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Braking deceleration of Airbus A320

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Braking deceleration of Airbus A320

Old 15th Jun 2019, 07:01
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iat
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Braking deceleration of Airbus A320

Hi, I have an assignment in which I need to find the time and distance it takes for an A320 to brake from 30 knots to a full stop.

To do so I need to know the braking deceleration during taxi.

I have only been able to find the deceleration values for autobrake i.e.

LO - 2m/s2
MED - 3m/s2
MAX - 6m/s2

Am I right to say that autobrakes are off during taxi? What are the deceleration values of manual braking during taxi?

Thanks in advance.
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Old 15th Jun 2019, 11:30
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We'll leave this query here for the time being. If it doesn't get much in the way of useful responses, we might look at moving it to Questions.

A difficult question as, for maximum braking, you would put everyone through the next seatback and for the range from nil to max braking is pretty broad.

Possibly you might do better with your assignment running the sums for the autobrake settings and qualifying your answers by reference to those values as examples within the available range. The other problem with the question is that, while the autobrake generally is set up to maintain a constant decel until the aircraft is stopped, the human pilot will modulate brake pressure throughout the stop with an associated varying decel.
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Old 15th Jun 2019, 18:33
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Something of a strange assignment iat!!

Taxying out - after a certain stage Autobrake MAX will be selected for the RTO case but it is not available until 72kts IRS GS.

Taxying in - the PF will have disconnected Autobrake in order to be able to taxi in - otherwise it will bring you to a halt on the RW!!

I am not aware of how you would calculate the decel rate from 30kts to a standstill with max manual braking in an A320. Experience tells me that it would be almost instantaneous as JT suggests. - an unpopular move!!

I would be interested to know the context of the task you have been set.

Cheers

Mcdhu
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Old 15th Jun 2019, 18:55
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Autobrakes are used for stopping at high speeds. At 30kts it would be down to the pressure being manually put on the brakes pedals by the pilot and I personally try to be smooth and not excessive. Obviously if you were trying to avoid a collision you would use full braking.Below 20kt the antiskid is deactivated.

Last edited by tubby linton; 15th Jun 2019 at 19:47.
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Old 15th Jun 2019, 19:02
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I guess it will also be contingent of the load of the plane and the tarmac conditions. I don't think you will get antiskid active at such low speed and in any case it will be a fairly quick stop...
As others have mentioned more context would be interesting.
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Old 15th Jun 2019, 19:32
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Yes, the auto brakes are off during taxi. Otherwise, they would be constantly slowing down the A/C, negating the objective of taxying.

Towards the end of the landing roll, the ABRK would be disengaged by the pilot at low speed, in order to prevent a locked wheel situation once the anti-skid (a.k.a ABS) deactivates (technical low-speed threshold for that). Actually 30 knots is about the right moment, after which you'd assume normal pedal braking.

The answer to your question is pretty useless: the distance depends on how hard you press on the brake pedals, and what is the surface friction available. If I pressed the pedals to the metal, the brakes lock giving max effectiveness on a dry surface, and we'd skid to a stop inside 30 meters. Possibly even sooner - and trash all the 4 tyres.
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Old 15th Jun 2019, 19:56
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Originally Posted by iat View Post
Hi, I have an assignment in which I need to find the time and distance it takes for an A320 to brake from 30 knots to a full stop.

To do so I need to know the braking deceleration during taxi.

I have only been able to find the deceleration values for autobrake i.e.

LO - 2m/s2
MED - 3m/s2
MAX - 6m/s2

Am I right to say that autobrakes are off during taxi? What are the deceleration values of manual braking during taxi?

Thanks in advance.
MAX and manual should give the same values (5m/s/s in my FCOM.) MAX canít be used for landing, itís only for an RTO, but the point of MAX is that it is maximum braking. You donít do an RTO using less than full brake pressure. However, the published deceleration rate is presumably an average for a full stop from high speed, ie either landing or a high speed RTO, and might not tell you much about stopping from 30 knots.
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Old 15th Jun 2019, 21:04
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You can use max for landing, it is just not recommended.
And it cannot be armed in flight (any more).
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Old 15th Jun 2019, 22:12
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Some other pointers/ corrections /additions to above. (Caveat these are A319 figures but all should be the same).
The MAX decelerations setting commands 6m/^2 deceleration rate but wont actually achieve this as itís greater than whatís the aircraft can do. (The anti skid on the day will determine deceleration rate)
Low starts after 4 seconds after ground spoiler extension. The rate is 1.7m/s^2
MED after 2 seconds @3m/2^2.

