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 conform 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 15C, pressure as 1013.2 hPa, density as 1.225 kg/m3 with a lapse rate of temperature of 1.98C per 1000 ft till 36089 ft beyond which it is assumed as constant at –56.5C. For example, at 5000 ft above mean sea level, in standard atmosphere conditions, temperature will be 5.1C, 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 inflight is 10C (50F) 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 10C (50F) 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 (C Lmax) 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 V S1G 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 V C 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 for landing as limited by aircraft strength/applicable regulations. 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 vice-versa. 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 - 60 Type of pavement (F) - Flexible Category of subgrade below the runway surface (B) - Medium Tire pressure (W) - Medium (up to 217 psi) Evaluation method (T) - 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. V MCA 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 twinengine 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 V 2 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
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 this 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 737-400s 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 go-around thrust. For three or four engine airplanes, VMCL-1 with one engine inoperative and VMCL-2 with two engine 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 (V REF) 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. enroute 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) * √TK = 39 * √TK 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).
