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Check your brakes or it could get expensive!

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Check your brakes or it could get expensive!

Old 9th Jan 2018, 02:33
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Check your brakes or it could get expensive!

https://www.rt.com/news/415305-plane...damaged-scoot/
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Old 9th Jan 2018, 04:54
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Forgive me if you already have heavy aeroplane time, but I have assumed not.

Not every helicopter has wheel brakes, so perhaps you might not be aware that it is common for larger aircraft with wheels to release brakes for improved cooling after the chocks have been installed. You might even have noticed some light aircraft with a padlock that prevents easy chock removal.

I have heard of other cases where untrained or careless people have removed chocks for other uses and placed the aircraft at great risk. In particular, I am aware of a RAAF C130A at Kainantu, PNG where chocks were temporarily removed to ensure the safe unloading of a vehicle. One of the pilots sprinted to the flight deck to re-apply the brakes and prevented the aircraft from continuing to roll off the airfield and down a hill.

And people steal chocks, which is the reason the aircraft registration is sometimes etched onto the chocks.

If there is a slope, the aircraft may roll onto the downhill chocks. Depending on the taxi out direction, it might be difficult to then remove them for departure.
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Old 9th Jan 2018, 05:04
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Strange that the aircraft started to roll when the chock was removed as this would suggest the weight of the aircraft was previously against the chock, in which case it would have been virtually impossible to remove, wouldn't it? Normally the chocks would not be removed until the tug has attached.
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Old 9th Jan 2018, 06:37
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Indeed. It's not uncommon for the Tug to take the weight off the chocks to get them out.

The article's a bit light on details. It's entirely possible there was a significant time delay between when the chocks were removed and the Aircraft rolled back.
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Old 9th Jan 2018, 07:36
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I thought it was normal for hydraulic brakes to "bleed off" over time once the hydraulic pumps (engines) are shut down. There's an "accumulator" to hold pressure for a while, but even that bleeds down eventually.

Of course, the 787 has electric brakes - but where does the electrical power come from if the aircraft is parked with engines off for some period of time?
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Old 9th Jan 2018, 07:51
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The lithium battery. Oh, wait...

What's a Bo-eng?
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Old 9th Jan 2018, 08:24
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Originally Posted by parabellum View Post
Strange that the aircraft started to roll when the chock was removed as this would suggest the weight of the aircraft was previously against the chock, in which case it would have been virtually impossible to remove, wouldn't it?
Parabellum is spot-on !
It would be interesting to see this airline's procedures re; docking.
There was a time when a headset man would be there on arrival and once on chocks and the P/B was released , would give a 'thumbs-up' to the jetty operator. In many airlines , the bean counters have seen that one off..

Some years back , our Eng. Dept. looked after a 'large Asian based' airline at LGW , some of whose flt. deck crews had a seriously deficient grip of the English language. One particular incident involved a pilot repeating several times , "release park brake" over the headset , upon which the A330 proceeded to roll back unchocked. It was only when sternly told that he was not clear to carry this out that an incident was averted .

A large proportion of todays problems stem from the fact that too many career-minded but inexperienced individuals are making decisions in compiling procedures that they are not in the position to be making in the first place !
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Old 9th Jan 2018, 08:48
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I once flew for a B737 operator that had a problem, grounding the a/c, when the front steps were fully extended, then chocks put in place, loosely to avoid jamming, and the brakes released. The a/c rolled, twisted the stairs just enough to prevent retraction. There then came an SOP to ensure chocks were in & brakes released before the stairs touched the ground; visible from LHS. A simple problem and to my knowledge not addressed in B737 manuals. The brakes were released due to quick turnarounds, but generally there did not seem to be a brake calling problem.
I then moved to another operator whose SOP was not to release the brakes. However, for what ever reason, they then introduced a captain's discretion to release brakes if there might be a bike coolling issue. I then mentioned the risk & SOP about the stairs from my previous operator and was met with blank stares similar to a suggestion that they should double salaries. I'd always thought we prevented problems by avoiding those already experienced by others. But then perhaps not, by some.
That same operator had an SOP manual so thick it could have been a chock, but they forgot to include 'brakes ON' in the final shutdown check if the crew left an a/c unattended. It had to happen that one day an a/c shifted. There was a big 'oops' moment and the SOP manual expanded. Airmanship seemed to be insufficient.
Generally, it used to be the ground personnel's SOP to confirm with Flt Deck that brakes were ON, or tug connected before removing chocks.
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Old 9th Jan 2018, 11:22
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Originally Posted by pattern_is_full View Post
Of course, the 787 has electric brakes - but where does the electrical power come from if the aircraft is parked with engines off for some period of time?
The 787 requires no active power to maintain the clamping force of the parking brake.
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Old 10th Jan 2018, 06:45
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Interesting

How is this achieved?
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Old 10th Jan 2018, 07:33
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Instead of hydraulic pressure, which tends to dissipate when power is removed, the 787's brake pistons are driven by geared elecromechanical actuators which effectively freeze in position when power is removed.

