Does the avoidance of thrust reverser save fuel?

Question

On 2017-06-17 I took an AirAsia Airbus A320-200neo from Hong Kong (VHHH) to Kuala Lumpur (WMKK).

The flight crew did not use reverse thrust for slowing down the plane.

Trying to find an explanation to this, I would like to know so much as possible about the use of thrust reversal by commercial flights, including low-cost airlines, which would probably like to use any feasible technique for saving fuel.

Answer 1

Without further knowledge of the procedures/state of the (a) airport, (b) airline and (c) aircraft , I don't think any conclusive answer could be made. To throw some thoughts in:

• All of Kuala Lumpurs's runways are over 4000 meters long, and braking an A320 (at landing weight) can easily be done without thrust reversers over that distance.

• Depending where the gate was, they may simply have wanted use more of the runway to speed up the taxi, in which case excessive braking would have been undesirable. Most standard procedures call for a minimum of idle reverse thrust however.

• The thrust reversers may have been inoperable, i.e. broken. You are allowed to fly an A320 in that state.

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Update: Since you said you didn't sense any thrust reversers (vibration/sound, i.e. no visual confirmation), here's a video to show that they can be pretty quiet even if the doors are opened. If you follow the spiral painted on the spinner note the speed doesn't appear to change a lot even after the plane leaves the runway and the reverser doors are closed.

^{(Touchdown at 6:50)}

Answer 2

There are two ways in which not using reverse will save fuel. First is the obvious, in that when the engines are throttled up to fairly high RPM for the reverse thrust, that burns significantly more fuel than leaving them at idle. So forgoing reverse thrust saves that fuel burn.

Beyond that, engines can be shut down after landing after they've had whatever cooling interval is recommended by the manufacturer, and in some cases, the power setting on approach is low enough that this time "counts" as cooling time, so if reverse thrust isn't used, then an engine (in the case of a twin) or a pair of engines (in the case of a 4-engine aircraft) can be shut down shortly after clearing the runway. There is fuel savings here as well, not having to run the engine/s for the minutes required to give them the recommended cooling.

Additionally, not using reverse thrust can save on noise, which can be a "good neighbor" policy at urban airports at night. Some airports & carriers will be more concerned about this than others.

Depending on where the aircraft will park, it may be advantageous to use the entire length of the runway (i.e. if you're landing to the west and you will park at the west end of the airport), and for a really long runway with ample room to slow down without reverse thrust or heavy braking, it can make sense to "coast" out to the end of the runway.

Three reasons that using reverse can be worthwhile:

• Using more reverse means using the wheel brakes less. They may be more expensive than the fuel & wear on the engines from using reverse thrust.
• On a slick runway, the brakes may be ineffective at high speed, and the reverse thrust may be necessary to stop in the available runway. Some aircraft are certified in such a way that they have to be able to stop without the use of reverse thrust, so getting the TR's out is simply "bonus" stopping margin. Others, however, are allowed to take credit for reverse thrust in the stopping calculations, so for those aircraft on sufficiently slick/short runways, using the TR's is vital to stopping within the available runway.
• Finally, some operators require the pilots to use reverse thrust on every landing, even when it isn't needed. This is to preserve the habit patterns & muscle memory so that they deploy the reversers every time, not just "when needed." The logic here is that if you're making a decision at touchdown ("did I need them this landing?"), you may make the wrong decision when you really do need them, and the consequences in that case are very bad. So rather than risk a bad decision, the airline policy is, don't make that decision -- always deploy them.

Answer 3

Thrust reversers (T/Rs) are often not deployed due to mechanical failure; if a T/R is defective (won't retract, for example), depending on the airline, the mechanic can lock out the defective T/R. In my experience, this is by far the most common reason for not deploying a T/R.

It is far more expensive to replace brake assemblies than it is to pay for the additional fuel to stop the aircraft with a T/R. Reduction of wear on brakes is one reason to always deploy the T/R whenever possible.

Source: I was a Maintenance Technician for Delta Air Lines for 16 years.