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View Full Version : Short and Slippery runways and use of B737 Autobrake MAX selection.


Tee Emm
2nd Jul 2007, 14:25
When landing on a slippery performance limited runway in a B737 it is considered prudent to select the autobrake to MAX. But as reverse thrust becomes effective the amount of brake pressure applied via the autobrake is reduced in order to give a certain decelleration rate.

On the other hand if the brakes are applied manually by the pilot under the surface conditions described above, it is expected that he would apply full pedal pressure in conjunction with full reverse thrust.

Full manual brakes application gives greater stopping efficiency than with autobrakes at MAX.

That being so, perhaps it would be advisable when landing on a short slippery runway to plan on applying full manual braking immediately after touch-down (as well as full reverse) in lieu of setting auto-brake at MAX and letting the autobrake do its work?

Discussion invited?

alexban
2nd Jul 2007, 19:05
Suggest using this: select MAX autobrake, then full reverse and full 'manually ' braking. You'll be surprised how hard you'll have to apply brakes in order to override the MAX setting.
Why select MAX first? in order to obtain immediate braking after touchdown.You won't be abble to apply brakes as quicly as the autobrake.
And it applies brakes evenly on both sides.
Also,on slippery runways be very carefull with reverse usage in case of crosswind-due to cornering forces you may encounter some control problems.

rubik101
2nd Jul 2007, 19:13
The Autobrake in MAX applies almost full braking as soon as the wheels spin up to 60kts. The application is smoothe, balanced and very positive. The aircraft will decelerate very quickly and as the aircraft approaches taxy speed, if left engaged, will bring the aircraft to an abrupt stop.
Applying manual brakes will NEVER be as effective on landing, particularly in crosswind or slippery conditions. Uneven braking will result if the rudder pedals are offset even a small amount. Let the Autobrake do its job and you will never be dissapointed.
MAX manual brakes should be applied only if the Autobrake application is insufficient. Most pilots who disengage the Autobrakes on landing do so far too soon, before the brakes are allowed to function at their optimum. Leave MAX Autobrake engaged and you will be pleasantly surprised at the results.
As you reach taxi speed then disengage the brakes by the preferred method, restow the speed brakes.

sudden Winds
3rd Jul 2007, 05:57
excellent rubik, good knowledge of the system...
The only thing against max autobrake is ..be ready for some cooling period after that...
The shortest Ive landed a 737 on was a 1500 m rwy. The technique we use is autobrake off flaps 40, little fllare, immediate reverse and then manual braking as soon as weve decelerated enough to prevent excessive warming, or autobrake 3 + all the same, deselecting it using the switch or the speedbrake. We rarely use max.
Regards,
SW.

Tee Emm
3rd Jul 2007, 14:49
Applying manual brakes will NEVER be as effective on landing, particularly in crosswind or slippery conditions

Interesting point of view. I always thought that max manual brakes (and I mean maximum, not partial) would pull the aircraft up in a shorter distance than the MAX position with autobrakes. Which may be why certification of landing distances are conducted by test pilots using manual braking.

In the simulator and when practicing all flaps up landing on a limiting length runway, instructors occasionally observe over-runs despite accurate touch-downs and use of max autobrake as well as full reverse. This is because the autobrake selects a decelleration rate. Pilots are observed to leave max autobrake engaged expecting the aircraft to stop quickly and are somewhat taken aback when the over-run occurs. Those that use max manual braking immediately after touch-down and full reverse usually pull up with room to spare.

rubik101
3rd Jul 2007, 15:48
The thread led with the situation of a short and slippery runway. Couple this with wet conditions and/or a crosswind and MAX Autobrake will be your best option. Attempting to apply even and consistent braking under the conditions described will result in uneven and intermittent braking. Have you ever tried to apply full braking with anything other than the rudder pedals centred?

If I have to land with all flaps up then I will certainly use the longest available runway. Unless the aircraft is burning, then divert to a longer runway rather than use a limiting runway. There are enough of them around!

alf5071h
3rd Jul 2007, 18:39
See a recent Boeing presentation on the problem - Boeing Slippery Runways "Update on landing performance", downloadable here. (http://uk.geocities.com/[email protected]/alf5071h.htm )
The problem is not as simple as it might first appear.

Selected text from the accompanying notes:-

With manual brakes, the brake system applies whatever brake pressure is commanded by the crew. If autobrakes are used, the autobrake system targets a predefined deceleration level and will apply the wheel brakes only as required to reach target level.

In summary, the deceleration from reverse thrust is always additive when using manual brakes, whether on a dry or a slippery runway.

Conversely, the deceleration from reverse thrust may be additive when using autobrakes, depending on the autobrake setting and the runway conditions.

