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View Full Version : No such thing as a ‘good’ wet runway.


safetypee
4th Jan 2016, 16:08
The recent SAFO 15 009 Turbojet Braking Performance on Wet Runways (www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/media/2015/SAFO15009.pdf) suggests that the term ‘good’ braking action on a wet runway only exists by definition.

If wet ‘good’ is given then the braking action used in the pre-landing landing distance assessment should always be downgraded to mitigate the effect of the unknowns and variability in landing as per the SAFO.

BBK
4th Jan 2016, 21:43
Safetypee

Interesting paper, thanks for the link. If I recall the "good" figures are factored whereas the dry ones are not. Certainly food for thought if it's peeing down. No pun intended!

rgds

BBK

donstim
5th Jan 2016, 01:02
Not exactly what the SAFO says. The SAFO says that if active moderate or heavy precipitation exists on a non-grooved/non-PFC runway or if active heavy precipitation exists on a grooved or PFC runway, then additional conservatism like what you mentioned should be considered.

safetypee
5th Jan 2016, 14:54
BBK, re factors, I agree.
See the Boeing example in fig 2 (http://vipa.asn.au/sites/default/files/pdf/member/130923%20tech%20bulletin%20slippery%20runways.pdf) and compare the distances for the dry and good braking action when using max manual brakes. Comparing other braking levels may be impossible due to the use and interaction of reverse and autobrake.
The root of the FAA’s concern appears to be that the ‘good’ wet factor is insufficient for all ‘good’ wet runways. This may be due to the operational assessment of ‘good’ being incorrectly reported or that the assumed level of friction used to calculate the wet data is too high.
There is some support for the latter view in AC25-32 standardised advisory landing data (www.faa.gov/documentLibrary/media/Advisory_Circular/AC_25-32_Final.pdf), where for future advisory dry distances, (the baseline friction for wet calculations) should be reduced to 90% of the certificated dry value.


don, you are correct, but my ‘suggestion’ was to invoke thought about the accuracy of determining the type of runway surface and the intensity of rainfall, irrespective of touchdown point, speed, etc.
Runway surfaces change with time, rainfall reports may be inconsistent, or not agree with piloted interpreted WXR returns. Thus the suggestion is for additional ‘conservatism’ to that already required in the SAFO to account for the variability or unknown aspects in determining the landing distance.
The SAFO provides the theory, pilots have to put this into practice.

Thus ‘no such thing as a good wet runway’; i.e. ‘medium’ braking action could be the baseline for a conservative assessment of the landing distance on any wet runway.

P.S. technically it is not the precipitation (rainfall) rate which determines the runway braking action, but the amount of water already on it – perhaps the most important of the unknowns.

alf5071h
6th Jan 2016, 16:33
Perhaps the committee of manufacturers and regulators should consider the detrimental effects of using PIREPS when investigating the shortfall in landing performance.

PIREPS are inherently subjective and restrictive. In addition, they can be very compelling because they originate from a fellow pilot, even if only given for info.
If the runway is slippery and thrust reverse is used with autobrake, then there is no reliable way for the crew to judge the braking action. Any assessment of braking action depends on the decelerating systems used, their selection and setting, and the aircraft characteristics. The range of variables is so great that there is little or no value in the reports.

PIREPS are also subject to reporting bias; having landed on a runway where the achieved performance was very much less than anticipated, perhaps there is a tendency to report medium/poor thus avoiding any embarrassment in acknowledging an error by reporting poor braking action.

The TALPA recommendation was for PIREPS to be used only to downgrade the braking action, but AC 25-32 appears to accept them as primary information.

The FAA should look to other inconsistencies in information and definition; whereas TALPA follows the ICAO definition of a dry runway, AC 25-32 appears to redefine it such that 25% of a dry runway can be wet or worse, which is not conducive to safety if landing on a limiting runway.

