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Sekkha

Deceleration rate provided by Autobrake

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Apologize if that has been discussed already some time ago..

As far as I understand Autobrake provides for a certain deceleration rate taking into account the usage of reverses. That means that the deceleration rate will be the same with or without reversers, the reversers thus mainly helping to reduce wear on brakes ... At least on a dry runway with normal braking action I suppose

Am I right that this is currently not modelled in the PMDG 744?  (At least some tests I just did suggest that as the stopway with reversers using the same autobrake level is significantly shorter than without reversers) 

 I can imagine that to be something quite complex in the given P3D environment though...

 

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I‘ve never checked this, I typically always land with idle reverse, but you have to do that on a landing rollout.

If you did it during an RTO the autobrake system would apply full pressure. 

I don‘t think that PMDG wouldn‘t model this due to its complexity. They have done much complexer systems and since they bypassed the sim’s default brake system this should be done I suppose. 


,

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If you check the landing distance tables there is a bit of variation in Autobrake landing distance with and without reversers. Particularly at low Autobrake settings it is entirely possible that the drag from spoilers/reversers etc will be greater that the AB1 deceleration rate in the early part of the landing roll, thus resulting in a shorter overall landing distance.

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Simon Kelsey

sig_FSLBetaTester.jpg

 

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Sekkha,

I hope this will bring some light to your concern:

Automatic Brakes
Use of the autobrake system is recommended whenever the runway is limited,
when using higher than normal approach speeds, landing on slippery runways, or
landing in a crosswind.
For normal operation of the autobrake system select a deceleration setting.
Settings include:
• MAX AUTO: Used when minimum stopping distance is required.
Deceleration rate is less than that produced by full manual braking
• 3 or 4: Should be used for wet or slippery runways or when landing
rollout distance is limited
• 1 or 2: These settings provide a moderate deceleration suitable for all
routine operations.
Experience with various runway conditions and the related airplane handling
characteristics provide initial guidance for the level of deceleration to be selected.
Immediate initiation of reverse thrust at main gear touchdown and full reverse
thrust allow the autobrake system to reduce brake pressure to the minimum level.
Since the autobrake system senses deceleration and modulates brake pressure
accordingly, the proper application of reverse thrust results in reduced braking for
a large portion of the landing roll.
The importance of establishing the desired reverse thrust level as soon as possible
after touchdown cannot be overemphasized. This minimizes brake temperatures
and tire and brake wear and reduces stopping distance on very slippery runways.
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.
After touchdown, crewmembers should be alert for autobrake disengagement
annunciations. The PM should notify the PF anytime the autobrakes disengage.
If stopping distance is not assured with autobrakes engaged, the PF should
immediately apply manual braking sufficient to assure deceleration to a safe taxi
speed within the remaining runway.

 

Basically reversers to a certain point modulate the AB pressure to a certain point, unfortunately not enough space and time to elaborate.

And yes this function was developed accordingly within the flight simulator capabilities. 

Edited by killthespam

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1 hour ago, killthespam said:

For normal operation of the autobrake system select a deceleration setting.
Settings include:
• MAX AUTO: Used when minimum stopping distance is required.
Deceleration rate is less than that produced by full manual braking
• 3 or 4: Should be used for wet or slippery runways or when landing
rollout distance is limited
• 1 or 2: These settings provide a moderate deceleration suitable for all
routine operations.

I'm going to push back on this.... do you have a source or reference?

Regarding thrust reverse with autobraking:  This was a hot topic for about a week during beta and resulted in careful measurements and adjustments to autobraking.  PMDG has the autobraking deceleration rates down to within points of a percentage from Boeing engineering documents.  The problem is thrust reverse.  The simulator platform does not realistically model the effects of reverse thrust.  Realistically, the effects should be greatest at landing speed and rapidly diminish with speed but the simulator treats reverse thrust at 160 the same as at 60 kts.

To keep things real, I always remove the reverse thrust between 60-80 kts and use only autobraking below 60 kts.  You'll find that if you keep reverse thrust on all the way to taxi speeds your aircraft is acting more like a C-130 during a tactical landing that a Queen.


Dan Downs KCRP

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1 hour ago, downscc said:

  The problem is thrust reverse.  The simulator platform does not realistically model the effects of reverse thrust.  Realistically, the effects should be greatest at landing speed and rapidly diminish with speed but the simulator treats reverse thrust at 160 the same as at 60 kts.

Thanks Dan , I know that reverse thrust is most effective at high speeds . So the overall result is not fully realistic due to sim limitations. Thats exactly what I thought...

 

6 hours ago, Ephedrin said:

 but you have to do that on a landing rollout.

If you did it during an RTO the autobrake system would apply full pressure. 

of course I refer to landing rollout with AB 1-3

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2 hours ago, downscc said:

 

To keep things real, I always remove the reverse thrust between 60-80 kts

That is 100% correct, SOP is at 80 KTS PNF will call "80 KNOTS" and PF starts getting easy out of reverse slowly to be @ 60 kts idle reverse or stow them based on conditions. 

Re:

For normal operation of the autobrake system select a deceleration setting.
Settings include:
• MAX AUTO: Used when minimum stopping distance is required.
Deceleration rate is less than that produced by full manual braking
• 3 or 4: Should be used for wet or slippery runways or when landing
rollout distance is limited
• 1 or 2: These settings provide a moderate deceleration suitable for all
routine operations.

The above statement it's a quote from the Boeing training manual which you not aware of it?

And by  the way internally during development that was available for everybody as a reference.

 


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30 minutes ago, killthespam said:

The above statement it's a quote from the Boeing training manual which you not aware of it?

I never had access and still do not have a copy of the FCTM for the B747.  The use of 1 or 2 for all routine operations is counter to what I've heard from operators (I'm not rated in these types), whom shared that the use of 1 or 2 with reverse thrust may cause the braking to modulate and this is not a good thing for ceramic brakes.  Autobraking 3 will ensure that the brakes do not release during reverse thrust application at high speed.


Dan Downs KCRP

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1 is a well known problem for 400 and -8, will modulate pretty hard and during real ops we don't use it.

All the operators train pilots not to use 1 (just for the grabbing issue) , 2 and 3 is the most used or whatever you get from performance (ACARS) data. 

Unfortunately there is lots of wrong information around.

As for the other settings for the brakes  to release during reverse thrust at high speed it's only if you do it or, a malfunction.


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