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When/how Are Autobrakes Applied?  
User currently offlineLemmy From United States of America, joined Dec 2004, 258 posts, RR: 0
Posted (9 years 9 months 2 weeks 2 days 3 hours ago) and read 3189 times:

First post. Be nice.

I have read that, on airplanes that are so equipped, autobrakes apply either after a squat switch on the landing gear trips, or once the wheels spin up to a certain speed. I've always assumed that this switch or sensor is located on the main landing gear, but I'm not sure about this.

So my question is, at the moment when the MLG hits the ground, the nose wheel is usually still in up in the air. If the autobrakes were to kick in immediately when the MLG hits, wouldn't the nose of the plane be in an awful big hurry to slam into the ground? For example, if I do a wheelie on my bicycle, and then tap the rear brake, the front of the bike comes down quickly. Why don't airplanes do the same? With the nosewheel off the ground, it would seem as if the torque caused by the brakes would push the nose over very hard, especially at higher autobrake settings.


I am a patient boy ...
12 replies: All unread, jump to last
 
User currently offlineLimaFoxTango From Antigua and Barbuda, joined Jun 2004, 789 posts, RR: 1
Reply 1, posted (9 years 9 months 2 weeks 2 days 2 hours ago) and read 3165 times:

From the little I know about this subject, the squat switch is located on the main wheels. When the MLG is firmly on the ground, the autobrakes are applied. Now seeing as though FULL brakes is not needed as what you are thinking, the nose gear will not slam down as in the bicycle scenario. In anyway, that will be counteracted with a little pulling back on the yoke. There are different settings on the autobrake system. On the 777 for example, there are 5 (i think) different brake intensity settings, with 1 being the least and 5 is giving it all she's got!!

Ill let someone else give you a much clearer explanation.



You are said to be a good pilot when your take-off's equal your landings.
User currently offlineAvioniker From United States of America, joined Dec 2001, 1109 posts, RR: 11
Reply 2, posted (9 years 9 months 2 weeks 2 days 2 hours ago) and read 3155 times:

Here's more than you ever wanted to know about an MD80  Smile/happy/getting dizzy

System Operation
A. Landing Mode Operation
(1) The landing mode operation of the ABS allows the crew three selectable levels of deceleration - MIN, MED or MAX. The MIN decel position produces a decel level of 4 ft/sec2 (1.2 m), MED produces a level of 6.5 ft/sec2 (2.0 m) and MAX corresponds to the maximum deceleration consistent with single system antiskid braking limited by the tire pavement interface. The system
compares actual aircraft deceleration derived from wheel speed with pilot selected deceleration to modulate brake pressure to achieve the fixed deceleration level.
(2) The landing mode operation automatic braking activity is initiated by spoiler deployment with throttles retarded after the crew selects and arms the ABS. Throttle lever position information is used as a brake inhibit function until spoilers are deployed. If throttle levers are advanced beyond 20 degrees, ABS operation is prevented. With throttle levers retarded and spoilers
deployed, automatic braking is implemented after a delay of one second if MAX is selected or 3 seconds with MIN or MED. With another second allowed for hydraulic delays including brake fill, the total delay to the beginning of braking is approximately 2 seconds and 4 seconds respectively. The delays are intended to allow for a normal nose touchdown sink rate while maintaining a predictable stopping distance.
(3) Only one hydraulic manifold is employed for braking in landing mode operation. The landing mode manifold differs from the takeoff mode manifold in that it contains a solenoid valve and servo control valve, while the takeoff manifold contains two solenoid valves. During landing mode operation the first solenoid on the takeoff manifold is activated simultaneously with the landing mode solenoid. This feature provides a check of the takeoff manifold integrity. Pilot takeover of the landing mode ABS is available when spoilers are deployed by either advancing throttle levers (one or both), depressing brake pedals (beyond 25%) or selecting the ABS off by the control panel. Advancing throttle levers produce a DISARM condition in an identical manner to depressing either brake pedal. A DISARM by either brake pedals or throttle levers verifies the integrity of the control box fault monitoring and detection circuits. The MASTER CAUTION indication is inhibited if a DISARM condition is produced by either brake or throttle activation with ABS glareshield lights only coming on to advise the captain that the system has been overridden and is no longer ARMED.
B. Takeoff Mode Operation
(1) The takeoff mode operation of the ABS provides maximum brake pressure on both hydraulic systems. The available deceleration is limited only by antiskid braking consistent with tire pavement interface.
(2) The takeoff mode operation automatic braking activity is initiated by spoiler deployment and throttle retarding. Brakes are applied immediately upon spoiler deployment. The "T.O." position on the control panel, flap angle and ground speed information are employed to differentiate between takeoff and landing mode operation. The flaps must be raised above 22-1/2 degrees
and ground speed must be in excess of approximately 70 knots. If brakes are commanded above 70 kts., full dual system antiskid braking will beapplied to a full stop or until pilot takeover. If commanded below 70 kts. in the takeoff mode the system will apply landing mode braking at the MIN deceleration level.
(3) As in landing mode operation (spoilers deployed), pilot takeover is available by either advancing throttle levers 1 or 2 beyond 20 degrees or depressing brake pedals beyond 25%. Both conditions produce a DISARM which turns on the "ABS" lights on the glareshield without a MASTER CAUTION indication. A MASTER CAUTION indication will occur only if a system failure
occurs.