In addition to the accident / incident data which triggered SAFO 15009, the committee could also consider those EMAS overruns which were not investigated (previous thread somewhere?); add to that those overruns into an area where the aircraft could taxy out (how many of those), and the number of landings where the stopping distance was longer than calculated / anticipated.
What were the factors in these incidents?

A timely SAFO.

Intruder
6th Jan 2016, 17:22
If the runway is slippery and thrust reverse is used with autobrake, then there is no reliable way for the crew to judge the braking action. Any assessment of braking action depends on the decelerating systems used, their selection and setting, and the aircraft characteristics. The range of variables is so great that there is little or no value in the reports.
I disagree.

There is significant value in braking PIREPs especially when they report degraded braking. If a 747 reports Fair or Poor braking, you can be relatively confident that the braking is NOT good!

I have 17 years experience in the 747 Classic, -400, and -8. The -400 'stops on a dime', considering its size and weight. I can easily feel the brake releases when the antiskid is working hard. If I can feel those releases while using full reverse, the braking action is Fair AT BEST. If I can feel them when using Autobrakes 2, the braking action is definitely Poor.

As for this particular SAFO, I think it's more than a bit wishy-washy. Over the past few years I've read an increasing body of evidence that grooving and friction-enhancing surfaces may not be as good as once believed, especially after a few months or years of wear. I certainly am NOT a believer any more in the older teaching that a wet grooved runway is as good as a dry smooth runway.

When in doubt, ALWAYS use the more conservative performance calculations when dealing with wet runways.

alf5071h
7th Jan 2016, 15:58
Intruder, -“value in braking PIREPs especially when they report degraded braking”, agreed. Similar agreement with the comments on grooving and friction-enhancing surfaces, where even without wear, grooves can hold water if the wind is strong enough.

Your example is less convincing. The root of the problem is the definition of braking action – ‘normal (braking) for the conditions’. You describe ‘your’ ‘normal’, normal for the aircraft type, normal for the level of autobrake selected, normal for the amount of reverse used, but this is of little value to other pilots as they do not have your definition of normal.

You appear to asses the braking action from the apparent deceleration. When using auto brake there is no foot force / angle feedback to judge the amount of braking used, and similarly with auto brake and reverse it is not possible to assess the reverse contribution to deceleration, which is not related to the level of braking.

Your description of the antiskid working suggests that you may be sensing a deep skid vice normal, smooth antiskid operation. This of course depends on the aircraft and type of antiskid system installed, which is another difficulty in achieving a ‘universal’ standard of assessment. The various types of antiskid performance are shown on P 358 onwards (http://easa.europa.eu/system/files/dfu/CS-25%20%E2%80%94%20Amendment%2017_0.pdf), where the design aim is to minimise the occasions and duration of the dips in brake pressure so to maximise braking.

Intruder
8th Jan 2016, 21:37
I agree with alf5071h that my "normal" is not the same as everyone else's "normal". However, the 744 is EXTREMELY stable, capable, and well-behaved on wet runways. I still suggest that if a 744 has a problem stopping in its 'normal' distance, any other airplane will have similar problems or worse.

However, I do NOT assess braking action "from the apparent deceleration". I assess it from the amount of antiskid release action (i.e., the "deep skid" he describes) I can sense through the brake pedals as well as any yawing tendency from asymmetric braking/releases.

I am very much a "seat of the pants" pilot (from my old glider and Navy carrier days, and more recent Champ time) despite 17 years of 747 flying. I have a good sense of what is "normal" on landing roll in the various configurations of Flap, Autobrake, and Reverser. I am also aware of the relationship of the input of Runway Condition into the performance calculations, and the Autobrake setting at which there will be no further landing roll reduction when using only Idle Reverse. From those inputs, I believe I can give a reasonably accurate assessment of actual Runway Braking Action.

I also realize that every airplane will respond differently, and any 5 pilots may give 3 different Braking Action assessments. That's why ATC gives the time and airplane type when reporting those PIREPs. It takes little brain power to assess consistent reports, but a bit more experienced-based translation to assess a handful of differing reports. If the information is available, it will still serve to warn and assist the following pilots.