View Full Version : Question on A320 Autobrakes selection v runway length


A37575
30th Jun 2013, 04:56
What prompts this question is the recent experience of a colleague who was a passenger on a Jetstar A320 flight that landed at an East Coast runway 6000 ft long dry hard surface sea level. He is a former 747, 727 and 737 captain so he is no amateur PPL. Immediately after touch-down there was very severe braking accompanied soon after by the normal sound of full reverse thrust.
Passengers adjacent to him were quite alarmed when they were jerked forward by the aircraft deceleration and several grabbed the back rest of the seat in front of them.

My colleague said he had never experienced such savage braking before and was startled. He assumed it was the autobrakes operating on a high setting or in the case of the A320 Medium setting which is harsh. Soon after he thought he detected the typical slight jerk when the autobrakes were disengaged. Then the aircraft proceeded to taxi quite sedately along the rest of the runway until reaching the end. Meanwhile down the back, passengers calmed down as the aircraft taxied to the terminal, but there was no shortage of muttered bitching comments from frightened passengers wondering what the heavy braking was all about. There was no PA from the crew apologising for the unusually heavy braking. Maybe that was considered normal braking by the pilot ?

The comment by an experienced pilot as a passenger should not be dismissed as a typical cynical remark from a pilot way past his use-by date. In fact he owns his own twin engine aircraft which he flies regularly.

I can understand the reactions of the passengers on that flight. The whole trip had gone smoothly, the service was good and no doubt many were looking forward to a nice time at this well known holiday resort. The touch down was smooth then without warning their comfort turns to momentary fright as the brakes are hit hard. Those few seconds can turn some passengers against travelling on Jetstar in future. Why was the hard braking necessary when the runway length was well beyond the performance limiting length and the runway was dry?

Was it company policy to set the automatic brakes for all landings regardless of excess runway length available? It is my understanding that Qantas for example require the autobrakes to be used on all landings on their 737 fleet even though a runway could be 10,000 ft or greater. Why is this so, I wonder? What happened to giving the captain his discretion to use manual brakes instead of autobrakes. Are crews not to be trusted to use their experience in such basic matters as use of wheel brakes for landing?

In the case in point, what on earth did the captain have to gain in landing performance by using such hard sudden braking on touch down only to coast along the rest of the runway after reducing reverse thrust and reverting to ordinary use of brakes.

If it was company procedure to select autobrakes for landing with runway lengths well in excess of that needed operationally, then surely the minimum setting would be sufficient. Airlines push the advertising publicity line that passenger comfort is paramount. Fine until the point of touch-down, then all bets are off while some pilots put the passengers into the next row of seat in front of them with unnecessary harsh use of brakes - automatic or manually applied. Is this piss-poor airmanship or company mandated procedure?

For normal landings on a dry hard surface, in something like a 737 or A320, is it true that 6000 ft and below is considered a short runway and a maximum performance landing technique is mandatory? And even 8000 ft or more is considered by some the companies as marginal and requiring special considerations Have crews forgotten the art of touch down on correct speed and correct point and the art of judicious gentle braking along with correct use of reverse thrust to bring the aeroplane and its load passengers to a safe stop or turn-off?

Or are we so brain-washed by the manufacturers and perceived risks of litigation that operators cover their backsides to the ridiculous degree on the use of automatics in all our operations, that pilots have lost the basics of how to apply brakes smoothly for passenger comfort? And I am not talking about landing on short wet surface runways that do require positive braking automatic or otherwise?



Metro man
30th Jun 2013, 05:14
A 6000' runway is plenty for an A320 even at MAX landing weight with tailwind, so runway conditions probably weren't a consideration. Auto brake commands a deceleration rate rather than a specific amount of braking, so if full reverse thrust is used little demand is made on the brakes once deceleration has commenced.

It's possible that they were practicing for more limiting runways, Captain showing a new F/O how quickly the aircraft could be stopped or the F/O practicing his short field technique in an environment were it wouldn't matter if he didn't quite get it right.

