Background to wet runway RTO - single reverse

[alosaurus]

Why is it that wet runway performance assumes one engine thrust reverse is used...but dry uses no reverse?

[Old Smokey]

Because it is a concession allowed for wet runway takeoffs, otherwise viable weights could not be carried.

The "Normal" rules, i.e. Dry Runway operations require that one reserve means of stopping the aircraft be available. The same rules require a screen height of 35 feet, whereas a further wet runway concession is allowed reducing this to 15 feet.

These two wet runway concessions eliminate entirely ANY margins for Accelerate-Stop performance, and reduce to less than half the screen height required for the GO case.

And yet, there are still jerks out there who would use wet runway data for dry runway operations. :ugh:

Regards,

Old Smokey

[hawk37]

Smokey, is there any data to confirm that the stopping distance with brakes and the asymmetric reverse thrust situation (from one engine in rev) is less than the distance with brakes but without reverse thrust (and no asymmetry problem)?

Assuming a wet runway, of course.

[Old Smokey]

Yes, there is data hawk, the AFM Wet Runway charts.:ok: These charts / data are 'geared' to the use of full reverse thrust as well as maximum braking and ground spoiler deflection.

For a direct comparison, now take a look at the same wet runway data but with reverse INOP.... :eek:

Regards,

Old Smokey

[hawk37]

OK thanks OS. I didn't know airlines had wet runway take-off distance charts for all reversers available, as well as a chart for the one reverser inop case. Guess I thought that maybe it was a generic "subtract xx% to the max allowable take off weight", or something similar to cover all cases.

[Old Smokey]

Hi hawk37,

You're partly right. Older certified aircraft may well have a generic correction to apply for wet runway operation to comply with the relatively recent requirement. Newer aircraft will have Wet Runway charts which are separate to the Dry Runway charts.

A point to not be confused with, for Dry Runway operation it is mandatory that a RESERVE means of stopping be available. (Don't confuse RESERVE with REVERSE). As Reverse Thrust is the least effective means of stopping, the manufacturer typically chooses Reverse Thrust as the RESERVE means of stopping. If no RESERVE means of stopping are available, a 15% penalty applies to the Accelerate-Stop distance.

A point often missed is that if Reverse Thrust is INOP for Dry Runway operations, a 15% ASDA penalty does apply, even though Reverse is not considered during Dry Runway certification. VERY OFTEN MISSED!!

Regards,

Old Smokey

[Ocampo]

As Reverse Thrust is the least effective means of stopping, the manufacturer typically chooses Reverse Thrust as the RESERVE means of stopping. If no RESERVE means of stopping are available, a 15% penalty applies to the Accelerate-Stop distance.

So, if I understand correctly, let's say, for example, the BAe RJ-100, which has no reverse, uses the +15% penalty for all performance calculations? :confused:

[hawk37]

Ocampo, what OS has said is certainly new to me. However, I think the 15 percent rule applies to the all engines case. And so, the min runway length allowable is the longest of the balance field (assuming engine failure), and 115% of the all engine take off distance.

Seems like in only very light weight conditions would the 115% distance be the greater.

[john_tullamarine]

a 15% ASDA penalty does apply

This requirement was in the Australian design standards (101.5/6) although it is years since we have read them. I probably have a file copy in the dusty drawers somewhere but I would be hard pressed to put my hand on it.

As I recall the value was 10% rather than 15%. Certainly applied to the F28 as I recall.

[Old Smokey]

"This requirement was in the Australian design standards (101.5/6)"..... and also buried in the FAR design standards.:ok:

"although it is years since we have read them"..... Shame on you J_T:=

T'was 15% in the FAR Design Standards, I cannot recall whether the Australian design standards (101.5/6) differed..... Shame on me :rolleyes: (Particularly as the last time that I applied 15% was on an Australian registered aircraft)..... whoops.:eek:

Regards,

Old Smokey

[Mansfield]

I don't see any reference to a 15% penalty for inoperative reverse in FAR 25.109, Accelerate-Stop Distance...nor is there any mention of such a penalty in the 757 MEL...

[Max Angle]

I agree although other companies may use different data. On our Airbus aircraft no performance correction is required by the MEL for one or both reversers inop. on a dry runway, the book states very clearly that the figures are computed without reversers. Wet runway requires a correction and contaminated is not permitted.

[mutt]

A point often missed is that if Reverse Thrust is INOP for Dry Runway operations, a 15% ASDA penalty does apply

Totally missed :):) I dont remember that little gem showing up in ANY of our MELS or AFM's (FAA and JAA). Can you dig out the FAR reference for it.

Thanks.

Mutt