The Advisory landing distance data contained in the Boeing QRH includes the effect of reverse thrust for Max Manual braking and for Autobrakes (whether additive or not).
As a reminder, reverse thrust becomes the most effective deceleration device as runway conditions deteriorate.

blackmail
3rd Jul 2007, 19:57
hello alf & others,
consider a normal dry rwy.
to add to your post & put some figures on it; boeing autobrakes selection on landing are 1,2,3 & max. to approximate the deceleration rate in knots/second, add 1 to the numerals & max is double the deceleration rate of autobrake 3.
so autobrake 1 = 2kts/sec deceleration rate, a/b 2 = 3 kts/sec, a/b 3 = 4kts/sec & finally a/b max = 4 x 2(double a/b 3) = 8 kts/sec. for info a one(1)g deceleration rate = approx. 19.1kts/sec deceleration(also 9.81m/sec square).
so autobrake max is circa slightly under half a g deceleration rate, which is not bad. as said in other posts, using reverse & autobrake together, will yield the same deceleration rate as selected. just more noise with more reverse & cooler brakes & vice versa. manual brakes + reverse on the other hand are additive.
kind regards,
bm::rolleyes:

alf5071h
3rd Jul 2007, 23:39
blackmail, I have difficulty in following your point.
I cannot comment on the values you suggest, but I interpret them as the autobrake demanded deceleration (on a dry runway?), or the theoretical cumulative deceleration with reverse. Thus they would not represent the achievable deceleration rate, which on a slippery runway may not be the same and in the worse case being considerably less.

A somewhat technical description of the aspects of autobrake are in CS 25 (www.easa.eu.int/doc/Agency_Mesures/Agency_Decisions/CS-25%20Consolidated%20version.pdf), see page 209. I assume that the 737 system is a fully modulating system.

I would not disagree with the view that when landing on a short slippery runway, the use of max brake and full reverse is a sensible option. However with a crosswind and a slippery runway where the majority of the deceleration comes from reverse, then any adverse effects of the reverse / crosswind combination will increase; thus in these circumstances many manufactures recommend that landings are not attempted or are subject to very restrictive crosswinds.

General references:-
Several papers on crosswind, contaminated, and tailwind landings - 2001 and 2005. (www.nlr-atsi.com/publications.php)
Select Landing Techniques - Crosswind Landings. (www.airbus.com/en/corporate/ethics/safety_lib/index.html)
Managing the Threats and Errors during Approach and Landing. (www.flightsafety.org/ppt/managing_threat.ppt)

blackmail
4th Jul 2007, 09:20
hello alf,
by selecting a deceleration rate with the autobrake, the system will try to keep this selected deceleration rate constant. by using wheelbrakes only, obviously, they have to do the job alone & will heat up accordingly. now by using reverse the system initially, senses an additionnal deceleration rate which it does not want & reacts by releasing & modulating the brake pressure as to keep the selected decel rate constant. so the combination of auto wheelbrakes & reverses will keep the same decel rate as selected. i think the figures in my previous post come from the boeing FCTM. now for slip rwy's, obviously, the system does not know before landing on which surface you gonna land. so, once on the ground, the system will try to keep the selected decel rate & when sensing a locked wheel, the anti-skid will release the brake pressure accordingly & will then keep the brake pressure just enough & optimum, as to not lock up the wheel again. you will never be able to beat the system in using manual brakes in this case. so sop's for many companies, is to use autobrakes with a minimum of 2, when anticipating slip rwy's. for crosswind landing on slip rwy"s, indeed, the lateral component of the reverse forces may be undesirable, when starting to slip or skid sideways & it is advisable to stow the reverse to regain directional control.
hope this clarifies a bit.
bm

alf5071h
5th Jul 2007, 01:08
blackmail, thanks; your description correlates with the Boeing presentation above (#7).
I am not familiar with the 737 autobrake system, but in many aircraft the wheel-lock protection that you describe is part of the antiskid system, which works with or without autobrake.
MAX autobrake does not necessarily provide the maximum wheel braking torque deceleration.
With manual brake the pilot can demand full torque (feet hard on the pedals); in this circumstance, and if the runway conditions permit, the cumulative effect of manual braking and reverse could be slightly greater than when using autobrake. Alternatively, if the runway is slippery (antiskid still functioning), the net braking effect with manual brake may only be the same as autobrake (mainly from reverse). This could be one reason that a manufacturer might recommend the use of a particular autobrake setting, another is that it removes pilot variability during critical conditions. Brake heat is not an issue.

If slipping sideways in a crosswind wheel braking can be reduced to increased cornering forces, but on slippery runways there is very little wheel braking so the effect is small. Similarly reverse idle will reduce the sideways force, but most of the deceleration is coming from reverse on a slippery runway. Thus, if you correct the sideways drift there is an increased risk of overrun, or if you dont check the drift then the risk of leaving the side of the runway remains high (catch 22)! Thus the better course of action is to avoid these situations - dont attempt to land on a slippery runway in a crosswind. This point can be reinforced by the ambiguities in reporting wind velocity (ref 2001 #9 above) and the difficulty in assessing runway condition see various threads on this subject. The latter point might result in the assumed safety margin for stopping within the length of the runway is non existent.

Dont forget that slippery is not a certification performance term and even when used, it is ill-defined and does not relate to any description of the runway condition or expected braking effectiveness. Most regulatory authorities recommend avoiding landings on a contaminated runway, which in my book equates to slippery.

Belgique
5th Jul 2007, 09:45
As you commence max manual braking, feed in the backstick for maximum weight-on-wheels/most effective wheel braking
.
per:
this link (http://www.pprune.org/forums/showthread.php?t=204897&highlight=backstick+braking)