Starbear
30th Jun 2013, 05:56
This sounds like the use of Max Autobrake setting (whether intentional or otherwise!) which is indeed harsh. It's meant to be. It is (not recommended for landing but not prohibited either. Medium on an A320 is not harsh at all.

Lookleft
30th Jun 2013, 06:57
Sounds like a typical MCY landing! Always is a ridiculous argument to land with autorbrakes medium then straight away kick them off and use manual brakes to ease it up to the turning node at the end of the runway. For me if you land on the touchdown zone then auto brake lo will slow you down in plenty of time before the end of the runway. It also means that you are not dealing with hi brake temps on the turnaround.

Bevan666
30th Jun 2013, 07:01
Back before Sept 11, 2001 when I used to enjoy a few jump seat visits we were on final to runway 16 in Melbourne in a 737. Conversation went like this

"I bet you cant turn off at Echo"

"I bet I can"

Pilot flying then reaches forward, bumps the autobrake up a notch or two.

We turned off at Echo.

Thats always the explanation I have when I have similar experiences now.

Bevan..

slam_click
30th Jun 2013, 07:15
"I bet you cant turn off at Echo"

"I bet I can"

I'm used to hearing:

"You BETTER configure so you can vacate by Echo". (Different airline)

waren9
30th Jun 2013, 09:14
arming all automatic retardation devices is mandatory at jetstar unless otherwise reqd by fcom/qrh procedures or mel dispatch reqmnts

a couple of the older 321s had fierce brakes that were hard to modulate a smooth transition from autobrake to manual braking via peddles esp when commanding a higher braking force than the autobrake was giving

cant speak for the rest of it

It's possible that they were practicing for more limiting runways, Captain showing a new F/O how quickly the aircraft could be stopped or the F/O practicing his short field technique in an environment were it wouldn't matter if he didn't quite get it right.

poor form if true. thats what sims and type ratings are for.

compressor stall
30th Jun 2013, 10:27
Mandatory auto brake seems to be a QF group thing. I've been admonished by a QF long haul capt for suggesting that there may be a time and a place for zero auto brake (ie vacating at far end of 3000m runway).

chimbu warrior
30th Jun 2013, 11:01
Mandatory auto brake seems to be a QF group thing. I've been admonished by a QF long haul capt for suggesting that there may be a time and a place for zero auto brake (ie vacating at far end of 3000m runway)

Sounds like a knee-jerk reaction to the Bangkok cross-country.......going from one extreme to the other.

waren9
30th Jun 2013, 11:04
mandatory arming is not the same as mandatory use

compressor stall
30th Jun 2013, 11:49
Yeees, so....

So you arm it, then disarm it before landing?

Or you touchdown, then call for it to be disarmed?

Or you touchdown then tap the brakes to disarm?

Capt Fathom
30th Jun 2013, 12:56
Talk about Sunday morning quarterbacks!

Since when do the people down the back start questioning the procedures employed by the people up the front?

Mister Warning
30th Jun 2013, 13:46
The OP stated that touchdown was smooth. What he doesn't say is how far down the runway that touchdown occurred.
We've all sat bedside someone who sacrificed runway length for an impressively smooth touchdown then stomped on the anchors to pull it up.
Unfortunately the only thing most pax remember is the landing and how long it took to get their bags.
There is a big difference between a smooth landing and a safe landing.

sheppey
30th Jun 2013, 13:52
Since when do the people down the back start questioning the procedures employed by the people up the front?

The fare paying punters down the back (bogans or otherwise) certainly do have the right to start questioning procedures by the people up the front, particularly when their teeth or dentures are now firmly implanted within the headrest of the seat in front of them as a result of harsh braking:ugh:
auto brake medium & full rev, to a pax with NO upper body restraint device it does feel alarming, does to me when I pax.

Case in point?

Checkboard
30th Jun 2013, 14:37
This sounds like the use of Max Autobrake setting (whether intentional or otherwise!) which is indeed harsh. It's meant to be. It is (not recommended for landing but not prohibited either.

Back to the books for, Starbear. :rolleyes:
Auto Brake - General
GENERAL

The purposes of this system are :

to reduce the braking distance in case of an aborted takeoff

to establish and maintain a selected deceleration rate during landing, thereby improving passenger comfort and reducing crew workload.