A bit like trying to push your car when it's in 4th gear.
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Old 10th Jan 2018, 08:31
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Our outfit has gone the route of leaving the parking brake set at all times. Similar to a previous poster, the company had the procedure of loosely chocking the wheels. After a couple of jetbridge dings with rolling jets, we switched to the new policy.
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Old 10th Jan 2018, 09:00
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Well....

I guess we could have 10 pages of "I leave them on", "I leave them off". It depends on type and Company SOP. I previously flew 75/76 and generally left them on, but now on the 321/330 take them off once one of the pilots has checked they are in. A sensible turnround time after a Med auto brake landing on the 321 or the the 330 in Vegas (without fans) would not be achievable if left on. After signing the a/c over to the engineers; that's up to them.
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Old 10th Jan 2018, 09:13
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Originally Posted by DaveReidUK View Post
Instead of hydraulic pressure, which tends to dissipate when power is removed, the 787's brake pistons are driven by geared elecromechanical actuators which effectively freeze in position when power is removed.

A bit like trying to push your car when it's in 4th gear.
Do the brakes auto brake on gear retraction and how do you know they have released on extension?
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Old 10th Jan 2018, 09:14
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but as the brakes cool down they shrink. With a Hydraulic system which applies pressure, even when turned off via an accumulator, the pressure keeps the brakes clamped. An electric brake works by movement, so when the electrics are turned off, there needs to be some sort of adjustment to compensate for the shrinking brake pack.
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Old 10th Jan 2018, 09:44
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Originally Posted by It's only Me View Post
Well....

I guess we could have 10 pages of "I leave them on", "I leave them off".
Absolutely right there IOM, so before it gets too heated and repetitive , my feelings are that the one idiot-proof method is to 'headset' the aircraft in and get it chocked with P/B off prior to ANYTHING else happening.

The most likely exception here is if it's blowing a hooley and Capt & Grd. Eng. agree to leave 'em on.
Of course , I understand that in LAS that could be a dilemma in itself , as it could be a case of smokin' brakes , OAT hot as hell and blowing a howler . But for normal circumstances , I stand by my first paragraph.
I do also realise that for some airlines , 'normal circumstances' means that there's not an engineer to be seen unless called for on the VHF.

Re: leaving it for the engineers to decide ? Well , the dreaded, "Brakes Hot/Release PB/Pref Chocks" message normally pings up about 3 minutes after docking and invariably ends up with the poor old Capt hanging out of the DV window trying to get the Eng.s attention.

As for me ? I'm old-school ! My career is going nowhere in the airline..
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Old 10th Jan 2018, 10:29
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Originally Posted by glad rag View Post
Do the brakes auto brake on gear retraction and how do you know they have released on extension?
I don't know, though I imagine it would be fairly easy with a geared electromechanical actuator to be able to detect and signal the position of the piston(s).
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Old 10th Jan 2018, 23:44
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but as the brakes cool down they shrink. With a Hydraulic system which applies pressure, even when turned off via an accumulator, the pressure keeps the brakes clamped. An electric brake works by movement, so when the electrics are turned off, there needs to be some sort of adjustment to compensate for the shrinking brake pack.
I do not know specific details but I would be very surprised if there is no deliberate "compliance" in the parking brake mechanics.
Compliance of course being an obfuscating a word for the technical term 'springy bits'.

As the brakes are applied the spring (in whatever form) deflects 'storing' reserve force in the spring. Without this the driving motor would come to a hard stop with risk of mechanical damage.

This provides continued pressure as the brakes cool, will drop slightly depending on the spring constant of system.

The accumulator provides the analogous spring function for hydraulic brakes.
Even without leakage the pressure will drop (possibly imperceptibly) as the brakes cool and slightly more fluid is required to maintain contact.

On self locking:

A better model than pushing a car in fourth gear (which while hard is theoretically possible) is a scissors car jack.

Turning the threaded rod moves the jack up or down but the geometry is such that the weight of car cannot turn the rod.
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Old 11th Jan 2018, 09:31
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Many years ago when I actually used to fix these things we had the following procedure.

1. Aircraft stops
2. PB on
3. Chocks under wheels
4. PB off

Once the a/c was ready for departure PB back on. Chocks away and push back.

The reason for doing this was not because of brake pressure failing, it was for cooling purposes. A set of brake pads clamped to the hot disks caused that section to stay hotter for longer and could result in warping of the disks (that's what the manufacturers used to say 30+ years ago anyway).
In addition, once the system did cool down, everything contracted and the pressure of the pads on the disks decreased meaning there was a possibility of them no longer holding the aircraft still if on an incline.

I am sure with the introduction of carbon brakes etc these things have changed slightly but its probably good practice to follow this still.
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Old 11th Jan 2018, 10:33
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Indeed. I worked for a similar outfit. Often I'd do the walk round and see a 1-2cm space between chock & wheel. Front steps extended. I asked the ground why the gap? "Because they become stuck under the wheel if we chock 'em too tight." So the a/c could roll enough to perhaps twist the steps. Captain at fault, naturally. Chocks were in.
And then we get the chestnut of SOP's that said 'Main Wheels' need to be chocked. Of course the agent who operates much on the right side of the a/c chocked only the right mains. Ok for me, but not for some. Then they started chocking just the nose wheel fore/aft, because that was by the headset for push back. OK for me, but not for all. You couldn't win.
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