B777 Max Autobrake vs Manual
The FCOM states that the max auto braking on a DRY runway is less than max manual braking. From that I am taking it that on a wet runway max auto is more than max manual....
Is this correct and if so why? |
Why would you not take it that max manual is also greater than max auto on a wet runway?
From an engineering perspective... Autobrakes give you a fixed deceleration schedule*. If you add reversers and speedbrakes into the mix, you still get the same deceleration rates (although with less work for the brakes). Manual braking gives you maximum braking plus you have extra deceleration c/o reversers and speedbrakes. Manual braking is similar to RTO braking. *assuming there is not too much intervention by torque-limiting/antiskid/anti-aquaplane circuits. |
Your assumption appears to be inaccurate according to the info below; in wet conditions the distances would most likely be the same. See:-
http://vipa.asn.au/sites/default/fil...%20runways.pdf Also http://www.vipa.asn.au/sites/default...%20runways.pdf |
NSEU the reason I took it that max manual wasn't as great as max auto on a wet runway is because thats what the FCOM says.... It makes the point stating that max manual is greater than max auto on a DRY runway...so by inference it is not on a wet.
Ref FCOM Section 20 Autobrake System, Landing Five levels of deceleration can be selected for landing. However, on dry runways, the maximum autobrake deceleration rate in the landing mode is less than that produced by full pedal braking. |
Your assumption appears to be inaccurate according to the info below; in wet conditions the distances would most likely be the same. See:- I probably could have used a better word than "assuming". I meant "providing". I was trying to say that Autobrake deceleration rates are targets only. Depending on how wet the runway is, these target rates won't be achieved. |
Not 777 specific...
Full manual application provides more pressure, with a full duty cycle (provided anti-skid is not triggered) to the wheels on aircraft I know of with Autobrakes by design, therefore the DRY decel rate would be higher, Mr. Cat.
From that I am taking it that on a wet runway max auto is more than max manual.... |
Practically speaking, I suspect auto brakes would actually give a better (lesser) stopping distance on MAX than the average line pilot could achieve with manual braking, especially in adverse conditions. Why? The QRH figures were achieved in dedicated test sessions by using fairly assertive techniques, to put it mildly.
It’s a big ask in a strong crosswind to get maximum braking pressure on both pedals immediately on touchdown and keep it that way to a stop, while simultaneously keeping the aircraft on the runway with the rudder. The geometry is not favourable and I think most people would find it surprisingly difficult... |
Personal take on this statement.
Boeing are trying to tell you that when landing on a dry runway, you can get significantly improved braking (if you need it) by applying max manual braking. ie. if you land long and think it may be possible that you might overrun, press the pedals to the stop. The OP's assumption that the reverse is true is simply a step too far... Full wings - I partially agree. If attempting a max perf landing on a dry runway - I would land on MAX and when under control apply full brakes... |
That raises an interesting point. Does MAX A/B go for a fixed but high deceleration rate in the rollout or does it take all the wheels to the point of triggering the ABS, i.e. what would happen with full manual braking? As there’s no credit for reverse, it would seem more likely to be the former. One wonders why this might be the case as RTO braking works the other way round, so it’s obviously possible. Maybe Boeing should add another position on the knob labelled “Absolutely, fully, completely MAX”, or just “11”.
The difference between dry MAX and manual distances might have some influence from the way brakes are applied after touchdown. MAX on the 777 is still quite smooth, compared with slamming on the anchors. (I’ve experimented on positioning flights...) |
Does MAX A/B go for a fixed but high deceleration rate in the rollout or does it take all the wheels to the point of triggering the ABS, RTO is simply 3000psi on the brakes until the antiskid/torque limiter cuts in. |
Any difference in braking between max manual and max auto on a wet runway is relatively small. The practical point is more about how the capability - deceleration as per Boeing diagrams - is used to minimise distance.
Any delay in applying manual braking has to be balanced against autobrake's advantaged of quick and consistent application. Boeing highlights this in their technical bulletin, where in-service manual brake application may take 4 - 5 secs vs the 1 sec assumed in the data, with an increased distance required of up to 1000ft. A greater difference, and safety hazard, is an inappropriate choice of autobrake setting and/or misjudgement of the runway conditions, which may require the use max manual brake late in the landing. Another issue with autobrake is the human 'automatic' functioning which may reduce emphasis on the need to assess runway conditions, to consider distance additives greater than the minimum 15%, and use a higher autobrake setting. |
(I’ve experimented on positioning flights...) Best accept what the performance manual gives you and leave the experimenting to the test pilots?:ok: |
I have always accepted the fact that Max Manual was the shortest stopping distance regardless of the runway state. That was why I asked the question when I had read the Boeing FCOM statement that specifically alluded to the dry as opposed to a blanket statement.
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Originally Posted by NSEU
(Post 9410422)
on Boeings, MAX is a fixed decel rate (although the rate is ramped up just after touchdown).
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With all due respect to your undoubted enthusiasm, it is possible your "experimenting" would have cost your company $$$ in terms of brake and tyre wear and tear not to mention hot brakes and risk of tyre failure. Best accept what the performance manual gives you and leave the experimenting to the test pilots? Landing at empty ferry weights is very different from doing the same at MLW, in terms of Vref, stresses and energy dissipation. I feel it beneficial in the long run to (safely) explore the handling envelope of any aircraft I fly, in case one day I should need use of one of the corners: at least I’ve been there or thereabouts and know what to expect. Simulators are good for a lot of that but sometimes the real thing is better. :) |
FW - On the 787 you get a display of deceleration vs auto brake settings in the HUD. Landing in the sim with max and then pushing the pedals harder certainly gave greater deceleration which is pretty much what NSEU is saying.
Agree with your statements about carbon brake wear too... |
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