplanes, VMCL-1 with one engine
inoperative and VMCL-2 with two engines inoperative are also defined.
APPROACH CLIMB SPEED
It is the climb speed which ensures the minimum approach climb gradient of
2.1% in the approach configuration with go-around thrust on operating
engine. To demonstrate this compliance the speed should not exceed 1.40
VS1G.
LANDING CLIMB SPEED
It is the climb speed that ensures the minimum landing climb gradient of 3.2%
in the landing configuration with all engines operating. To demonstrate this
compliance the speed should not exceed 1.23 VS1G.
VREF (LANDING SPEED)
The minimum speed at the 50 feet height over the threshold in a normal
landing. This speed is equal to 1.23 times VS1G in the landing configuration.
LANDING DISTANCE AVAILABLE (LDA)
It is the length of runway declared, available and suitable for the ground run
of airplane whilst landing. LDA equals the length from threshold to end of
runway.
ACTUAL LANDING DISTANCE (ALD)
ALD is the distance from 50 ft over threshold at landing speed (VREF) till the
complete stop of an airplane with maximum manual braking.
LANDING DISTANCE REQUIRED (LDR)
LDR is 1.67 times the distance required to land from 50 ft over threshold at
landing speed, touching down at the 1000 ft marker, and to come to a
complete stop using speed-brakes and maximum manual braking.
FAR LDR = 1.67  ALD
Or, ALD = LDR  1.67  LDR  0.60
The LDR (or, FAR LDR) on WET runway will be 15% more than the FAR LDR on
DRY runway.
FAR LDR (Wet) = 1.15  FAR LDR (Dry)
Enroute Climb Speed
This speed also gives best Lift to Drag ratio with maximum continuous thrust in
case of one engine failure. En-route climb speed gives best rate of climb and
is to be maintained during climb whenever obstacle clearance is required
after takeoff in case of one engine failure. The aircraft will gain more altitude
for a given distance. During climb 1000 feet clearance above obstacle is
mandatory.
Drift Down Speed
It is the speed, with maximum continuous thrust in case of one engine failure,
which gives best Lift to Drag ratio. This means that there will be least rate of
descent and thus the aircraft will lose less altitude for a given distance. Drift
down speed is to be maintained whenever obstacle clearance is required.
During descent 2000 feet clearance over obstacle is mandatory. However
after level off and during further climb, 1000 feet clearance over obstacle is
called for except in mountainous areas where 2000 feet clearance is
required.
JET TRANSPORT CHARACTERISTICS
The Jet Transport Performance capability results in normal operations at
altitudes and airspeeds where compressibility effects occur. At higher flight
speeds, flow separation causes buffeting and conventional ASI can not warn
when this occurs. Hence the need for a machmeter arises to show the high
speed buffeting.
A machmeter indicates an aircraft's true airspeed (TAS) to the local speed of
sound. This dimensionless parameter is very much important for high speed
airplanes. Expressed in a formula this is:
Mach No. = TAS  C
where, C = Local speed of sound
= (662/√288) * √TK = 39 * √TK
T = Outside Air Temperature
As altitude increases, outside air temperature decreases and therefore Mach
number increases. For a constant Mach number, TAS reduces with increase of
altitude (Fig. 1). Thus, a machmeter (Fig. 2) in fact gives the pilot a continuous
indication of the ratio of TAS to the local speed of sound.
As discussed above, Mach number is ratio of two elements - TAS and Local
speed of sound. TAS is the function of dynamic pressure, P - S, and the density.
Speed of sound is the function of static pressure, S, and the density. Density
being a factor to both sides of the fraction, the equation may be re-written
as:
Mach No = (P - S)/S
where, P – Pitot (Total) pressure, and
S – Static pressure
P - S suggests ASI capsule and S suggests altimeter capsule and when placed
90 apart to give a ratio, by interlinking their movement to pointer, Mach
number can be read.
Actually this information is of vital importance to the pilot of high speed
aircraft. As the flight speed approaches the speed of sound (sonic speed) it is
found that the behaviour of the aircraft changes. All the high speed airplanes
fly at high altitudes. At higher flight speed, the air flow can reach speed
higher than the local speed of sound over some portion of wing (thickest
section of the wing) due to accelerated flow over the wing surface. This
results in formation of shock waves towards trailing edge of the wing due to
deceleration of flow from sonic to subsonic (speed less than Mach 1). These
shock waves are lines of abrupt changes of pressure, temperature and
density where the speed of airflow reduces and flow separation takes place
resulting in loss of lift which is termed as buffeting (Fig. 3).
An aircraft which is designed for very high speeds generally employs very thin
wing sections and the wings themselves are well swept back. These features
delay the onset of the shock wave, and when it does occur, it is well in the
rear.
The Mach Number which produces the first evidence of local sonic flow over
the wing surface, is called Critical Mach Number (Fig. 4). A pilot should not let
his aircraft exceed this speed unless the aircraft is designed to fly beyond it.
Thus, a knowledge of the mach number is of vital importance.
LOW SPEED AND HIGH SPEED BUFFET
At low speeds, the onset of initial buffet and stall are primarily determined by
the angle of attack. At high angles of attack, flow separation of the wing
causes buffet. As the separation proceeds over the entire wing, stall occurs.
For a low indicated initial buffet speed (light weight), very little change takes
place at low altitudes and increases rapidly at high altitudes.
A high indicated initial buffet at a high gross weight, starts to increase rapidly
at higher altitudes. The stall associated with the initial buffet varies almost
proportionally with altitude and therefore the stall margin from initial buffet to
stall remains relatively unchanged.
BUFFET BOUNDARY
If high speed buffet and low speed buffet are plotted against altitude and
indicated airspeed on the same graph, the two buffet curves will meet in an
area where one type of buffet is indistinguishable from the other. This is a
buffet transition zone and occurs at the theoretical maximum altitude or
buffet altitude for the given airplane weight (Fig. 5).