System Arming

The system arms when the crew presses the LO, MED, or MAX pushbutton switch if:

Green pressure is available.

The anti-skid system has electric power.

There is no failure in the braking system.

At least one ADIRS is functioning.
Note

Auto brake may be armed with the parking brake on.

MAX autobrake mode cannot be armed in flight.

Tee Emm
30th Jun 2013, 14:49
Since when do the people down the back start questioning the procedures employed by the people up the front?

Presumably the same people who are encouraged to report to the flight attendants if they see anything that could affect the safety of the flight. Flight crew call it CRM

Capn Bloggs
30th Jun 2013, 15:38
Since when do the people down the back start questioning the procedures employed by the people up the front?
When the questioner is a former 747, 727 and 737 captain, as stated by the OP.

porch monkey
1st Jul 2013, 12:21
Don't know about the A320. On the 737, use of the optional carbon brakes will nearly always result in heavier application of brakes, part of the regime for efficient use.

ANCPER
1st Jul 2013, 14:08
Make sure you nail the app spd and landing point, 6000' is 1828m, Scheduled LD at 64.5 (mlw) with 10kts TW 1702m.

I doubt I've known a 320 driver who would choose to do that, doable? Yes, but not by choice!

Metro man
1st Jul 2013, 15:36
We operate into one runway that's less than 2000m, Captains only landing and usually has no tailwind. Full flap, auto brake medium, and max reverse will give a decent arrival provided, as you said, approach speed and touch down point are nailed.

2500m with 10kts tailwind is allowable for our F/Os and can get a bit interesting if they float a bit.;)

waren9
1st Jul 2013, 15:49
Scheduled LD at 64.5 (mlw)

whose airbuses are those? jetstars are 66?

DeltaT
2nd Jul 2013, 08:34
With Airbus Autobrake application based upon wheel deceleration turn rate, if you landed and had no reverse thrust, would you expect to stop at the same point on the runway due to autobrake alone, compared to if you had reverse thrust and autobrake together?

compressor stall
2nd Jul 2013, 09:52
wheel deceleration turn rate

I think you'll find the brake system commands a deceleration rate of the aircraft measured in m/s^2, not RPM of the wheel.

DeltaT
2nd Jul 2013, 11:10
ok, however it measures the decel, where will it stop?

Found this from FCOM which is what I was thinking of, perhaps more part of the antiskid system:
The speed of each main gear wheel (given by a tachometer) is compared with
the aircraft speed (reference speed). When the speed of a wheel decreases below 0.87 time [sic] reference speed, brake release orders are given to maintain the wheel slip at that value (best braking efficiency).

Not hard to convert RPM to decel rate when you know the circumference of the wheel by the way! :}

Metro man
2nd Jul 2013, 12:01
A "DECAL" annunciator on the auto brake switch illuminates when 80% of the commanded deceleration rate is achieved. If landing on a slippery runway it may not illuminate at all as the desired rate may not be achieved even though the aircraft is slowing down.

If you landed with full reverse and max manual braking you would get the best stopping performance as the deceleration rate wouldn't be limited and anti skid would prevent wheel lock up. An aircraft lands in the shortest distance with the wheels on the runway and brakes applied, not floating along above the ground with the speed slowly decaying.

However this should never be needed as landing performance requirements need to be met. Once you are on jet airliners the rules are very strict. Use the correct technique and stay with the aircrafts certification limits and you shouldn't have any dramas. No one will be impressed with your skill if you manage to squeeze into an airfield with less than the required runway length unless it is a dire emergency, e.g. uncontained fire and need to get on the ground ASAP.

ANCPER
2nd Jul 2013, 14:02
If at your LW and speed the brakes can meet the selected decel rate using RT will only result in the brakes backing off. So in that situation it won't matter if you use RT or not, you'll stop at the same point, theoretically speaking as nothing is ever the same!

waren9
3rd Jul 2013, 02:55
hi wally, yes aware of that.

who runs 320s in oz with that lower ldg wt?

