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Reduced Thrust Takeoff

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Category: Flight Technical Flight Technical
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Definition

A reduced thrust takeoff is a takeoff that is accomplished utilising less thrust than the engines are capable of producing under the existing conditions of temperature and pressure altitude.

Rationale

AOM limitations, the runway specific criteria of length, altitude and obstacles factored against the actual aircraft weight and existing environmental conditions allow the calculation of the actual amount of thrust necessary to meet Regulatory requirements for takeoff. In a significant percentage of cases, the required thrust is less than that which the engines are capable of producing.

Advantages

The primary advantage to a reduced thrust takeoff is cost savings through increased engine life and reduced overhaul costs. Secondary advantages include fuel savings and that, under certain circumstances, it may be possible to increase the maximum takeoff weight for a specific runway by using a reduced thrust profile.

Jet Engine Limitations

The principal limitations of a jet engine are the maximum internal presure that the casing can withstand and the maximum allowable operating temperature. At low altitudes and cooler outside air temperatures (Operational Air Traffic (OAT)), engine pressure is the limiting factor as the engine is capable of producing more pressure and, consequently, more thrust than the engine can contain. The Full Authority Digital Engine Control (FADEC) will be programmed to flat rate the engine at a thrust corresponding to a safe internal engine pressure. This thrust value is the rated or maximum thrust that the engine will produce. Engines are flat rated by the manufacturer by referencing a specific environmental limit or flat rated temperature that is expressed as an ISA+xx° value. At an OAT of ISA+xx° or below, the engine is capable of producing its rated thrust. The FADEC compensates for varying OAT and presure altitude by adjusting fuel flow and limiting the rotational speed of the engine. As the OAT increases above the flat rated temperature, the engine is no longer capable of exceeding its limiting pressure, can no longer produce its rated thrust and therefore becomes temperature limited. At this point, the FADEC limits the internal temperature of the engine so the maximum temperature limit is not exceeded. As the OAT increases from flat rated temperature to the maximum allowed for engine operation, there is a linear reduction in the amount of thrust that the engine produces.

If the amount of thrust that the engine can generate under given environmental conditions exceeds that required for takeoff, the FADEC can be "instructed" to reduce the amount of thrust to be produced by the engine.

Methodology

There are two methods of achieving an acurately controlled reduction in engine thrust:

  • Derate, and
  • Assumed temperature (sometimes referred to as Flexible temperature or FLEX).

Dependant upon the engine manufacturer, one or both of these thrust reduction methods may be available to the operator. When both possibilities are available, the engine design may allow that both derate and assumed temperature can be used simultaneously or, conversely, the two methods of reducing engine thrust may be mutually exclusive.

Derate

A derate selection electronically reduces the the rated thrust of the engine to either one or more prespecified values or by a selectable percentage of the normal flat rated thrust. As this new thrust limit cannot be exceeded during the takeoff phase, critical speeds such as Vmcg and Vmca change from those associated with full rated thrust. Consequently the AOM must include performance data for all permissible derate selections.

Assumed Temperature

As stated above, at outside air temperatures above the flat rated temperature, there is a specific thrust value (variable by pressure altitude) for each temperature. If the thrust requirement for the takeoff is known, the associated temperature at which this thrust would be produced can be extracted from the applicable charts. This "assumed" temperature is then entered into the FMS. Note that regulations limit the amount by which normal takeoff thrust can be reduced to a maximum of 25%. For assumed temperature thrust reduction, the full thrust values of Vmcg and Vmca must be considered limiting as the full rated thrust for the actual OAT can be commanded by moving the thrust levers appropriately.

Operational Consequences

A reduced thrust takeoff will result in a slower acceleration on the runway, a longer takeoff roll and a reduced initial climb rate.

Threats

The principal risks associated with a reduced thrust takeoff are the potential of miscalculating either or both of the derate or assumed temperature values and the possiblity of entering incorrect values into the aircraft Flight Management System (FMS). Either of these errors could result in the engines producing insufficient thrust to safely execute the takeoff.

Defences

When derate or assumed temperature calculations are made by the pilots utilizing either manual or electronic means, each of the pilots should make the calculations independently and then compare the results. Differences must be reconciled. Likewise, all FMS entries should be crosschecked by both pilots for accuracy.

Accidents & Incidents

  • B738, Belfast International UK, 2017 (On 21 July 2017, a Boeing 737-800 taking off from Belfast was only airborne near the runway end of the runway and then only climbed at a very shallow angle until additional thrust was eventually added. The Investigation found that the thrust set had been based on an incorrectly input surface temperature of -52°C, the expected top of climb temperature, instead of the actual surface temperature. Although inadequate acceleration had been detected before V1, the crew did not intervene. It was noted that neither the installed FMC software nor the EFBs in use were conducive to detection of the data input error.)
  • A345, Melbourne Australia, 2009 (On 20 March 2009 an Airbus A340-500, operated by Emirates, commenced a take-off roll for a normal reduced-thrust take-off on runway 16 at Melbourne Airport. The attempt to get the aircraft airborne resulted in a tail strike and an overrun because insufficient thrust had been set based upon an incorrect flight crew data entry.)
  • A320, Porto Portugal, 2013 (On 1 October 2013, an Airbus A320 took off from a runway intersection at Porto which provided 1900 metres TORA using take off thrust that had been calculated for the full runway length of 3480 metres TORA. It became airborne 350 metres prior to the end of the runway but the subsequent Investigation concluded that it would not have been able to safely reject the take-off or continue it, had an engine failed at high speed. The event was attributed to distraction and the inappropriate formulation of the operating airline's procedures for the pre take-off phase of flight.)
  • B773, Auckland Airport New Zealand, 2007 (On 22 March 2007, an Emirates Boeing 777-300ER, started its take-off on runway 05 Right at Auckland International Airport bound for Sydney. The pilots misunderstood that the runway length had been reduced during a period of runway works and started their take-off with less engine thrust and flap than were required. During the take-off they saw work vehicles in the distance on the runway and, realising something was amiss, immediately applied full engine thrust and got airborne within the available runway length and cleared the work vehicles by about 28 metres.)
  • B737, Southend UK, 2010 (On 21 Nov 2010, a Boeing 737-700 being operated by Arik Air on a non revenue positioning flight from Southend to Lagos with only the two pilots on board carried out a successful take off in daylight and normal ground visibility from runway 06 but became airborne only just before the end of the runway.)

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