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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

  • A320, Basel-Mulhouse-Freiburg France, 2014 (On 6 October 2014, an A320 crew requested, accepted and continued with an intersection take off but failed to correct the takeoff performance data previously entered for a full length take off which would have given 65% more TODA. Recognition of the error and application of TOGA enabled completion of the take-off but the Investigation concluded that a rejected take off from high speed would have resulted in an overrun. It also concluded that despite change after a similar event involving the same operator a year earlier, relevant crew procedures were conducive to error.)
  • A332, Montego Bay Jamaica, 2008 (Prior to the departure of a Thomas Cook Airlines Airbus A330-200 from Montego Bay Jamaica during the hours of darkness and in normal visibility on 28 October 2008, incorrect takeoff speeds had been input to the FMS by the flight crew without this being recognised. When rotation during take off was, as a consequence, initiated too early, the aircraft failed to become airborne as expected. The aircraft commander, acting as PF, quickly selected TOGA power and the aircraft became airborne before the end of the available runway had been reached and climbed away safely.)
  • 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.)
  • 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.)
  • H25B, vicinity Owatonna MN USA, 2008 (On 31 July 2008, the crew of an HS125-800 attempted to reject a landing at Owatonna MN after a prior deployment of the lift dumping system but their aircraft overran the runway then briefly became airborne before crashing. The aircraft was destroyed and all 8 occupants were killed. The Investigation attributed the accident to poor crew judgement and general cockpit indiscipline in the presence of some fatigue and also considered that it was partly consequent upon the absence of any regulatory requirement for either pilot CRM training or operator SOP specification for the type of small aircraft operation being undertaken.)

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