Lancair70
3rd Jul 2013, 08:13
Ive pax'd in and out of YBNA on JQ flights a few times. The braking on landing in Ballina is very noticeably MUCH harder than arriving at YMML or YSSY and is even more noticeable arriving on 24 as they seem to be hell bent on making the turnoff rather than rolling a few hundred meters further and turning around. As is the acceleration on Departure. I always figured it was due to shorter runway length at YBNA.

*Lancer*
3rd Jul 2013, 12:56
A320s have 3 settings: low is -1.7mss, med is -3.0mss. You can't select max in flight.

Sometimes low isn't quite enough, depending on runway config / touchdown point / weight / wind etc, so it's a balance between starting with medium and backing off, or starting with low and increasing.

Boeings have 5 settings, plus RTO (max).

compressor stall
4th Jul 2013, 04:05
They activate after nose wheel touchdown, so there's a chance it feels more severe depending on when that actually happens.

On my A32F aircraft the auto brake begins 4 or 2 seconds for LOW or MED respectively after the ground spoilers activate which is when both mains are on the ground... Lowering the nose has nought to do with it.

WhyByFlier
4th Jul 2013, 13:06
The pilots on this flight were quite possibly trying to make an early exit from the runway and were overly optimistic / floated! If the next vacate point was 1000m further on then it could be 2km+ of taxiing they could've saved. It has no relevance that the aircraft was an A320.

I've done this with the A319 at Berlin (SXF) before - there's an exit point at about 950 m and another at 2500 m. We did a short field landing- firm touch, full reverse and man braking and made it! If we'd missed it it would've been a long, slow taxi down the runway!

There's are captains that get very upset about firm braking in case it upsets the passengers - as far as I'm concerned good, firm braking should be reassuring to them!

As for your colleague/ friend - as a regular commuter on aircraft I know it's easy for a pilot to assume, calculate and assess from the back and difficult to let go and not compare against how you may have done! But your experienced colleague's observations sound spurious and subjective. There are as many different styles and ways as there are pilots.

The autobrake on the A320 is as sweet as a nut.

With Airbus Autobrake application based upon wheel deceleration turn rate, if you landed and had no reverse thrust, would you expect to stop at the same point on the runway due to autobrake alone, compared to if you had reverse thrust and autobrake together?

Full reverse will give an immediate deceleration. Low auto brake kicks in 4 secs after touch down and MED after 2. The auto brake modes are designed to give a set deceleration rate. By using full reverse during auto brake application on a dry runway you're just wasting fuel and making noise. I go full reverse and then to idle once the auto braking has begun on a short landing. In the QRH CONF FULL landing distance chart for a DRY runway they offer -10 m per operative reverser with manual and LOW auto brake and -0 m with MED auto brake. Clearly with degraded braking action the reverse thrust has more effect.

Centaurus
4th Jul 2013, 15:24
I go full reverse and then to idle once the auto braking has begun on a short landing

I don't know about the Airbus series but common sense would dictate it would be most unwise to use this technique on a damp/wet runway. And the brakes must surely get hot with only reverse idle.

I recall the 737 FCTM warning that "the use of minimum reverse thrust as compared to maximum reverse thrust can double the brake energy requirements and result in brake temperatures much higher than normal" Presumably that principle applies equally to the Airbus as to a Boeing?

Is there any danger to ground personnel if you park at the Terminal with hot brakes? From all that have read over many years hot brakes are not desirable especially as the heat affects the tyres which in turn will degrade tyre reliability. The next crew to fly the aircraft might cop the problem if a tyre decides to fail due excessive heat.

Isn't good airmanship a lovely term? Don't hear much of it nowadays:ok:

ANCPER
4th Jul 2013, 15:48
Your last sentence was somewhat unfair if directed at WBF. Wasn't QF doing no reverse landings on their 744s sometime back until Bangkok? I think your problem is your time on the B733 without brake temp indication, which if the same as the 300s I flew also had steel brakes.

Carbon brakes work better when hotter (to a point) and most 320s these days have brake fans. I've not heard of tyre failure being an issue due to brake temps and there are also brake temp limits for T/O and also max limits for maintenance.