One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
User currently offlineDeltaGuy From , joined Dec 1969, posts, RR:
Reply 3, posted (9 years 9 months 2 weeks 2 days 2 hours ago) and read 3141 times:

Very nice description Avioniker!

Funny part about the Mad Dogs....the AB switch is on the aft end of the pedistal, right next to the rudder trim switch...kind of a random spot compared to the Boeing's logical positioning right near the LG lever...oh well  Smile/happy/getting dizzy

DeltaGuy


User currently offlineLemmy From United States of America, joined Dec 2004, 258 posts, RR: 0
Reply 4, posted (9 years 9 months 2 weeks 2 days 2 hours ago) and read 3133 times:

Wow! Thanks Avioniker! "The delays are intended to allow for a normal nose touchdown sink rate while maintaining a predictable stopping distance." That definitely answers my question.

Two more questions:

1) It looks like the MD80 applies autobrakes only after the throttles are retarded AND the spoilers deploy. I assume that ground spoilers won't deploy if the airplane is still in the air, which means that some sort of squat switch is in the loop here somewhere. Is that correct?

2) As LimaFoxTango points out, I'm sure you can counteract the pitching forward with some elevator. Is that amount of elevator needed to smoothly lower the nose noticeably different at different brake settings? In other words, do higher brake settings require more elevator?



I am a patient boy ...
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 5, posted (9 years 9 months 2 weeks 1 day 20 hours ago) and read 3025 times:

On the B732s.The Autobrake was functionally connected thru the Squat switch & the Wheel speed sense of the Main gear.To avoid Thrust Reverser Deployment with the nose gear off the ground the Air/Grd sense for it was parallelly given to the Nose gear too.
In the case of Autobrake the mode of deceleration is selected & subsequently Air/Grd sense and/or Wheel speed sense enable autobraking.
regds
MEL



Think of the brighter side!
User currently offlineAvioniker From United States of America, joined Dec 2001, 1109 posts, RR: 11
Reply 6, posted (9 years 9 months 2 weeks 1 day 10 hours ago) and read 2968 times:

1) Right. The ground spoilers employ circuits that senses wheel spin up as well as gear compression to ensure that they do deploy but not before the aircraft is firmly on the ground.

2) The elevator on an MD80 isn't hydraulicly powered in the up direction and the stall recovery system is disabled on the ground so no down power either. That way as the aerodynamic forces lessen with speed decrease the nose settles very smoothly and the pilot can look good with relatively little effort compared to a Boeing driver.

Interesting to note that many airlines, TWA was one, don't have autobrakes installed. There's a few reasons for this. The thing to remember is brakes and tires aren't cheap. The plane can use reverse and aerodynamic braking very effectively and economically and the company saves a lot of money on maintenance and parts. Also there's a considerable savings to be had in not having to maintain the manifolds, switches, relays, and black boxes associated with the autobrake system.

(Ah, the old days when experience counted more than a 25 year old bean counter's opinion....)

