Are Thrust Reversers essential safety equipment?

[uitauoesnth]

Just perusing through the tu-204 crash thread, I have noticed that possible thrust reverse failure has been mentioned several times. I want to know if thrust reversers are generally considered equipment that adds to the safety of an aircraft?

My own guess is that thrust reversers are not considered to be such essential equipment, on the understanding that landing distances are always calculated with thrust reversers inoperative. This means that as much as thrust reversers do help slow a plane down, if an overrun does occur and the thrust reversers were inoperative, then something else must have gone wrong for that overrun to occur?

Can anyone else lend an informed opinion?

As a corollary, assuming I am correct, and TRs are not safety equipment, why do planes have them? Are they really only there for saving money on brakepads?

[PEI_3721]

Yes, but ….
Suggest amalgamating this thread with http://www.pprune.org/tech-log/505269-overrun-safety-issue.html and discuss there.

[Tom!]

On a wet runway thrust reversers are taken into account. For landing on a contaminated runway they are essential, if the wheels have no grip on the ice or snow the thrust reversers are all you have for stopping power until the wheels get grip.

[uitauoesnth]

hmm, if TRs are necessary for stopping on poor runway conditions, does this mean that aircraft become undispatchable if they have inoperative thrust reversers?

[Fuddy Duddy]

Not so on A320 variants. FCOM2 states actual landing distance does not consider the use of reverse thrust . It then lists contaminated R/W states vs landing distance.
This distance may be decreased by a factor if reversers are operative, but they are not essential.
HOWEVER they are required for contaminated R/W take off, to be used in the engine out decel phase following a rejected takeoff.
So yes they are essential but only for contaminated R/W take off.
Fuddy Duddy

[Wizofoz]

HM, if TRs are necessary for stopping on poor runway conditions, does this mean that aircraft become undispatchable if they have inoperative thrust reversers?

Aircraft with unservicibilities are dispatched with reference to the Minimum Equipment List which lists the requirements to dispatch with whatever the particular unservicibility is.

It may be that a particular problem leaves a particular aircraft serviceable for some flights but not others.

In the case of Reverse thrust, it would most likely be a case of calculating the dispatch landing capability of the aircraft WRT the conditions forcast at the destination with the TRs unservicable, and if sufficient runway was available, the flight could dispatch.

It is impossible to make a general statement, as it would require the particular combination of aircraft/MEL/destination conditions to be assessed.

[safetypee]

Thrust reverse use is not normally considered in certificated landing performance [CS 25.125 (c) (3)]; rejected takeoff requirements differ.
Credit for reverse may be given when landing on contaminated runways, in which case reverse would be mandated according to the information in the AFM [CS AMC 25.1591]; note the many caveats for these operations.

The differences between use and nonuse represent the various levels of risk in each operation and the assumed frequency of exposure; hence for contaminated runways, avoid the conditions unless the operation has approval, and there are specific mitigating factors, e.g. training/procedures.

Many normal operations suggest that reverse thrust is a valuable aid to safety, and that in some circumstances crews rely on this without consideration of the certification assumptions or additional risk – e.g. reverse fails to engage due to flight technique / squat switch operation.

The European Action Plan for the Prevention of Runway Excursions (www.skybrary.aero/bookshelf/books/2053.pdf) discusses landing performance from page 69, also see subsequent appendices.
The data formats for certificated (factored) performance and QRH advisory ‘actual’ performance can be misunderstood, where the latter may quote reverser use.

There is increasing evidence that thrust reverse has helped avoid overruns in normal operations. However these near misses should not have occurred according to the current certification standards and assumed operating procedures, i.e. reverse thrust can hide operating ‘errors’ while still maintaining safety.
Whilst the main source of error (variable human performance) appears to be crew related, particularly knowledge and judgement (http://www.pprune.org/tech-log/505269-overrun-safety-issue.html#post7642479) #9, there are also significant problems with the assessment and reporting of runway condition and thence deduction of braking action. Reverse helps mitigate some of these, but perhaps less so if contaminated operations already assume the use of reverse.

Thus, if certification does not consider reverse thrust credit due to the lower likelihood of availability/reliability – system or human variability, then operators must not depend on it and act accordingly.
Where thrust reverse is allowed, then the additional risk must be mitigated by specific activities such as training, knowledge, procedure, and appropriate human performance.

[Ollie Onion]

Well according the the A320/21 MMEL no performance calculation required with BOTH thrust reverser inop in the case of Takeoff on Dry and Landing on Dry and Wet. It also says that you MUST calculate performance using the thrust reverser inop table for Takeoff on WET and CONTAMINATED and Landing on CONTAMINATED runways.

My performance manual issued by the company does NOT include a table for thrust reverser inop, so therefore I can't dispatch on a wet or contaminated runway with both thrust reverser inop or land on a contaminated runway.

[john_tullamarine]

In the olden days, certification generally required either an additional means of retardation (ie reverse) or a distance penalty to be applied to the AFM charts for routine operation .. affected aircraft such as the F28. The answer to the OP's question will depend very much on what is in the AFM and the particular Regulators' requirements.

Ref the suggestion to amalgamate this and the other thread, I fear it would prove more confusing than illuminating ?

[gusting_45]

The answer to the basic question is no as not all aircraft (jet or otherwise) have reverse thrust in the design.

Simple.

[JQKA]

Thrust <reverser are not essential safety equipment and as you can check in the performance section for Boeing for example, they not make a big clue to shorten the landing distance...just a couple of hundred of feet.
<<<<<<<<<<<differently from the Speedbrake which allow a very useful reduction in landing distance.