Credit for reverse thrust use on dry RW's

[Jumeirah James]

Ladies and Gents,

I'm trying to find the reason why there is no credit for using reverse thrust on landing on a dry RW, but its taken into account on wet and contaminated figures. The answer seems to be it's not a regulatory requirement, but I'd like to know a bit more please.

I've searched PPRUNE and the nearest link on reversers is here, but i couldn't find a bit more of a detailed answer:

http://www.pprune.org/tech-log/54638-thrust-reverse.html

I'd love a bit more info though.

Thanks in advance

JJ

[BOAC]

This might be of help? (http://www.pprune.org/tech-log/380984-background-wet-runway-rto-single-reverse.html)

[lomapaseo]

Reversers are great for a safety redundancy add-on. However their reliability tied together with piloting does not yield the level of reliability necessary for an acceptable level of safety for all landings.

They are one of those systems that are nice to have but if they malfunction at a "critical" time they are "gothchas" If you start depending on them then they become "critical" to the landing.

[PEI_3721]

Any credit for reverse is covered by the certification requirements - CS 25 or FAR 25.125
Means other than wheel brakes may be used if …
(i) Is safe and reliable;
(ii) Is used so that consistent results can be expected in service; and
(iii) Is such that exceptional skill is not required to control the aeroplane.

IIRC most aircraft fail to meet the requirement of ‘reliable’, but combinations of all three might apply – see Midway 737 accident.
Thus, in general, thrust reverse is not included in dry or wet landing performance.

However, where reverse is included in contaminated data CS (AMC) 25.1591 7.4.3. (not in FAR) “Performance information may include credit for reverse thrust where available and controllable”, the exposure to the higher risk is assumed to be balanced by operational caution, and by minimizing the number of landings, e.g. CS AMC 25.1591 8.1
“8.1.1 Operation on runways contaminated with water, slush, snow, ice or other contaminants implies uncertainties with regard to runway friction and contaminant drag and therefore to the achievable performance and control of the aeroplane during take-off, since the actual conditions may not completely match the assumptions on which the performance information is based. Where possible, every effort should be made to ensure that the runway surface is cleared of any significant contamination.”

And

“8.2 … the AFM should also include recommended procedures associated with this performance information.”

“8.3 … information relating to the use of speeds higher than VREF on landing, that is speeds up to the maximum recommended approach speed additive to VREF, and the associated distances should also be included.”

Note that these assumptions are in a certification document (CS 25) not normally used by operators.

[Jumeirah James]

Thank you all very much for your replies. I'm a wiser man now :uhoh:

[muduckace]

Cost savings, a balance between fuel usage and break ware.