DERATED THRUST

 

By Patrick Oria

1. INTRODUCTION

 

When operating an aircraft, the airline always tries to increase the profits made during each flight. This can be achieved by two means:

 

The first one is to increase the revenue of each flight.

 

For that, the commercial loads carried by the aircraft, and consequently the takeoff weight of the aircraft must be as high as possible.

The derated takeoff thrust option will help to increase the takeoff weight when the aircraft is operated from a short runway.

 

The second way is to reduce the cost of each flight.

 

This is partly achieved if the operating costs of the engines are decreased.

To reduce the wear of the engines and consequently their operating costs, it is important to make an optimized use of them. This means that the crew should only use the thrust needed for the current phase of flight i.e. during takeoff, climb…

 

As a first step to reach this goal during takeoff, Airbus Industrie developed the flexible takeoff thrust method. But this method cannot be used on contaminated runways.

 

To further optimize the thrust for takeoff on short or contaminated runways and during climb, Airbus has developed features permitting to automatically reduce the thrust during takeoff and climb:

 

- the derated takeoff thrust

- the derated climb thrust

 

The goal of this article is to give some data on the benefits of using derated thrust and to describe how these features are implemented on the aircraft.

 

 

2. TAKEOFF

 

To avoid any confusion for the discussion below, it is important to clearly define both terms: flexible takeoff thrust and derated takeoff thrust (ref: AC 25-13 from the FAA).

 

 

 

Flexible takeoff thrust:

 

It is a thrust level less than the maximum takeoff thrust for which the takeoff performance is established by approved simple methods such as adjustments or corrections to the maximum takeoff performance.

In this case, the thrust setting parameter, which establishes thrust for takeoff, is not considered as takeoff operating limit.

 

Derated takeoff thrust:

 

It is a thrust level less than the maximum takeoff thrust for which exists in the Airplane Flight Manual a set of separate and independent, or clearly distinguishable, takeoff limitations and performance data that complies with all the takeoff requirements, of FAR Part 25.

In this case, the value of the thrust setting parameter, which establishes thrust for takeoff, is presented in the Flight Manual. The established thrust setting is considered as a new operating limit for takeoff.

 

As discussed above, the most common method for reducing the takeoff thrust is the flexible takeoff thrust method. But this method cannot be used on contaminated runways.

 

2.1. Performance

 

The derated takeoff thrust is to be used when the takeoff weight is limited by the minimum control speed as it enables to take benefit of the reduction of VMCG associated to the lower rating.

 

Minimum control speed limitation

 

VMCA is the minimum control speed in flight at which the aircraft can be controlled with 5° maximum bank, in case of failure of one engine, the other engines remaining at takeoff power (takeoff flap setting, gear retracted).

 

VMCG is the minimum control speed on ground from which a sudden failure of the critical engine can be controlled by use of the primary flight controls only, the other engines remaining at takeoff power.

 

The minimum control speeds vary with the temperature, the altitude and the thrust. VMCA and VMCG are lower for lower takeoff thrust. The regulation does authorize to take benefit of this reduction of minimum control speeds in case of derated takeoff.

 

 

 

2.1.1. Effect on takeoff performance

 

The regulation requires for the takeoff:

 

VR >= 1.05 VMCA and V2 >= 1.1 VMCA

V2 >= 1.13 Vs

V1 >= V1 limited by VMCG

 

The first two conditions affect the takeoff distance, the last one affects the accelerate stop distance.

When plotting the possible takeoff weight as a function of the runway length, the following curve is obtained.

 

 

2.1.2. Example on A340 with CFM 56-5C4 and on A330 with RR Trent 772

 

If the curve shown above is made for the A340-313E (A340-300 equipped with CFM56-5C4 engines) on a runway covered with ¼ inch of water, the results are the following:

 

 

 

 

 

The gains in performance associated to the use of derated thrust on an A340-313E can be as high as 17100kg on a 2800m runway covered with ¼ inch of water at ISA or even 22900kg on a 2500m runway. At ISA+20 with the same contaminant, the gain is 20200kg on a 3000m runway and 32800kg on a 2600m runway.

 

The same curves drawn for the A330-342 (A330-300 fitted with Rolls Royce Trent 772 engines) would show that the weight gain can be as high as 12200kg on a 2100m runway covered with ¼ inch of water at ISA or even 26600kg on a 1900m runway. At ISA+20 with the same contaminant the gain is 12000kg on a 2300m runway and 26900kg on a 2100m runway.

 

 

2.2. System

 

Six levels of derated thrust are proposed to the pilot.

 

The level of derated takeoff thrust is selected on the MCDU PERF TAKEOFF page. The pilot enters D followed by the level of derated thrust; as an example, if the pilot selects a 24% derated thrust level, he enters D24 in the DRT TO – FLX TO field. This entry is only possible in the preflight phase.

 

The system will recognize the entry as a level of derated thrust or as a flex temperature entry thanks to the letter preceding the figure: D for derated takeoff, F for flexible takeoff.

