View Full Version : Do not override autobrake to shorten the landing distance??

18th Nov 2009, 01:48
I just read the article "Landing in conf3 - Use of reversers" presented by Hélène REBEL Head of A330/A340 Operational Standard from the 15th Performance & Operations conference 2007

Anybody knows why Airbus not recommend to override autobrake to shorten the landing distance? Is this apply to A320/319 also?


Sir George Cayley
18th Nov 2009, 21:40
Is it something to do with brake pedal pressure demand? I think that exceeding the pressure commanded by a/b actually disengages the system inc anti skid.

So, further pressure on the brake pedals takes you to the point where the tyres lock and you are now friction limited.

Just a wild guess:)

Sir George Cayley

19th Nov 2009, 09:24
I cannot find the reason not to override the autobrakes but my guess is that the auto system is going to be evenly applied on both sides and more controlled. But then I have read some that would say manual is better than auto and so I will not go down that road sir..

Time for me to read that Airbus Journal and do some swotting...:ok:

If footbrake pressure exceeds a pre-determined value the autobrakes are disconnected and manual braking is restored with full anti-skid protection.


19th Nov 2009, 09:41
Sir George Cayley is incorrect.

19th Nov 2009, 10:02
Hi Alton77,

I've just watched the ppt again and think the emphasis is on reduced fuel and DMC (Direct Maintenance Cost).

It suggests that overiding the autobrakes increases the brake temp. and increases tyre wear and the subsequent costs. They also say that for performance issues (I guess earlier runway turn off ?) then you should consider using Flaps full etc.

celtic mech
19th Nov 2009, 19:57
Just in reference to Autobraking and an above comment...If you are in Autobrake and you press the brake pedals and request an input that is greater than the autobrake commanded rate of decel, you will indeed disengage the Autobrake and will be manually braking...BUT you will NOT lose Anti-Skid. You Cannot have Autobrake without Anti-skid.

Doors to Automatic
19th Nov 2009, 20:20
In the A320 series is the MAX autobrake setting used during landing or is it solely for RTOs?

Does the aircraft need to be able to stop on the MED setting if it is to be allowed into a particular runway? I'm thinking here specifically the A318 at London City.

19th Nov 2009, 20:37
Hi. In the A320 series the MAX autobrake is not recommended for landing if you read the QRH. And it cannot be set in the air anyway....so that pretty much answers that I suppose.

Always set for take off for RTO - yes.....

Yes medium autobrake is very useful for landing on shorter runways and our airline certainly uses it at Jersey !!

Cheers WM

19th Nov 2009, 23:10
In the A320 series the MAX autobrake is not recommended for landing if you read the QRH. And it cannot be set in the air anyway.

Must be new modification. We used to operate 320 with serial no in low 200s which once landed with max autobrake armed. The crew swore it was genuine mistake and they absolutely were not bent on experimenting. They were surprised by rapid derotation immediately following touchdown, but otherwise rollout was normal and nose gear suffered no damage.

20th Nov 2009, 00:01
Re “Does the aircraft need to be able to stop on the MED setting if it is to be allowed into a particular runway?” (#8)
All aircraft must be able to stop within a % the total runway length available depending on the conditons (60% for a dry runway). There is no mechanism for the autobrake, at any setting, to guarantee any particular stopping distance.
Auto brake can only demand a particular, selectable deceleration by modulating the brake pressure and then it is limited by the anti skid system.
The stopping distance depends on the runway conditions, the runway friction, and whether any additional deceleration device is available, e.g reverse thrust, spoilers, or airbrake. Where the runway friction is very low, then even with max brake pressure, the selected deceleration may not be achieved. In these conditions the aircraft may not be able to stop safely, i.e. contaminated/slippery runway operations not allowed at LCY.

Ref: Landing on Slippery Runways (www.csoca.org/index.php?option=com_docman&task=doc_download&gid=76&Itemid=58[/url) - Slides 16-17

20th Nov 2009, 05:13
Have seen time and time again, one gear's brakes hotter than the opposite. Uneven pressure has to degrade systems and performance in my assumption. Human factor of being one leg/foot dominant is probably normal.

Wonder if this is a significant factor... Or possibly they want to create a controlled environment to further remove human interaction from the operation of an aircraft as is the goal of the industry, to prove automation is safer. Airbus is big on this stuff.

20th Nov 2009, 11:37

I would agree with you and that is why I considered that the auto brake should be used fully for the runway conditions.

I am waiting for some left footed pilots to land for me to check this theory, but certainly a very high proportion of aircraft appear with much hotter RH gear temperatures.


20th Nov 2009, 12:14
Paradoxically, there can be a situation where use of autobrakes can get you into trouble and that is when a directional control problem occurs during a landing roll-out on a slippery runway with a crosswind. The Boeing 737 FCTM gives advice on how to correct a sideways slide when using reverse thrust. It advises the pilot to correct back to the centreline by reducing reverse thrust to reverse idle and release the brakes. In practice this is quite tricky since from full reverse to idle reverse of 23 percent N1 can take as long as 15 seconds to reach a stabilised idle. At the same time a quick stab on the rudder pedals to disengage the autobrakes can cause a further yaw unless the autobrake switch is used to disengage the system.

