Question on A320 Autobrakes selection v runway length

[waren9]

Scheduled LD at 64.5 (mlw)

whose airbuses are those? jetstars are 66?

[Wally Mk2]

There are options like most A/C for MLW & MTOW, depends on factory specs at time of ordering.

64.5T & 66T are the two most common numbers about with A320's

Wmk2

[DeltaT]

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]

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]

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]

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]

DeltaT

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]

hi wally, yes aware of that.

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

[Lancair70]

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*]

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]

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]

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]

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

[ANCPER]

Centuarus

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]

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]

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

1. Runway condition
2. Flap config
3. Autobrake mode, and
4. 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]

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]

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.