Why do modern jetliners provide Thrust Reversers at all in their design decisions?

Question

Let's start with the following knowns (please correct me if I am wrong about any):

1. The braking force provided by the wheel brakes is much larger than that by the thrust reversers
2. One is not allowed to does not take credit for reversers when calculating landing distance in most aircraft. (A few exceptions exist)
3. Thrust reversers add quite significantly to the weight and complexity of engines
4. An improperly deployed thrust reverser can be critically dangerous and hence many redundant systems must be provided to prevent unwanted deployment
5. Modern civillian aircraft never rely on reverse thrust during flight
6. In the history of aviation incidents quite a few have been related to reverser problems or procedure errors.

In the light of this why aren't more large civilian aircraft designed that use only wheel brakes + spoilers to do all their braking?

More specifically, suppose you had to re-assign the braking force provided by the reversers in the initial rollout to spoliers alone how much more spoiler area would one need to provide?

Overall, spoilers + wheelbrakes seem far more docile and benign than the complexity and risks associated with reverse thrust?

What gives?

PS.

Are the any modern large passenger jet aircraft that don't have reversers at all?

Answer 1

There are a couple of reason for the presence of thrust reversers:

• To reduce wear on the brakes.

• To provide a margin of safety (for example in wet conditions).

In 1995, NASA carried out a survey on airlines Why Do Airlines Want and Use Thrust Reversers?. The results of the survey, though dated, give an indication of presence of thrust reversers.

• The main reasons given by the airlines was to provide additional stopping force in adverse weather and to reduce brake wear.

The first question posed to the airlines was "why do we have thrust reversers?" (...) The number one reason given by the airlines was to provide additional stopping forces in adverse weather conditions (i.e. on wet, slushy or slippery runways). The airlines also use thrust reversers to reduce brake wear and to provide directional control and additional safety margins during an aborted takeoff (RTO).

• According to the airlines, the thrust reversers were there because they wanted them there. This meshes with the fact that the thrust reversers are not used in calculating stopping distances during certification.

When the airlines were asked who they perceived as the principal organization behind the installation of thrust reversers on commercial transport aircraft, the airlines responded that they were. That is, the aircraft manufacturers put the systems on board the airplanes because the airlines want them.

• Most airlines recommend using the thrust reversers early in the landing to minimize braking.

• The airlines weigh the economic benefits of using the thrust reversers (and engine wear) against the use of brakes and the results seem to indicate a significant amount of money saved by using thrust reversers. The report notes:

... the savings in brake system costs due to the use of thrust reversers was estimated. The results show that the total yearly savings in brake system costs is about \$8.6 million, or an average of about $17,700 per airplane for airline 17.

Though this is almost certainly an exaggeration, the numbers indicate that the airlines would definitely prefer thrust reversers over increased braking.

In the end, the reverse thrust can be important in reducing the runway requirements in case of a wet runway. In 1999, a Qantas 747 overran the (wet) runway in Bangkok and landed up in a golf course. Qantas has used only idle reverse thrust rather than full power during the landing as per the operational procedure. The official accident report included data from Boeing 747-400 Flight Crew Training Manual, which stated that,

Immediate initiation of reverse thrust at main gear touchdown and full reverse thrust reduce brake usage, thus minimising brake temperatures.

To minimise wear of carbon brakes, minimise the number of brake applications. Use maximum reverse thrust and operate the autobrakes normally.

The importance of establishing the desired reverse thrust level as soon as possible after touchdown to minimise brake temperatures, tyre and brake wear, and to reduce stopping distance on very slippery runways cannot be overemphasised..

As a result, Qantas revised their operational procedures, requiring full reverse thrust. Brake wear is an important reason for having thrust reversers. For example, in Airbus A380:

... unlike the thrust reversers on most airliners, including the Boeing 747 jumbo, they do not stop the aircraft in a shorter distance than brakes and spoilers alone. They do, however, take some of the strain off the brakes and are useful if water or snow makes the runway slippery.

The BAe 146 lacks a thrust reverser; it makes do with an airbrake in the tail and spoilers all along the wing for braking (which should give an indication of how much spoilers is required to eliminate reversers). The 146 got away with that because of the low touchdown speeds (~90 kt) and the huge airbrake, which is not avialble in the other commercial airliners. For the same amount of spoiler area in other commercial airliners (in absence of airbrakes), they (spoilers) would occupy most of the wing surface, increasing weight and complexity. Some Embraer ERJ 145s also lack thrust reversers (they are optional).

Answer 2

The added complexity is a valid point. However, the additional braking option provides enough benefits to offset the added cost. Also, it is not accurate that credit is never given for reverse thrust. Per 14 CFR §25.109, when calculating accelerate-stop distance, reverse thrust is not considered for a dry runway, but:

May be included as an additional means of deceleration using recommended reverse thrust procedures when determining the accelerate-stop distance on a wet runway

So at least from an FAA certification standpoint, reverse thrust is considered as a benefit in wet conditions, which is generally the more critical case.

Yes, the wheel brakes provide the majority of the braking force. But that is also a function of your second point. Credit is not always allowed for the reversers, so that means their use merely reduces stopping distance. It is often preferable from an operations standpoint to not use up the entire runway when landing. Exiting the runway earlier can reduce delays and taxi time, and makes room for the next aircraft in line for that runway. If a longer landing roll is still acceptable, the thrust reversers help to reduce the strain on the brakes. While modern brakes are much improved over older designs, they are still an item that needs replacing after use. Harder braking also results in higher brake temperatures, which may require extra time for the brakes to cool.

It's helpful to have an extra layer of safety in landing distance calculations. If you calculate landing distance that is just within the available distance, there are still many failures that can prevent the aircraft from stopping. The thrust reversers offer additional braking force that can offset unplanned deficiencies in other areas.

Other options for providing braking force have their own problems. For an example, look at the spoilers on the 737 NG. You can see that they already occupy all of the available spanwise area. That means the chordwise area would need to be increased. This would also increase weight and complexity. Additional panel area would be needed, and the supporting systems would need to be reinforced. Also, while thrust reversers offer some amount of braking force at any speed, the effect from spoilers drops off more significantly at slower speeds.