Undercarriage Retraction.
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Undercarriage Retraction.
As a humble wannabe I was leafing through a good friends B757 tech notes the other day and I came across the section on undercarriage retraction. It said that there was a braking pressure exerted on the wheels during the retraction (obviously to slow them down from the V1 speed before they sat in the fuselage)
It made me wonder though - would it theoretically be possible for the brakes to "jam" on during retraction and remain jammed until the undercarriage is lowered during the approach? Methinks that this could cause a little bit of the brown trouser type scenario up front!!!!
So could anyone shed some light on what would happen, is it possible, any ideas to sort it etc etc etc - been bugging me for days!!!
Thanx in advace
Tunny
PS Do all aircraft have this braking thing going on during retraction or is it unique to the 757???
It made me wonder though - would it theoretically be possible for the brakes to "jam" on during retraction and remain jammed until the undercarriage is lowered during the approach? Methinks that this could cause a little bit of the brown trouser type scenario up front!!!!
So could anyone shed some light on what would happen, is it possible, any ideas to sort it etc etc etc - been bugging me for days!!!
Thanx in advace
Tunny
PS Do all aircraft have this braking thing going on during retraction or is it unique to the 757???
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This is common on lots of airplanes. When you select the gear up there is a small amount of pressure sent to the brakes to stop the rotation before they go into the wheel wells.
That pressure will dissipate anyway, but the valve that causes the braking is disabled once down is selected.
On some other airplanes there is a requirement to apply the brakes before selecting up.
The nose gear normally hits skid pads once it is almost up and the rotation is stopped.
That pressure will dissipate anyway, but the valve that causes the braking is disabled once down is selected.
On some other airplanes there is a requirement to apply the brakes before selecting up.
The nose gear normally hits skid pads once it is almost up and the rotation is stopped.
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As I recall it was a potential problem on the venerable BAC 111. The system as I recall used the parking brake pressure line to apply brakes after gear retraction. If the parking brake set light remained illuminated after gear retraction there was a procedure to use try remove this pressure. My BAC systems memory is alittle hazy after 15 years. The BAC 111 had brake temperature gauges though and I once recall the brakes being on and the need to lower the gear because of excessive (or so I thought) heating of the brakes, tires, and all that other other stuff in the wheel wells. I seem to recall a 727 (Mexicana?) that suffered an inflight loss of hydraulic pressure due to tires exploding in the wheel well becase of extreme temperature. The 737- 200 (the entire family probably) does not have enclosed main landing gear. The tires and brakes are subsequently cooled by the slip stream with the aid of a hole in the wheel covering. The B737-200 also does not have brake temperature gauges. The way around knowing whether the brakes a cool enough for takeoff is to consult a brake cooling chart.
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As has been said, most modern high performance jet aircraft with higher take off speeds have this system. On the A320 family, all the wheels are braked. The reason for this is that the wheels rotate at a high enough speed that centrifugal forces make the tyres expand enough that they could prevent the wheels entering the bay correctly. The A320 ground air logic enables the normal braking system to stop the main gear wheels rotating, and a meatl snubber band in the "roof" of the nosegear bay slows the nosegear. The Fly By Wire system will self check the Autobrakes/Anti skid system on gear deployment. Just fyi., if the brakes do need cooling the A320 may be supplied with brake fans.
Hope that helps.
Tailwinds
Hope that helps.
Tailwinds
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There is also a huge gyroscopic effect on the main gear while retracting if wheels were still spinning.
The main gear is retracting sideways.
With spinning wheels , the "gyroscopic" force acts 90 degrees on this motion.
This would impose a torque on the strut which would damage the gear structure.
The nose gear is retracting "in-line" with the wheel spin direction (not sideways).
So this effect does not occur , and nose wheel braking is not neccessary before retraction cycle begins. (757)
Cheers
The main gear is retracting sideways.
With spinning wheels , the "gyroscopic" force acts 90 degrees on this motion.
This would impose a torque on the strut which would damage the gear structure.
The nose gear is retracting "in-line" with the wheel spin direction (not sideways).
So this effect does not occur , and nose wheel braking is not neccessary before retraction cycle begins. (757)
Cheers
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In comment to the original post regarding the likelihood of system failure, it must be noted that any gadget ..whether mechanical or electrical/electronic ... can fail or function contrary to the design intent and do silly (and often tragic) things.
As a result the manufacturer has
(a) a set of design tasks to determine failure modes, their probability of occurence, likely outcomes, and to put in place defences against undesirable outcomes.
(b) a routine review group to assess in service problems which often leads to the various SBs which beset operators.
It must be kept in mind that all of this takes place against a backdrop of potential litigation - the manufacturer is not always in an easy situation.
Sometimes things slip through the net for whatever reasons. For instance,
(a) the Concorde accident may be in that category.
(b) As another, I was involved in the investigation of an unpleasant fatal several years ago where it appeared that a rather unfortunate STC design decision led to a quite unexpected system problem which, in turn, led to a very confusing set of operational symptoms - which the pilots, in turn, misinterpreted with dire consequences when their handling of the immediate problem was inappropriate.
(c) The Sioux City DC10 is another pertinent example.
(d) How many times, in the early FMS glass ship days, did we hear the catchcry "What is it doing now ?"
Aviation, like any other area of human endeavour, is imperfect. A well trained and intelligent pilot (CRM plays its role) is the current best last line of defence. The human can be innovative when non standard situations require new and imaginative strategies for successful resolution.
Doesn't always work, and we just have to accept that there are times when it would have been better not to have got out of bed that day.
From such cases the lawyers develop their investment portfolios.
As a result the manufacturer has
(a) a set of design tasks to determine failure modes, their probability of occurence, likely outcomes, and to put in place defences against undesirable outcomes.
(b) a routine review group to assess in service problems which often leads to the various SBs which beset operators.
It must be kept in mind that all of this takes place against a backdrop of potential litigation - the manufacturer is not always in an easy situation.
Sometimes things slip through the net for whatever reasons. For instance,
(a) the Concorde accident may be in that category.
(b) As another, I was involved in the investigation of an unpleasant fatal several years ago where it appeared that a rather unfortunate STC design decision led to a quite unexpected system problem which, in turn, led to a very confusing set of operational symptoms - which the pilots, in turn, misinterpreted with dire consequences when their handling of the immediate problem was inappropriate.
(c) The Sioux City DC10 is another pertinent example.
(d) How many times, in the early FMS glass ship days, did we hear the catchcry "What is it doing now ?"
Aviation, like any other area of human endeavour, is imperfect. A well trained and intelligent pilot (CRM plays its role) is the current best last line of defence. The human can be innovative when non standard situations require new and imaginative strategies for successful resolution.
Doesn't always work, and we just have to accept that there are times when it would have been better not to have got out of bed that day.
From such cases the lawyers develop their investment portfolios.