 

 

 

 

 

 

 

 

 

 

 

 

 

The selection is transmitted to the FADEC via the FMGC, the FCU and the EIU.

Associated indications are provided on the ECAM: in the example, D24 is displayed as thrust rating mode.

 

 

 

 

 

 

 

 

 

 

 

The selection on the MCDU can be made on ground only. It is latched when the thrust levers are moved forward of the CL position.

 

Setting the thrust levers in FLX/MCT detent activates the derated takeoff function. It is deactivated when the thrust levers are moved out of the FLX/MCT detent.

With the derated thrust function activated, the Flight Warning Computer (FWC) may generate the following warnings:

 

ENG T.O THRUST DISAGREE is triggered when at least one FADEC selects a thrust rating mode different from the one selected by the other(s) FADEC on ground. This warning may be activated by the takeoff configuration test).

 

ENG THR LEVERS NOT SET is triggered at takeoff thrust application if the FADEC detects a disagree between the thrust levers position and the takeoff mode selected by the FADEC:

 

if the thrust levers are at FLX/MCT and no flexible temperature and no derated thrust level has been selected, the ECAM will require to set the thrust levers to IDLE;

 

if the thrust levers are at TOGA and a derated thrust level has been selected, the ECAM will require to set the thrust levers to FLX/MCT position.

 

 

In case of failure during takeoff, the crew must not set the thrust levers to the TOGA position as this may lead to fly at a speed below the minimum control speed corresponding to the full takeoff thrust.

 

 

2.3. Availability

 

The derated takeoff option is available on A340 and on A330. Any customer interested by this option must raise a RFC (Request For Change).

 

On A320 family, the option is being developed on aircraft fitted with the CFM 56-5-B engines. Any customer interested by this option must raise a RFC.

 

The embodiment of the derated takeoff option on these aircraft requires the following standard of computers:

- FMGC standard called FMS2

- FCU and EIU: current standards are capable of the option

- ECU standard 5BI to be certified by November 98

- FWC standard E3

- DMC standard V40

 

The derated takeoff thrust option may also become available on aircraft fitted with CFM 56-5-A engines or IAE V2500-A5 engines.

 

 

3. CLIMB

 

As for the takeoff, the derated climb option is aimed at reducing the engine maintenance costs of the aircraft.

Selection of lower levels of engine thrust during initial climb, when full max climb thrust is not required by operational constraint enables to decrease engine temperatures and thus to increase the engine life.

 

The thrust reduction is limited in altitude so that the top of climb performance of the aircraft is not affected, whatever the flight conditions and aircraft weight. In particular, the max climb ceilings remain unchanged from the full rating. But the climb times are slightly increased.

 

3.1 Benefits

 

As an example, an A330 fitted with RR TRENT 772 engines at 220000kg needs 24 minutes to climb to FL 350 with max climb thrust whereas the same aircraft needs 29 minutes with "derated climb 1" thrust (between 5 and 10 % less thrust than max climb thrust). It will burn 350 kg more fuel.

If a cruise segment is added to the climb phase to build a leg of about 2.6 hours, the total time would be 0.4 minute longer if derated climb thrust 1 is used instead of max climb thrust. The fuel burnt would also be increased by less than 10kg. Taking into account the maintenance costs, the overall benefit of using the derated climb thrust is around 24 $ per flight.

 

 

 

3.2 System implementation

 

The crew selects the level of derated climb thrust that he wants to use on the MCDU PERF CLB page. He can do it during preflight, takeoff, climb or go around (following a change of destination) phase. During the other parts of the flight, the DRT CLB field is not available.

 

 

 

 

 

The selection is transmitted to the FADEC via the FMGC, the FCU and the EIU.

Associated indications are provided on the ECAM: in the example, DCLB1 is displayed as thrust rating mode.

 

 

 

 

3.3 Availability

 

The derated climb function is available on A330 and A340. Any customer interested by this option should raise a RFC.

 

Studies made on the A320 family in cooperation with an engine manufacturer have shown that the benefit of this option on the A32O is very low, whatever the level of derated thrust that could be chosen.

In addition, modifications of the EIU and FCU hardware would be necessary to implement this function. The cost for an airline to implement the modifications on the aircraft would offset the benefits that the option would give. Therefore the economic benefit is not considered as justifying these modifications and, today, Airbus does not propose this option on the A320 family.

 

 

4. CONCLUSION

 

The benefits of the embodiment of the derated thrust options vary with the type of operations of each airline. The network as well as the costs of the maintenance of the aircraft/engines are two important factors which can influence the decision of the airline to choose these options.

 

As mentioned before, the derated takeoff and derated climb options are available on the A330 and A340 aircraft. The derated takeoff option is being developed on the A320 family for the CFM 56-5-B engines. Customers may request these options by raising a RFC.

 

The derated takeoff option can be made available on aircraft fitted with CFM 56-5-A and IAE V2500-A5 engines.