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Another airframe down...

Started by Jeroen Hoppenbrouwers, Wed, 7 Nov 2018 23:38

John H Watson

(EDIT: Sorry.. cross-posted)

QuoteIs the antiskid system of no help when braking manually?

I think Jon's comments are simply highlighting the limitations of human braking. Max manual brakes feels severe, so the pilots back off... then realise they have backed off too much.. and reapply max braking, repeating the cycle.. or may be hestitant to reapply max braking. Or, the pilots apply unequal pressure on the left and right pedals due to leg strength/length, so have to figure out the correct balance to stop them running off the side of the runway. The antiskid and torque-limiting systems are always in operation.

The autobrakes provide relatively smooth application and balanced left/right application pressure automatically. The braking should be straight (unless the wheels on one side of the aircraft cannot get a grip at all due to long sections of  ice/deep water only on that side). By balanced, I mean there is only one outlet pipe on the autobrake pressure control module. This hydraulic pressure goes through "T" pieces (pipes) to the normal antiskid valves for all bogeys. The pilot can apply different pressures to each side.


Hardy Heinlin

"Balanced left/right application pressure" -- does this aim at equal pressure or at avoiding any yaw? And if it's the latter, what are the control criteria? Sensed IRS yaw or sensed wheel speeds?

emerydc8

QuoteMax manual brakes feels severe, so the pilots back off... then realise they have backed off too much.. and reapply max braking, repeating the cycle.. or may be hestitant to reapply max braking.
+1

Quote
"Balanced left/right application pressure" -- does this aim at equal pressure or at avoiding any yaw? And if it's the latter, what are the control criteria? Sensed IRS yaw or sensed wheel speeds?

I've found the AB to do a great job keeping it on centerline, even if there's a momentary release on one side going over a patch of ice.

I tend to favor the right brake because I can rest my right knee up against the center console and I have a reference point for braking. If you play the guitar, it's like resting the edge of your your picking hand on the bridge to have more control. So for me, if one side is hot, it's usually the right side.




United744

I'm left-foot dominant. I'll lead with the left foot and balance with the right. If any brake needs to be modulated to straighten things out, I'll modulate the right foot to match the left once I've got the braking effort I desire.

emerydc8

Never thought about it from that point of view. I think I'm left-foot dominant too, but I have a heavy right foot for reasons explained above.

John H Watson

Quote"Balanced left/right application pressure" -- does this aim at equal pressure or at avoiding any yaw? And if it's the latter, what are the control criteria? Sensed IRS yaw or sensed wheel speeds?

No, by application pressure, I mean input pressure into the antiskid valves. What comes out of the antiskid valves will be unbalanced, depending on which wheels are skidding.

John H Watson

QuoteI've found the AB to do a great job keeping it on centerline, even if there's a momentary release on one side going over a patch of ice.

Perhaps (as I think was previously discussed in other threads), inertia is playing a key part here.  At higher speeds, the aircraft has a tendency to travel in a straighter line....  Probably why even the unbraked nose gear tends to skid at higher speeds when it's used prematurely.

Hardy Heinlin

Quote from: John H Watson on Sat, 10 Nov 2018 04:19
No, by application pressure, I mean input pressure into the antiskid valves.

I thought you meant the autobrakes, not the antiskid:

QuoteThe autobrakes provide relatively smooth application and balanced left/right application pressure ...

John H Watson

QuoteI thought you meant the autobrakes, not the antiskid:

I did mean the autobrakes. The autobrakes provide a controlled/metered hydraulic pressure (instead of the pilot) and it goes to all 4  normal antiskid modules and then to the brakes. The antiskid modules interfere with this pressure flow if a skid is detected.

hydraulic pumps > autobrake module > all antiskid modules > all brakes.

With manual braking, the pilots modulate the brake pedals which controls the flow of hydraulic fluid to the left and right pairs of antiskid modules.

hydraulic pumps > left/right brake pedal-operated metering valves > L/R antiskid module pairs > L/R brakes

Hardy Heinlin

I see. But it's the antiskid which maintains the aircraft track when an asymmetric skid is detected? (The antiskid doesn't "steer", but thanks to its asymmetric skid detection the track is pretty stable.) The metered pressure of the autobrakes is controlled by the IRS groundspeed (or wheelspeed if IRS invalid), and does not try to maintain the track by asymmetric braking, correct?

John H Watson

QuoteBut it's the antiskid which maintains the aircraft track when an asymmetric skid is detected?

No, the antiskid or the autobrake system doesn't have asymmetry protection. The only crosstalk between the brakes is between the wheels of the same bogey under certain conditions. This cannot help with balancing the left side with the right side. I think only the inertia of the aircraft keeps the aircraft going in a straight line (up to a point).
Perhaps if the plane (slowly) veered to the side of the runway and two wing gear wheels were on the grass, the aircraft would brake more on the opposite side and pull you back onto the runway... but if a complete bogey started to sink into the grass, the drag alone might keep you in the grass.

Wheelspeed cannot replace the IRU input. IRU failure will trip the A/B switch. Wheelspeed is only used for application logic.

emerydc8


QuoteFlat main gear tire(s) cause a general loss of braking effectiveness and a yawing moment toward the flat tire with light or no braking and a yawing moment away from the flat tire if the brakes are applied harder. Maximum use of reverse thrust is recommended. Do not use autobrakes.

B747 FCTM 6.24

Britjet

The aircraft could stop in that distance. No question about that. However given the nature of the gusty surface wind it would be difficult to get the aircraft down in the correct place, and apply adequate braking.
Manual braking should exceed the autobrake capabilities but it depends on how it is used. Dealing with a crosswind yaw AND applying maximum manual braking is extremely difficult. If they had set Max Auto or even level 4 the braking will not go to the stated rate until the pitch angle is below a certain amount anyway (to prevent the brakes crunching the nose in).
It is very tempting to try and brake differentially to help with directional control. However there is a real downside to this. As soon as you release  brake pressure on a pedal that brake system will go through a slight delay before the brakes take effect again because they go through their anti skid sensing cycle again - the only way to beat this is to apply more pressure on the other brake - but if you already have full pressure applied - that won't work..
Also it's actually quite difficult to apply rudder for directional control without disengaging the autobrakes accidentally. This is not uncommon on significant crossword landings. If you don't spot this straight away you will have lost valuable distance..
Just a few possibilities.
Lots going on here. I can't help but feel the CVR will be very interesting with regard to CRM..
We shouldn't be pointing fingers really, but - hey - as no-one was hurt..