 Smile/happy/getting dizzy Blue on top

[Edited 2004-12-10 17:23:29]


One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
User currently offlineModesto2 From United States of America, joined Jul 2000, 2801 posts, RR: 5
Reply 7, posted (9 years 9 months 2 weeks 1 day 9 hours ago) and read 2931 times:

Each airline has a different policy. While some airlines prefer aerodynamic braking, others prefer brakes. At JetBlue, idle reverse is the norm except in specific short-field ops. Instead, JetBlue prefers the substantial use of brakes to save costs.

User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 8, posted (9 years 9 months 2 weeks 1 day 8 hours ago) and read 2929 times:

Hey Lemmy welcome to Tech/Ops.

It has been addressed above but the bicycle is not a valid comparison unless your bike has an elevator well aft of the rear wheel and a very small rear wheel. The geometry is there, when an airplane touches down, that we can apply fairly heavy manual braking and still hold the nose wheel off the ground if we choose.

The squat switches may be found on all three gear legs, depending on aircraft type. They have effects on absolutely every aircraft system, electrics, hydraulics, landing gear & brakes, pneumatics, air conditioning and pressurization, flight controls, flight instruments and on and on. Each system has a flight and a ground mode and some even divide it down further (above a certain speed, engines spooled up etc.) Some Boeing designs even had a cable from gear compression physically open a hydraulic valve for the ground spoilers in addition to the other triggers for their operation. In flight there was no hydraulic pressure to the system.

As a general principle of airliner design you want a good clean touchdown event. You want ground spoilers to operate and transfer the weight from the wings onto the wheels. Then you want autobrake and/or anti-skid braking available. You want reverse thrust available at that time. There are dozens of variables in all these designs but those are the basics.

Wheel speed transducers are an important part of autobrake and anti-skid systems because airspeed is just not specific enough. There are generally wheel speed transducers on each braked wheel. The systems are pretty complex and very, very good.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 9, posted (9 years 9 months 2 weeks 15 hours ago) and read 2866 times:

Although available,Most pilots tell me that they prefer not to use the Autobrakes & rely more on T/R & Manual braking.
Is that true.
regds
MEL



Think of the brighter side!
User currently offlineAAR90 From United States of America, joined Jan 2000, 3474 posts, RR: 46
Reply 10, posted (9 years 9 months 2 weeks 10 hours ago) and read 2848 times:

Why don't airplanes do the same? With the nosewheel off the ground, it would seem as if the torque caused by the brakes would push the nose over very hard, especially at higher autobrake settings.

It is called aerodynamics and pilots simply "fly" the nosewheel to the ground. The application of autobrakes, even at max braking, is just another torque to be taken into account as you fly the nosewheel to the pavement.

Although available,Most pilots tell me that they prefer not to use the Autobrakes & rely more on T/R & Manual braking. Is that true.

Up until I started flying AA's 738 I would have said: "yes, it is true." However, the vast majority of 738 FO's I fly with prefer using Brakes-2 for landings on even the longest of runways and they tell me the vast majority of CA's are the same way. Don't know why that is, but I prefer manual everything (or at least as much as I am permitted by AA policy/procedure) whenever possible. It just makes for a much smoother landing/rollout IMHO.



*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 11, posted (9 years 9 months 2 weeks 10 hours ago) and read 2838 times:

I agree with AAR90 on the manual braking. Of course some aircraft have better autobrake than others, but I still enjoy manipulating the aircraft controls myself once in a while.

Most landings, before you touch down it is pretty clear which taxiway you want to use to exit the runway. You do not want to slam on the binders and roll everyone up into first class, then have to add power to get to that taxiway. You'd like a nice smooth touchdown, gentle lowering of the nosewheel to the ground, enough reverse to get the job done and no more, and most of all, nice even braking and centerline tracking to bring you to taxi speed as you leave the runway. Do that and you "leave a good taste in their mouths" and should feel some personal satisfaction.

A trained ape could fly the plane, it is in these little things that our skills are showcased. Autobrake settings give you a deceleration rate and at some point in the rollout you are going to have to punch it off and modify that rate because of that exit taxiway.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineWbmech From , joined Dec 1969, posts, RR:
Reply 12, posted (9 years 9 months 2 weeks 9 hours ago) and read 2842 times:

Please don't forget that a 737 needs both throttles retarded. I had a captain write this up during an emergency landing with one engine out and the throttle still stuck near the top of the quadrant.

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