There is an option available now to reduce the time delay on LOW to 2 sec.

none of this is relevant for taxi but it might help you refine your question if you need.
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Old 15th Jun 2019, 22:46
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Originally Posted by safelife View Post
You can use max for landing, it is just not recommended.
And it cannot be armed in flight (any more).
If you canít arm it, it wonít activate. Or are you talking about using max manual braking?
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Old 16th Jun 2019, 00:18
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The same distance it takes a a truck to slow from 30mph...
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Old 16th Jun 2019, 03:32
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Originally Posted by Check Airman View Post
The same distance it takes a truck to slow from 30mph...
+10 And if the professor gives you A on that one, you know you chose the school right! https://www.albany.edu/faculty/miesi...sess/hell.html
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Old 16th Jun 2019, 06:02
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Originally Posted by AerocatS2A View Post
If you canít arm it, it wonít activate. Or are you talking about using max manual braking?
Try pressing max on your next landing.
I said, it cannot be armed in flight.
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Old 16th Jun 2019, 06:45
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iat, which course is being taken?
If a soft science, logic, then ask Airbus.
If a hard science, conduct a test and double integrate longitudinal deceleration. The latter being a technique used in fdr analysis to establish distance, or conversely braking action vs applied braking.
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Old 16th Jun 2019, 08:08
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Hi, I have an assignment in which I need to find the time and distance it takes for an A320 to brake from 30 knots to a full stop.
Why not use the appropriate equations of motion or a calculator such as Equations of Motion Calculator and simply insert your initial speed (30 kts), final speed (0) and choose sample deceleration rates (say 2 m/sec/sec.)? e.g. Answer = 59.55m, 7.7 secs.
At 6 m/sec/sec Ans = 19.8m, 2.6 secs etc.

Last edited by Goldenrivett; 16th Jun 2019 at 10:38. Reason: extra text
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Old 16th Jun 2019, 09:58
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Originally Posted by safelife View Post
Try pressing max on your next landing.
I said, it cannot be armed in flight.
Yes I know that. You said max can be used for landing but it canít be armed in flight. How are you proposing to use it without arming it?
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Old 16th Jun 2019, 12:50
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Originally Posted by iat View Post
Hi, I have an assignment in which I need to find the time and distance it takes for an A320 to brake from 30 knots to a full stop.

To do so I need to know the braking deceleration during taxi.

I have only been able to find the deceleration values for autobrake i.e.

LO - 2m/s2
MED - 3m/s2
MAX - 6m/s2

Am I right to say that autobrakes are off during taxi? What are the deceleration values of manual braking during taxi?

Thanks in advance.
In any academic assessment, exam or assignment, it is important to ask yourself "What am I being asked to demonstrate that I have learnt here?"

Is this a basic physics unit which teaches you to calculate time and distance to decelerate to a standstill from a known velocity given a known acceleration?
Or is it a unit which teaches you to track down typical performance parameters such as the likely deceleration of an A320 at taxiing speeds?
Or is it perhaps intended simply to get you to calculate an estimate (with no fixed "correct answer") to make sure that you thoroughly grasp and remember how much space is required to stop?

Once you've answered this, you can then set about providing the type of answer required.
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Old 20th Jun 2019, 22:10
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Originally Posted by AerocatS2A View Post
Yes I know that. You said max can be used for landing but it canít be armed in flight. How are you proposing to use it without arming it?
Actually you can Arm Max inflight but depends on MSN. Earlier ones certainly could as a colleague did arm it by mistake and it stopped very quickly. Certainly later MSNs donít let you Arm it.
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Old 21st Jun 2019, 19:00
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Originally Posted by AerocatS2A View Post
Yes I know that. You said max can be used for landing but it canít be armed in flight. How are you proposing to use it without arming it?
Arm MED in flight and select MAX during rollout?
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Old 21st Jun 2019, 20:01
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Originally Posted by 411A NG View Post


Arm MED in flight and select MAX during rollout?
No, on the old MSN a/c and indeed older sims, you can arm MAX just like MED in flight.. This is what my colleague did, He thought he was selecting MED during brief in flt but accidentally selected MAX. A/c stopped very quickly. In sim (old one) you can still do that - i train trainers and highlight the problem and tell them to check when trainee arms autobrake - of course it depends on which airline they are going to and which a/c they have as to whether to discuss it or not, but many a/c (usually the CRT ones) still let you.
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