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 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/50526...ml#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.