Autobrake Disengage Pilot Pressure (Question for Real Pilots)

[John H Watson]

A real pilot on a 737NG has stated that the pressure on the pedals required to disengage the Autobrake System is proportional to the pressure selected. There seems to be nothing in the 737NG or 744 A/B system which suggests this operation (although it would make sense to me to design it like this. At high braking pressures, you don't want the system to suddenly go to a lower pressure). Is this how it feels on the 744?

Thanks in advance.

(EDIT:  I don't know what the 500psi pilot input disengage pressure equates to in terms of A/B levels. If it equates to medium levels and the A/B system is at position 1, you're going to be overriding the autobrake shuttle valve physically before the 750psi is reached (but not overriding the A/B system electronically... i.e disengaging it).

[Britjet]

No idea, but I wouldn't have thought so.
I know that the pressure required to cancel the RTO autobrake application is nothing like the amount of braking being applied by the system at the time.
Also - cancelling Autobrakes with a dab on the pedals at the late stage of a rollout invariably causes a slight jolt, so again, it doesn't match the braking being applied.
Peter

[John H Watson]

Interesting. Thanks!

[IefCooreman]

On a 737, disengaging the AB with the pedals is a 'skill to be learned'  :-) Very hard to say, but I would say it disengages at equal or more brake pressure. In training you will see a lot of people looking for 'the disengage with slight pressure' which does nothing while at the same time the exit is approaching quickly. So all of a sudden they'will 'kick' it which feels like somebody stepped on the parking brake, it can be pretty rough.

I guess many will feel you always need more pressure than what the AB system applies. Many will therefore also request the PM to put the switch off manually. While aiming for'a'high speed taxiway, I'm not a fan of the switching off by the PM. If you put your feet on the pedals and slowly push them forward as if you were to 'brake to vacate' the AB will disengage without noticing when the pressures (applied by pilot vs AB) are equal.

[John H Watson]

Thanks, Ief. It does seem that the 737NG braking system is a little different.

[Magoo]

From experience yes, it is harder to dis-engage A/B 4 than A/B 2. You do need more pressure for higher settings.

[John H Watson]

From experience yes, it is harder to dis-engage A/B 4 than A/B 2. You do need more pressure for higher settings.

If this is the case, then there is something we have overlooked.

For Level 1~Max, the A/B system override is a fixed pressure value of 500 psi (airplane logic). The pressure is sensed by a switch fitted to a 3 way shuttle valve which determines which pressure reaches the brakes (the pedal-produced hydraulic pressure or the A/B-produced pressure). The disarm pressure switch is definitely on pilot/pedal input side of the shuttle valve.



All pressures on the A/B side of the shuttle valve are greater than 500 psi. I don't know if antiskid operation affects the pressures on the lower port of the shuttle valve.



What I had not realised or forgotten is that RTO requires pedal position switches (not pressure).  I don't know if this is modelled in PSX. RTO requires pedal travel of 12 degrees (full travel is 17 degrees). Also, I don't know if 500 psi corresponds to specific pedal angle in PSX.

[Hardy Heinlin]

In PSX:

RTO disengages when either pedal is 70% depressed, which corresponds to 12° of 17° total.

Autobrake disengages when either pedal is 20% depressed, which corresponds to 600 psi of 3000 psi total.

From experience yes, it is harder to dis-engage A/B 4 than A/B 2. You do need more pressure for higher settings.

Assuming this experience is a tactile feeling, could it be that this feeling corresponds to the pilot's inertia which pushes the human's spine away from seat-back during deceleration? The greater the deceleration, the greater the feel of flying-away-from-the-seat and the feel of need-more-body-expansion-between-seat-and-pedal.


|-|ardy

[b744erf]

Quote
From experience yes, it is harder to dis-engage A/B 4 than A/B 2. You do need more pressure for higher settings.

I am very lucky to fly a Level D 787 sim yesterday. And when I rolling on the rwy after landing, my instructor taught  me to press the brake pedal may disengage the auto brake.  As my experience in PSX, I may just need to lightly press the pedal. But instructor ask me to press a deeper pedal position, which must be more then the current a/b setting. Otherwise, A/B will not be released. And before my last playing around flight, we did some test for the a/b disengage logic:

1. A deeper braking pedal it need to disengage the a/b when it was set at 3 then 1.

2. If we press the same pedal angle as to disengage a/b 1 when setting a/b 3, the a/b will not disengage.

To make sure we don't affected by the different deceleration feeling during press the braking pedal, we switched  off the moving platform during the test.

Instructor said, 737 share the same logic as this. But he had never flight 747, he can't tell the 747 logic.

I hope this is useful. And maybe not.m, since it is not the same type. Or any one could have chance to test this on 747?

Regards
Jack.

[John H Watson]

Thanks, Jack et al...

We just have to understand the physics behind this.

With so much pressure going to the brakes at high Autobrake settings, perhaps there is less fluid flow/pressure available for the manual brakes, so you need to push the pedals harder to achieve the 500psi required to trip the electrical disconnect switch. Once the system is electrically disarmed and the A/B valves close, the manual system has plenty of pressure to move the shuttle valve out of its detent.