It could be argued if a strong crosswind is present on a slippery runway it might be worth considering not using autobrake at all but rely on tactile feel of manual rudder pedal pressure to know when to ease off braking in response to unwanted yaw. Boeing go on to say that when re-established on the centre-line apply max braking and symmetrical reverse to stop the aircraft. But you have to keep in mind that it takes several seconds to spool up from idle reverse to full reverse.

20th Nov 2009, 12:45
In the A320 series the MAX autobrake is not recommended for landing if you read the QRH. And it cannot be set in the air anyway.

Must be new modification. We used to operate 320 with serial no in low 200s which once landed with max autobrake armed. The crew swore it was genuine mistake and they absolutely were not bent on experimenting

Newish, we've operated A320 series from MSN0006 (No 6 off the production line) to 2009 deliveries and you could land with Autobrake MAX on the earlier aircraft but not on the later one. By no means definitive but the first aircraft I recall seeing this change was around 2004/2005 but please don't quote me on these dates.

As an aside, the early aircraft didn't have brake fans whereas the later ones did and there was a noticeable decrease in brake wear on the later ones. It was noticed that with the carbon brakes and number of applications affecting wear more than braking force applied, pilots weren't worried about heating them too much so tended to use them less timidly hence less appications, especially on short (30-35 minute) summer sectors with only 40 minute turnarounds.

21st Nov 2009, 14:12

Do you consistently land with a crosswind from one particular side?

I am left footed and most often find that it is the "downwind" brake assembly is hotter on arrival at the gate in what I can only assume is the extra braking required on that side to prevent weathercocking in the landing roll.

Nothing to do with left or right "footedness" in my experience.

Of course, other factors like taxi technique and which direction the wind comes from in your taxi to the gate will also affect brake temp differential etc.

Sir George Cayley
21st Nov 2009, 17:15
So is it the case with Airbus that once the autobrake has tripped, anti skid will continue to attenuate the brake pressure and maintain slip at Mu Max?

Sir George Cayley

22nd Nov 2009, 01:53

Yes. Antiskid is available down to 20kts.

As for max mu, my fcom says antiskid will allow wheelspeed down to .87 of aircraft reference speed (furnished from adiru's). In case of failure of all 3 adiru's, reference speed is equal to the greater of either main gear wheel speeds.


22nd Nov 2009, 03:20
All I know is in 7 years landing in Tegucigalpa Honduras, noted as one of the most dangerous airports in the world if you google it, you can land shorter using manual braking with full anti skid using the highest normal short of max setting. auto brakes 4 was what the procedure said was required for landing. The 757 stopped very well and with that short runway with a cliff at the end and a hill on final you didn't want to not do it right. That was the only airport we were required to land with AB 4 set. Our manual told us that. I have never flown an Airbus but assume it is the same. I don't think they can certify an airliner that you can push on the brake pedals enough to disable the antiskid.

Sir George Cayley
23rd Nov 2009, 11:42
So, even with anti skid working, if the the runway surface has reduced grip due to either contamination or poor maintenance the system could make you think the wheels have locked, as retardation will not be as expected?

Sir George Cayley

23rd Nov 2009, 22:53
Not at all.

In those circumstances, you'd know not to expect usual braking performance anyway.

Antiskid failure generates a master caution/single chime and is announced on ecam (inhibited below 80kts till 2nd engine shutdown) and is also shown on the SD Wheel page in amber.

So, in your scenario, one has every reason to believe its working until you are told otherwise.

24th Nov 2009, 02:32
SGC, in your scenario (#21) the wheels would not lock as the antiskid system will reduce the brake pressure to maintain wheel rotation whilst still attempting to achieve the ‘maximum’ retardation in the poor conditions. Hence some systems may be described as ‘cycling’ the brakes on-and-off on a low friction runway.
Retardation may not be as expected, but often crews are unaware of this at higher speeds particularly if thrust reverse is contributing a significant proportion of the retardation. However, after cancelling reverse, the relative ineffectiveness of the brakes may be sensed and could be interpreted as a brake failure due to the low level of deceleration.

For those who wish to know the Brake Pedal Force (www.britflight.com/wingfiles/performance/takeoffsafetytrainingaid.pdf) required to disengage autobrake (page 16).

24th Nov 2009, 06:04
Override the autobrake to shorten the runway .. for me, is just apply the more force contunuously on the pedal until the aircraft in the taxi speed..

Seems like the reason not to do that as Airbus said is just to balance the brake temp and wear. isn't it?

24th Nov 2009, 06:59
Next time you are out flying the 'Bus, simply print out the landing data. There, in black and white, from the aircraft itself, you will see that MANUAL BRAKING stops SIGNIFICANTLY shorter than either LOW or MED autobrakes.

That said, VERY FEW PILOTS would EVER think of standing on the brakes at touchdown and thus ANY runway shorter than MED brakes but longer than MAX MANUAL brakes should be viewed with significant skepticism as to landing performance (i.e. stopping on the hard surface), due to unknowns and unknowables.

There are certain runways we use that REQUIRE a go-around if the aircraft is not on the ground within the first 2000' because there is, IMHO, a rather large "fudge factor" that one must apply to any landing, due to the myriad conditions that can affect performance that may not be fully known or understood during the landing planning phase.

Happy Landings!