# What are the criteria for conceptual design of thrust reverser in the use of braking?

I'm doing a project: Study of aircraft braking and pre-sizing of associated subsystems. We tried to find criteria for the reverser in the braking phase. I read FAR 25, but I did not find it. Is there a criterion such as how many percent of the braking force must be applied by the reverser in certification documents?

• You won't find it because aircraft are supposed to stop with in-op reversers, and are not allowed to use them during certification. Reversers are just for convenience. Dec 16 '19 at 21:17

As @John's answer alluded to, the rules for actual landing distance (25.125) and the accelerate-stop distance on dry runway (25.109) effectively prevent a manufacturer from taking credit for thrust reverser (T/R) when demonstrating/calculating for these distances. However, 25.109 does allow a manufacturer to take credit for T/R when dealing with a wet runway. This means that having a (good) T/R gives an operator more flexibility when dealing with adverse weather conditions, and this translates to more money saved (vs sitting on the tarmac).

Therefore, just like any other performance numbers, the performance of T/R is subject to market objectives. There is no regulatory requirement on how powerful the T/R has to be at max reverse.

That being said, this NASA memo has some helpful information on why airlines want thrust reverser at all. The following two graphs are cited from this source, which outline the percentage of work done by the respective braking systems for a typical jetliner on dry and wet runways. You can perhaps back-calculate the thrust of T/R from your design point:

Other factors to consider for design:

• Foreign object debris ingestion from T/R deployment, especially at lower speed
• Compressor stall from hot reversed gas, especially at low speed
• Inadvertent deployment of thrust reverser in-flight. For most airplanes, this would be catastrophic.
• Controllability in single T/R deployment on ground
• For rear mounted engines, aerodynamic interference with rudder effectiveness
• I'm confused by the Icy runway graph. Is that saying that shortly after 4000 ft the reversers & inlet momentum suddenly lose all effectiveness in braking and that the total overall braking work suddenly drops off? Dec 19 '19 at 18:25
• @FreeMan The left and right are not connected. The first portion of the right graph isn't shown (or consider it's hidden beneath the first graph).
– JZYL
Dec 19 '19 at 18:48
• Ah, that makes more sense. Dec 19 '19 at 18:52

Thrust reversers are not accounted for in performance criteria for accelerate/stop and landing distance values, so there aren't really any cert requirements for what you would think of as minimum reverser braking performance. Cert requirements mostly focus on controlability during landing/reject with asymmetric reverse thrust, and the safety related aspects of (preventing) reverser function in flight, that sort of thing.

In terms of stopping performance, their use is a performance "bonus" and the pilot has to option to leave them stowed, unstow them but leave them at idle, or use any reverse thrust amount between idle reverse and max reverse desired during braking on any landing or reject.

• For wet/contaminated runway, however, manufacturers are allowed to take credit for T/R for performance.
– JZYL
Dec 17 '19 at 2:51
• Good point. I'm trying to recall if the CRJ's QRH had reverser/no reverser data for contam runways but no longer have access to those sorts of documents having been retired several years. Dec 18 '19 at 1:07