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How to determine braking action in the absence of TWR info?

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How to determine braking action in the absence of TWR info?

Old 18th Dec 2013, 04:52
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How to determine braking action in the absence of TWR info?

I have always found the lack of braking action advice from ATC a deficiency in the system when landing on runways during moderate or heavy rain, especially on runways that are not cambered or grooved.

I have landed several times during moderate rain on such runways in various ports in Asia/Pacific without being able to establish any advice regarding braking action from our friends in the tower.

Is there some way of using rain reduced visibility as a guide as to the expected depth of water sitting on the runway?
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Old 18th Dec 2013, 15:04
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You do not get any braking action advise from the tower because they have no way of measuring it. It has been established that previous methods such as Mu meters were very inaccurate and did not replicate how an aircraft would react in the same conditions. Now there is a runway matrix which uses reported braking action from landing crews converted to a code to give guidance.
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Old 18th Dec 2013, 17:28
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"You do not get any braking action advise from the tower because they have no way of measuring it."

Of course, the tower is simply passing on the report from previous crews, something that will happen in a perfect world.

But what if there are no previous crews? Often controllers in some provincial ports in Asia/Pac wont understand when you request info about braking action and I assume many crews do not report it.

So in the absence of braking action reports, is there another way to judge what the expected braking will be?
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Old 18th Dec 2013, 18:26
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So in the absence of braking action reports, is there another way to judge what the expected braking will be?
I would say, no... Any rule of thumb will be worthless, since there are other factors. Like, rubber deposits in the touchdown zones, length of runway (speed when entering opposite touchdown zone) etc. braking action will probably vary during the landing rollout.

Hence, local knowledge combined with a carefull though through performance calculation is probably the best answer.

Same goes for takeoff performance - runway wet and heavy rain. Reduced performance takeoff combined with a reject may have you braking on wet rubber deposits. So if you know the state of the normal dry runway, you may have a better possibility to make a good decision.

For both takeoff and landing calculations, slippery/braking action good will probably be more conservative than just wet. Plus a good margin on performance for takeoff and higher brake setting for landing, to ensure low speed in the opposite end may be a good idea in my opinion.
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Old 18th Dec 2013, 19:12
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Airbus provides a matrix as noted by Fly3 but it is based on the state of the runway - dry/wet/snow/slush etc, and gives an expected braking action to be used in performance calculations with the onboard software.

Compacted Snow (OAT at or below -15°C)
Good to Medium

If there are pireps they should be considered if they indicate a worse BA than given from the matrix.
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Old 19th Dec 2013, 00:20
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FAA Advisory Circular AC 91-79 provides a Braking Action Equivalence Table (Appendix 1 - Table 1) which lists ICAO braking action codes, Mu figures and runway surface conditions and the assumed braking action for each (based on agreed upon assessment of industry representatives and the FAA). Our company uses this to calculate braking, if a braking action report from a like kind aircraft is unavailable. The FAA considers this table more accurate than obtaining a Mu report from friction measuring equipment but less accurate than a PIREP.

That should provide you with guidance on braking action based on runway contamination.
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Old 21st Dec 2013, 15:01
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Table 1, Annex 1, of AC91-19 is not particularly useful; the industry’s current focus is on crew assessment based on the reported type of contamination and depth, as with the Airbus matrix. The matrix originated from TALPA which FAA has trialled, but is struggling with regulatory approval.

The braking action based on the AC table is of little value unless you know what ‘normal’ or ‘reduced’ deceleration means for your aircraft, - and in the same conditions as expected, and with a particular level of braking and/or reverse; i.e. was the deceleration from the brakes (relevant to BA/runway condition) or reverse (meaningless).
If the BA report is from a PIREP (TALPA only allows a down grade) then without knowledge of the reporter’s aircraft type, braking/reverser level, etc, the report is of limited value, unless you wish to bet your butt on the feeling in someone else’s (deceleration) – nor will the fact that they didn’t go off the end will not guarantee you stopping safely.
The measured accuracy of MU varies considerably with machine and contamination type; IIRC European ATC do not report MU, only contamination type, depth, extent, …

With heavy rain, WXR may help. A reasonably well-drained runway having experienced an area of ‘red’ WXR may take up to 15 mins to drain below a flooded category (NASA ref somewhere). This is subject to surface type, crowning/slope and crosswind. There can also be problems with ‘bitumen dams’ between concrete blocks or 'dishing' of old blocks where the center is lower than the edge.

Crews are expected to assess potentially inaccurate information and judge a safe course of action based on the expected landing distance. The recommendation is to add a minimum of 15% to the ‘actual distance’ for variable performance, but perhaps a bit more is well justified by the unknowns.

Edit - NASA tech memo 72650 ‘Wet runways’ W. B. Horn (http://ntrs.nasa.gov/archive/nasa/ca...1975012279.pdf)

Runway drainage: “In general, runway water depths increase with increasing rainfall intensity and drainage path lengths, and decrease with increasing runway transverse slope and surface macro-texture.”

“surface winds tend to increase drainage path lengths and can greatly increase water depths on the runway, depending upon the wind magnitude and direction.”

Braking is affected by surface micro texture and tires:
“… tire tread designs can improve tire/pavement drainage and reduce braking traction losses to some extent, but are not as effective in this regard as improving the pavement surface micro/macro texture by pavement grooving. Research indicates that tread designs tend to lose their drainage capability when the tread is approximately 80% worn.”

Know your tire condition; beware smooth concrete runways.

Last edited by safetypee; 21st Dec 2013 at 15:26.
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Old 21st Dec 2013, 22:28
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You have to use imagination/interpretation , and local knowledge. . .these guys are only flying in the Canarias, so I guess they just missed the 2nd part ?

Accident: Naysa AT72 at Tenerife on Dec 11th 2013, runway excursion on landing

The Norwegian CAA recently put out a circular advising that their study of runway "incidents" over the last 20 (? ) years identified several common factors . . .one being that the braking action reported usually didn't correlate with what was experienced, and therefore had to be treated as very approximate/inaccurate & with caution. . . .and then what ? well air operations continue day in day out & everyone is expected to "get on with it" Ah, the joy of "command responsibility"

Local knowledge/ caution/imagination/ a healthy scepticism regarding all aspects of reported BA / Oh, and a bit of luck

Funnily enough, I was just thinking the other day. . .when it is really tipping it down, or when the runway is slippery (perceived) as it starts to dry out, why are we (generally ) using "WET" in the EFB to make our performance calculations rather than "Degraded Braking Action" MED or MED/POOR or suchlike. . . I believe I may start to do so, as I seriously doubt whether "WET" is really doing it much justice on certain ungrooved runways.
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Old 26th Dec 2013, 11:30
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Why can't Airport Authorities experiment with tri-cycle vehicles using aviation standard tyres and brake systems on actual rainy conditions and compile reports based on empirical tests like that and disseminate as "advisory"..so as to give pilots some basic idea as to how conservative they should be in planngin the landing? Am sure it can be researched and developed as a simulation model? Just brainstorming here...
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