[Hardy Heinlin]

Looking at your illustration: When the autobrake input can really increase the pressure in the "green field", wouldn't this additional pressure push that switch even closer to its trigger point, so that even less pedal input is required to disengage the A/B?

Thank you for the sim test, Jack. Good idea to deactivate the motion platform for this test :-)


|-|ardy

[John H Watson]

When the autobrake input can really increase the pressure in the "green field", wouldn't this additional pressure push that switch even closer to its trigger point, so that even less pedal input is required to disengage the A/B?

Not sure. Perhaps the shuttle valve has endstops? Once the endstop has been reached, perhaps the only pressure in the green area can be brake pedal supplied pressure.

(EDIT: The manual text mentions detents. i.e. the shuttle valve has two positions. A 30~70psi pressure differential will change the position from one detent to another).

[fsam]

in 737NG the AB will disarm if normal brake pressure is more than 750 PSI, done by autobrake shuttle valve metered pressure switch with input to AACU. Best regards Ralf

[John H Watson]

in 737NG the AB will disarm if normal brake pressure is more than 750 PSI, done by autobrake shuttle valve metered pressure switch with input to AACU. Best regards Ralf

Understood. The operation is similar to the 744, but real world pilots are saying that the pedal pressure required to disarm the autobrakes varies when the settings are higher.

[fsam]

the disconnect triggerpoint 750 PSI is the same for the AB Selector positions, you need more normal metered pressure to override the highest AB pressure within the shuttle valves. Best regards Ralf

[John H Watson]

the disconnect triggerpoint 750 PSI is the same for the AB Selector positions, you need more normal metered pressure to override the highest AB pressure within the shuttle valves.

As you can see from the diagram, the pressure sensor is in the cavity where the brake pedal metered pressure is. It's not reading A/B pressure. If the switch didn't work, then you would need a greater pedal pressure to override the A/B pressure.

[fsam]

in my understanding you need in every case a higher pedal press than AB pressure to override AB by the shuttlevalves, so depending upon AB selector setting you need more and more pedal pressure for overriding, but the AB disconnect point is at 750 PSI, of course at the pedal pressure side. Best regards Ralf

[Hardy Heinlin]

So we have two different pressure references, Ralf?

One reference is the variable pedal pressure ("you need more and more pedal pressure for overriding").

The other reference is that fixed 750 psi ("but the AB disconnect point is at 750 PSI").

I don't understand how these two references relate to each other mechanically?


Cheers,

|-|ardy

[fsam]

above speaking for the 737NG we have the AB pressure with the preselection from the AB selector and regulated by the AB Pressure Control Module which then is splitted into the two AB Shuttle Valves and on the other hand you have the normal (manual) brake pressure by pedals regulated via the Brake Metering Valves also feeded to the AB Shuttle Valves. If the pedal input / Brake Metering Valve pressure output raises above the AB Pressure Control Module output pressure the Shuttle Valves giving the pedal input pressure to the brakes but at 750 PSI brake metering valve pressure output the AB disarms via the pressure switch input to the ACCU.

In short - AB press is reulated by AB Pressure Control Module and Pedal Pressure is regulated by the Brake Metereing Valves. The shuttle valves decide which is higher and disarm AB if 750 PSI is reached by pedal input. So it seems to be normal that you have to use more pedal pressure to override the higher AB selection and a disarm at lower AB setting is more abrupt.

I think in B744 it is more complicated since during RTO (only then) the brake pedal switches disarm AB if pedals pushed more than 1/3 distance and during landing roll with AB selector in Landing Mode the 750 PSI switches as well as an additional 750 PSI switch at the AB Control Valve Module came into the game (within the shuttle valves with the same function as B737NG) the additional 750 PSI switch disconnects AB if output is less than 750 PSI and the preselcted rate of deceleration is not reached...

Best regards Ralf

[John H Watson]

In short - AB press is reulated by AB Pressure Control Module and Pedal Pressure is regulated by the Brake Metereing Valves. The shuttle valves decide which is higher and disarm AB if 750 PSI is reached by pedal input. So it seems to be normal that you have to use more pedal pressure to override the higher AB selection and a disarm at lower AB setting is more abrupt.

With no pressure on the pedals, the A/B shuttle valve will be fully pushed over into one of the detents during autobrake operation. The 737NG autobrake pressure on level 1 is around 1285psi. On Max, the pressure is around 3000 psi.  Any pressure on the brake pedals would have to be greater than 1285psi to move the shuttle valve out of the detent. 750psi will be reached before the shuttle valve moves out of its detent. The same pressure should disengage the Autobrakes.

NG shuttle valves appear to be identical to the 744 valves.

So it seems to be normal that you have to use more pedal pressure to override the higher AB selection and a disarm at lower AB setting is more abrupt.

How is the lower AB setting more abrupt? If you pushed on the pedals with a force of, say, 751psi, the brake pressure will go from 3000psi (at Max) to 751psi.... the largest pressure differential.

Basically, the only differences (in this area) between the two aircraft are the disable pressures and the 744 RTO pedal switch.