I guess you could say that's where the airmanship/sops and common sense come in to play to achieve the best result!

WhyByFlier
4th Jul 2013, 20:28
Centaurus thank you for your comments.

I don't know about the Airbus series but common sense would dictate it would be most unwise to use this technique on a damp/wet runway.

Given I described a DRY scenario as shown by the:

In the QRH CONF FULL landing distance chart for a DRY runway they offer -10 m per operative reverser with manual and LOW auto brake and -0 m with MED auto brake. Clearly with degraded braking action the reverse thrust has more effect.

I don't think you're being terribly fair but perhaps that's just how I've read your post. For the record - our dispatch landing performance distance calculations consider ALL REVERSERS INOPERATIVE for a landing on WET runways. (DAMP is considered DRY from a performance point of view which also considers them inoperative).

I do agree that if performance is limiting, turn around times are tight, a strong TW is present, mass is high or if in any doubt then FULL reverse thrust should be used immediately on touch down to make best use of their effect.

From a brake wear point of view with carbon brakes a hotter temp (+275 degrees) will cause significantly less wear than a warm brake with peak wear on A320 brakes at around 150-250 degrees - max at 200 degrees. Full reverse all the time - when it's not needed - burns fuel, increases the risk of FOD ingestion potentially damaging blades and increases brake wear.

compressor stall
4th Jul 2013, 23:50
In the QRH CONF FULL landing distance chart for a DRY runway they offer -10 m per operative reverser with manual and LOW auto brake and -0 m with MED auto brake. Clearly with degraded braking action the reverse thrust has more effect.

Thanks WBF for the final bit of the answer to DeltaT's question from the last page. It's an oft asked question, but the truth is cant be answered accurately unless you know the

Runway condition
Flap config
Autobrake mode, and
Engine type


On my A32F aircraft with both reversers operative, the decrease in stopping distances are:

CFM
RWY DRY, A/BRK LO, CONF FULL: -20m
RWY DRY, A/BRK MED, CONF FULL: 0m
RWY DRY LO CONF3: -20m
RWY DRY MED CONF3: 0m

RWY MEDIUM A/BRK LO CONF FULL: -100m
RWY MEDIUM A/BRK MED CONF FULL: -200m
RWY MEDIUM A/BRK LO CONF3: -140m
RWY MEDIUM A/BRK MED CONF3: -240m

IAE
DRY LO FULL: 0m
DRY MED FULL: 0m
DRY LO CONF3: -20m
DRY MED CONF3: 0m

MEDIUM LO FULL: -100m
MEDIUM MED FULL: -240m
MEDIUM LO CONF3: -140m
MEDIUM MED CONF3: -300m

The fundamental intended point that the Autobrake will back off with reverse engaged stands as the reverse is taking up some of the deceleration rate that's commanded by the aircraft.

However, being pedantic, the Autobrake on LOW takes a few seconds to kick in so the reverse (CFM FULL, CFM &IAE CONF 3) has an, albeit brief, effect during this time. This also shows how the flap config has an effect here too with reversers operative when the runway condition deteriorates.

Which brings us neatly to the fact that the intent of the original question is only valid for DRY runways. Something that is usually omitted when asking it. The tyres' braking action can achieve the commanded rate on dry runways. Anything less than DRY and then the reverse kicks in to make up the shortfall.

As an aside, for MAX A/BRK for the RTO the commanded decel rate is 6m/s^2 which is more than the tyres could ever do in any condition, hence the assistance of MAX REV for the RTO in reducing the stopping distance.

Trent 972
5th Jul 2013, 00:27
Also remembering that the Airbus Landing charts make no allowance for reverse thrust on a dry runway but do (reverse) on a wet runway.

Lookleft
5th Jul 2013, 10:54
Centaurus the Airbus also has brake fans specifically for the hot brake you are concerned with. There are also brake temperature limits for take-off depending on the fans being on or off. The 737 gets around this by having the u/c exposed to the airflow. The OM for the airline in question specifies full reverse for wet/damp runways.