Autobrake usage 737 Classic

[Olendirk]

guys,

some use it, some not, some say it doesnt help, others say it helps. so when makes it sense to use it, when does it start braking, how does it work together with the reverser?

Thanks for the help!


OD

[InSoMnIaC]

Hi Olendirk,

The autobrake system is quite helpful. Ie both on landing and if you need to reject the T/O. We always use it in our daily ops

If you are rejecting the T/O. The autobrake system when armed starts braking when the the thrust levers are retarded to idle and wheel speed is greater or equal to 90kts. The Auto brake in this case applies the equivalent of full manual braking force.

On landing. the autobrake provides 4 selectable rates of deceleration. 1,2,3 or MAX. (btw MAX is less than full manual braking).

Braking begins when the thrust levers are reduced to idle and main wheel spin up of around 60kts occurs.

Autobrake doesn't directly give or receive a signal from the reverse thrust. It does however maintain (or try to maintain) the selected deceleration rate. so by selecting reverse thrust you are in effect helping the aircraft to maintain the selected deceleration rate with less braking.

I hope that helps a bit.

[alexban]

If you want to save the brakes you should use the autobrake .It was demonstrated that the use of autobrake,setting 1 or more ,till 80 kt will reduce brake wear.

[vwreggie]

we operate 737 -400 and 800 and the use of autobrake for RTO and landing is standard ops on both. The autobrake gives a programmed rate of deceleration and most people use a dry setting of 2 on a classic and 3 on the 800. We also use full reverse thrust which provides a deceleration rate equal to that of the autobrakes at touchdown so the autobrakes aren't actually braking until around the 100 - 90 knot speed. If you leave the autobrakes in too long then you have to apply brake pedal pressure in excess of that being applied by the autobrakes which can give you that lurching/whiplash feeling forward in your straps. So when I am happy with the touchdown and rollout I disconnect the autobrakes at around 90 and keep the pressure on the pedals as manual braking. We have the pilot not flying announce the " autobrakes disarm" light as standard.

[Cough]

Quite agree for the above cases.

Seperate thought of the day at the 'reverse idle' type airports, when heavy and its hot (and you have a long enough runway) then I use reverse idle, with autobrakes brakes off, applying brakes at the 80-90 knot stage. Why?

Max landing wt (for us on the 737-4, approx 55T) with reverse idle ,20 degrees at sea level with autobrake 2 puts the brakes in the 'caution zone' according the the QRH. (Caution zone=wheel fuse plugs may melt...)

Same condition, brakes on at 90kt gives a min turnaround time of 20 ish minutes and I normally vacate on the same exit as my autobrake 2 colleagues, sometimes just a little bit later.

Hope this gives a slightly different perspective...

[tonker]

I have recently flown with a captain who didn't use reverse thrust and no useful answer was given for not using it.

The brake cooling time based on the conditions with idle reverse was 57 mins, and nearer 17 with! Something to consider when our turn around time is 30 mins.

[Just another student]

I landed into LGW the other day onboard a 737-500, reverse was not used full-stop, or if it was, No1 had the doors locked out. Heavy manual breaking was used.

I will find out soon what this particular company requires, I start groundschool there next week.

[podbreak]

so when makes it sense to use it

I think the question should be when does it make sense not to use it?!

When its bloody hot, otherwise you'll be sitting on the ground for quite a while...

[Centaurus]

If you want to save the brakes you should use the autobrake .It was demonstrated that the use of autobrake,setting 1 or more ,till 80 kt will reduce brake wear

Maybe so,especially with carbon brakes. Historically it is a different story though. Shortly after the 737 was introduced into service around 1972 (?) Boeing conducted periodical Boeing 737 Operator Symposiums. Some operators complained that wear and tear on tyres and brakes had increased significantly with use of the auto-brake system. This was despite Boeing assertion in the then FCTM that wear and tear would be reduced with aauto brake use.

