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timers15

Activating auto brake during rollout: bug, feature, or for real?

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Hello all,Here's a procedure that I've been using. I do not arm the auto brake before touchdown. Then, after the reversers have slowed me to 80 kts or so I select autobrake 1/2/3 to activate the brakes. This works good, considering I do not have any analog pedal brakes, and it appears that the brake key is the equivalent of slamming on the brakes.Is this how it's suppose to work? Does the real 737 do this? Would this be a bad idea in the real world?Tim Dobrowolsky

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I set my autobrakes before landing so that when the plane touches down the autobrakes engage. I then use reverse thrusters to slow to 60knts then disengage both reverse thrusters and autobrakes and manually apply brakes to slow down enough to turn off the runway.Regards

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Guest Erups

Apart from the fact that for safety reasons autobrake is engaged in the air, your procedure may be sufficient on the runways you use, but in the world there are very short runways which need immediate brakeing action, otherwise you won't stop :)

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Guest boxjockey99

John At the risk of turning this into a techinical battle between pilot and engineer (I have no fear you guys know more about this aircraft than any of we members of the 2 winged master race!)Auto brake has nothing to do with Thrust Reverse, Autobrake put simply applies a level of braking required to stop the aircraft in a preset distance. so lets say for example Autobrake 2 is set to stop the aircraft in say 6000ft (These aren't real figures and I use them for illustration only). IF we were to land with Autobrake 2 then the aircraft will come to a halt in 6000ft whether we use reverse or not. The use of reverse is recommended because by using t we reduce the amount of braking force required to stop int hat 6000ft of runway. So inessence the autobrake system can ease off a little as it senses that it is decellerating much more quickly than it thought it would. I think that makes sense, the up and down of it is that using reverse reduced the brake wear which makes the bean counters happy! As for when to set the autobrake, do it when you do your approach brief (usually about 80miles before top of drop). In your brief remember the three B's - Bearings (setting the course bars for the ILS) Bugs (Vref speed bugs and altimeter decision bug) and finally Brakes (Autobrake and how much reverse you plan to use)Hope that helpsKris

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Guest Stefan Bergert

...maybe sth. in addition to Kris's good explanation:If you set Autobrake max or RTO, maximum pressure is applied regardless of distance.Stefan

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Guest Erups

>If you set Autobrake max or RTO, maximum pressure is applied>regardless of distance.Yes, but how much distance have you run while brakeing only with the reverse?I bet at least half a normal runway :)On short ones, unless you start a brakeing action immediately after touchdown, coupled with reverse, you may end up very far, or can even not make it.The safety reason i mentioned is that if you activate autobrake while still in the air, unless there is a failure in the autobrake system, you are sure that the brakeing action is initiated immediately; while if you leave the autobrakes disabled, there can be a delay in the brakeing.The distance needed to slow down also depends on the ac's load:the heavier you are, the longest you'll go.So you see, that when operating on short runways, and with passengers, you can't just slam full brakes, because FS doesn't even get near how hard full breaking bressure is... it nearly throws your head in the glass!BRClaudio

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Hi Kris,I think that my answer my have been missinterpreted. I never did say that TR and Auto Brakes were interconnected. Just that they are both used together to stop the aircraft. I said this becuase the original poster stated that he was using TR and then Auto Brakes after 80 knots. This is why I said to use both together as a correct normal procedure. Obviously this can change in real life if the TR's are MEL'd.As for how auto brakes work, all one is doing is selecting a deceleration rate and this is constant with the setting selected. The only difference being the aircraft weight and landing speeds which will effect landing distance. The auto brakes do not modulate.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

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Guest EDCS

Hi Kris,sometimes I read about the "rule of 9" for setting the Autobrake.(for Example: Distance to stop 8000 feet + Autobrake 1 = 9). Do you now about this rule?Landig on the great german airports with runway lenght over 10000 feet or runway goes uphill, I use no Autobrake. Best regards Stefan

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"The auto brakes do not modulate."Then how would the various deceleration rates be achieved, John?According to the Boeing Maintenance Manual (D&O), in 1, 2, 3 and Max, the autobrake system (during landing) modulates brake pressure until the airplane is at a full stop (Ref 32-42-00).Various maximum brake pressures are used for each setting, but braking pressure will be momentarily removed if the target acceleration is exceeded. Hence the reduction in brake wear if reversers are used.Perhaps I have misunderstood you???Cheers.Ian.

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Stefan, that's not quite correct (at least on other B-s (747, 757, 767))Autobrake max does not apply maximum pressure! The only setting that does this is RTO. Autobrake max has a very high deceleration rate (sometimes higher than achievable, therefore it may seem to be the same as maximum manual braking on a wet runway)Regards,Mark

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Guest Stefan Bergert

You're right...thought of Airbus where you have to arm autobrake - max before T/O.Stefan

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