Autobrake Manual Activation After Landing

[emerydc8]

Has anyone investigated whether the autobrakes would work if the landing was done with the autobrakes off and then the switch was moved to say autobrakes 2 during the rollout?

I can't seem to find anything on this except the old United Airlines' Doc Donaldson video where he warns not to move the autobrake switch if the autobrakes were off for landing. Not sure what it would do. I intend to try it in the airplane but just curious what to expect in advance if someone has already investigated this question. Thanks.

Jon

[Britjet]

I'm in the BA sim with Gary on Saturday, Jon. Will give it a try.
Peter

[emerydc8]

Thanks Peter.

Jon

[John H Watson]

Trying to read between the lines in my notes and the schematics....

I think it would be quite uncomfortable (if not risky) in real life. On the ground, the switch would have to be positioned quickly to the desired rate. If you moved the switch slowly between the settings, it would be seen as Autobrakes 1..  no input... Autobrakes 2.. no input... etc... No input may equate to OFF, so you might get very jerky operation (especially considering the A/B system applies slightly stronger initial braking to let the pilots know the system is working).

I don't think the autobrakes would take too long to activate, but the logic to latch the A/B switch on the ground until full stop (and after) requires the average velocity of the 4th, 5th, 6th and 7th fastest of the 8 forward wheels to exceed 60kts and stays above 30kts for 3 seconds.

Applying the brakes manually and holding the A/B switch in position stops the application of autobrakes (not just trips off of the switch).

[emerydc8]

I would have agreed with you, John, but a buddy of mine who was a classic captain (now on the -400 at UPS) said he used to use this technique every time he landed in KDH in the summer at max landing weight (usually more) to keep from thermaling tires. He would use reverse at high speed and then wouldn't have the FE turn the autobrakes from off to min until he got down to about 100 knots because the autobrakes did a better job of applying the brakes than manual braking. It was especially an issue there because it took what seemed to be an eternity for the military personnel to chalk us once we blocked in, so we'd sit there with the brakes set and the temps approaching 1000 degrees. The mechanics would look at the hottest brake and grab an armful of bottled water to start pouring on the rim (not directly on the brakes) to get the temp down. This picture below is what happens when you do it by the book and set medium autobrakes. Definitely not something you want to see when you block in. Manual braking that last 80-100 knots is enough to push them into the red if you have a heavy foot on one side, but with autobrakes medium most of the heat probably accrues shortly after touchdown.



My buddy thinks the same technique would work on the 744 but has never found the need to try it. On the classic, if you thermaled more than two tires (that's all we carried), you got to spend some time at the wonderful Hotel Kandahar until the next flight could come through with more tires. Almost as nice as the Crowne at DXB, eh?

[John H Watson]

Interesting..

I guess the A/B applies the brakes on the ground as long as the switch is held in position until the BSCU has had time to determine that wheel spin-up is ok. Then it latches the switch. Assuming the 744 is the same, I guess as long as the system senses that wheelspeed has reached 60kts and wheelspeed is maintained above 30kts for 3 seconds (744 logic), the braking will continue below 30kts. 

I seem to remember on the 744 during A/B tests on the ground, that you had to hold the switch for a moment or two to get it to latch. Is this the same in the air? Also, if the A/B switch is moved from, say, 1 to 2 in the air and you spend too much time halfway between those settings, will the switch drop back to 1 or to DISARM? If you plan to test this in flight, I'm not sure if I'd hold it between settings for too long. It might generate an A/B message. I think the A/B system needs to see an earth signal at all switch positions. If it doesn't see any earth for a certain time period, it might fault the switch.

Brake application on the Classic depends on front wheelspeed being greater than an average of 80kts. The A/B braking (until stopped and after) latch depends on this wheel spinup and an "on ground" signal being maintained for 3 seconds. If it loses the wheel spinup signal after this, the latch is maintained. Only an "in air" signal or "disarm" signal can reset this "3 second" latch. I don't know what resets the latch on the 744.

You can change the settings during rollout, can't you? If so, the A/B system must have some tolerance to loss of brake setting position.

[emerydc8]

Also, if the A/B switch is moved from, say, 1 to 2 in the air and you spend too much time halfway between those settings, will the switch drop back to 1 or to DISARM?

On the 767 (presuming a common system) it sometimes take a momentary pause in that position (e.g., AB 2) for the switch to stay there. I can't remember if it just goes to off or disarm. I'm thinking it will just snap to off.

You can change the settings during rollout, can't you? If so, the A/B system must have some tolerance to loss of brake setting position.

Absolutely. I do it when I land AB 2 and I want to make a certain intersection but I'm not decellerating hard enough. I'll go to 3 for a few seconds then back to 2 and it is seemless.

[John H Watson]

Thanks.

I can't remember if it just goes to off or disarm. I'm thinking it will just snap to off.

If it's between DISARM and MAX, it can't go back to OFF because of the purely physical detent at DISARM.... unless perhaps the physical detent is worn away (but that would be noticed after pedal application on the ground).

So, best guess... I think the A/B switch would need to be held for a little longer on the ground than it would in the air to engage autobrakes (from OFF) because it has to read wheelspeed. From then on, the autobrakes would maintain the selected rate until selecting the new rate (as long as you didn't linger between settings too long.. forcing the switch back to DISARM). The 3 second timer won't reset at each setting, unless it accidentally went back to DISARM perhaps.