Boeing explained to delegates at the Symposium that the advantage of autobrakes in a fleet of Boeings was that wear on the wheels and brakes would become more uniform and thus maintenance times of these units more predictable. The point being that pilots have differing braking cultures - some are lead footed and stamp on the brakes early and hard while others wait for the effect of reverse thrust to slow down the aircraft before applying brakes toward the end of the landing run. Thus maintenance periods on brakes and tyres depended largely on pilot technique and was difficult to plan on.

Boeing went on to say that autobrakes were designed to aid in stopping the aircraft and as the brakes were applied immediately on wheel spin up before reverse could build up, naturally there would be faster wear and tear and hotter brakes would result.

The B737 FCTM states "Boeing recommends that whenever runway limited, using higher than normal approach speeds, landing on slippery runways or landing in a crosswind, the autobrake system be used".

This suggests there is no operational requirement to switch on the autobrake system unless one or more of these parameters are present. To actuate the autobrake system for all landings regardless of excessive runway length is operator choice, but expect more wear and tear on the brakes and tyres.

[A37575]

We also use full reverse thrust which provides a deceleration rate equal to that of the autobrakes at touchdown so the autobrakes aren't actually braking until around the 100 - 90 knot speed

That's strange. I thought the autobrakes actuated immediately on wheel spin-up which at max landing weight on a 737 Classic could mean a ground speed of 132 knots or higher depending on flap configuration. For most landings reverse thrust does not really get going until 20 knots below actual touch down speed because the engines have to re-spin up from the closed thrust lever position and this takes at least 5-8 seconds depending on how fast the pilot actuates the reverse thrust levers.

[c100driver]

The 100 series and most (not all) 200 series B737 had analog auto brakes which worked slightly differently to the later model B737 digital autobrake system. The analog would apply a fixed brake pressure on wheel spin up then reduce or increase braking once the decel rate was calculated, hence the increase wear seen with the early intoduction on the B737. The initial brake application was always rather severe which made many pilots distrustful of the autobrakes.

[alf5071h]

Autobrake use on ‘slippery’ runways; see the Boeing presentations:-
Stopping on Slippery Runways. (www.pilots.or.kr/upfile/aip/9AAAA_StoppingonSlipperyRwys2.ppt)
and
Update on Landing Performance. (http://uk.geocities.com/[email protected]/alf5071h.htm)

[stilton]

On the 75 /767 I try to avoid that 'lurch' when the autobrakes disengage with manual braking by sloowly starting to retract the spoilers just a hair on the rollout.

It doesn't take much movement and you get a nice smooth disengagement.
I still believe in reverse thrust though and dislike the emphasis at my company and others to use idle reverse to 'save fuel and wear and tear'

My opinion is you should use every available stopping device on the aircraft and I still do.

Habits become ingrained. The Qantas 744 that slid off the end of the runway in Bangkok had a lot of issues, but one of the facts later discovered was they only used Idle reverse.

So what if it saves fuel and so what if carbon brakes work better when hot, they still work perfectly well in conjunction with reverse.

Furthermore, and especially in wet / slippery conditions you may select for example 'autobrake 3' but not achieve that deceleration rate as that is all the anti skid can give you.

Reverse will 'help' to achieve the desired deceleration rate.

One day you will be glad you used it, it's too late when you are in the 'red lights'

[Dani]

Modern carbon brakes do not wear per second of application but per cycle. Three short and light applications use the brakes more than one long hard application. That's why autobrake is favoured. If you can judge yourself where you gonna leave the runway, and apply only once, you might damage brakes less. With autobrake you brake a) when it starts and b) when you start braking manually.
There is also a dependence of temperatur: below 80 and above 250, the wear is least.
Nowadays airlines favour brakes instead of reverse, due to fuel and noise issues. Airbus wanted to design the A380 initially without any reverser, but opted against it after airlines wanted them. Now only two of four engines have reversers.
Dani