[Panos Bilios]

Hi all

I remember a video from ITTV where a Cathay 744 lands
in old Kai Tak airport and during the rollout the captain asks
the FO to turn the AB switch to 3 from 2 cannot remember if
he switched it back to 2.

Panos

[Britjet]

It's considered quite normal to back off a previously-set autobrake figure on the rollout. There is no difficulty with it - the braking change comes in immediately. It's much better to do it this way than try to override with manual braking, which tends to cause increased, jerky and possibly uneven brake use.
For info - we tried changed the autobrake setting today in the BA sim from a preset of OFF for landing to other levels during the rollout. It latches without delay and functions as selected.
Peter.

[cavaricooper]

Hi all

I remember a video from ITTV where a Cathay 744 lands
in old Kai Tak airport and during the rollout the captain asks
the FO to turn the AB switch to 3 from 2 cannot remember if
he switched it back to 2.

Panos

Panos-

Don Grange left it to disarm normally and made the turn off he wanted quite nicely. I still think of him departing Cardiff after a "C" or "D" check... "Max thrust, let's go!"

C

[emerydc8]

For info - we tried changed the autobrake setting today in the BA sim from a preset of OFF for landing to other levels during the rollout. It latches without delay and functions as selected.
Peter.

Thanks, Peter. Now I'll know what to expect when I try it in the plane.

Jon

[Jeroen Hoppenbrouwers]

For info - we tried changed the autobrake setting today in the BA sim from a preset of OFF for landing to other levels during the rollout. It latches without delay and functions as selected.

Peter, just curious: I am not exactly sure "who" is in the sim when you try these things. Are they trained pilots and you discuss these things before trying? Or are they "hapless tourists" that want to experience flight and you maneuver everything around them anyway and they don't even notice?

Hoppie

[Britjet]

Hoppie,
This particular sim was one that Gary and I and a couple of the other Simfest friends paid for specially so we could try some stuff out. I don't use my BA guests' allotted time for stuff that changes the flight in any way. It's expensive enough as it is..
For some switch response-type things - yes, I move the switch and they don't notice..
I don't do professional training any more.
Peter.

[emerydc8]

I seem to remember on the 744 during A/B tests on the ground, that you had to hold the switch for a moment or two to get it to latch. Is this the same in the air? Also, if the A/B switch is moved from, say, 1 to 2 in the air and you spend too much time halfway between those settings, will the switch drop back to 1 or to DISARM?

Presuming the 767 is the same as the 744 here, the AB switch will move to disarm on the ground if a thrust lever is moved forward when you have it set to 1-max, but if you get the switch between two settings (e.g., 1-2, 2-3), and the solenoid doesn't stick, it will fall back to the lower setting -- not to the disarm position (unless you are trying to set 1). This worked the same way in the air as far as when you are setting AB 2 and it doesn't stay (stick) at 2, it will drop back to 1. Likewise if you try to set AB 3 and it doesn't stay, it will drop to 2.

In the video, I am stopping short of the detent to get it to trip back to the previous setting. The solenoid grabs immediately so no need to hold it there momentarily in flight or on the ground to get it to stick. What happens very often is I try to set AB 2 but not quite make the switch detent at 2 so it drops to 1.

https://youtu.be/7ZpDCy7qF7E

[John H Watson]

This worked the same way in the air as far as when you are setting AB 2 and it doesn't stay (stick) at 2, it will drop back to 1. Likewise if you try to set AB 3 and it doesn't stay, it will drop to 2.

Interesting. Thanks. So at normal selection speeds, the system must be fast enough to latch the switch at each passed detent. This may mean the 3 second (brake to stop) timer starts at position 1 and might not reset until the switch goes back to DISARM.

[emerydc8]

I can confirm that on the 767, landing with AB OFF and then selecting AB1 or AB2 on the rollout, the AB will engage as selected.

Jon

[pilotngb]

For info - we tried changed the autobrake setting today in the BA sim from a preset of OFF for landing to other levels during the rollout. It latches without delay and functions as selected.

Peter, just curious: I am not exactly sure "who" is in the sim when you try these things. Are they trained pilots and you discuss these things before trying? Or are they "hapless tourists" that want to experience flight and you maneuver everything around them anyway and they don't even notice?

As a side note, I was one of those "hapless tourists" that had a flight experience with Peter in the BA sim near Heathrow last week. I have to say it was great fun and Peter added a wealth of experience and wit to the session. I refuse to discuss how I managed to land on 31R (GA runway) at KBFI when aiming for 31L with my 4th engine INOP!!  As a result, I am now back flying PSX and have just less than 12 months to get ready for my next session in 2019!

Thanks,
Neil

[Britjet]

Thanks for the kind comments Neil.
A very sensible decision of yours to divert to a nearby runway, if I may say. Very quick thinking on your part!
Peter.

[Jeroen Hoppenbrouwers]

I refuse to discuss how I managed to land on 31R (GA runway) at KBFI when aiming for 31L with my 4th engine INOP!!

HEEEEY!

I am currently trying to sleep about 1.07 km NW of the threshold of KBFI 13R so next time you try this kind of stunts, please be a bit careful, yes?

It's already ennerving enough to see/hear/feel heavy traffic for 13R come over at 250 ft with heavy traffic for KSEA crossing just above that.


:-D