Heat from Heavy Landings

[pulse1]

Some years ago I was on a business visit to BA Engineering at LHR. I was shown an undercarriage assembly with the lower end of the oleo quite blue from the effects of heat.

I was told that this was caused by a single heavy landing, which generated sufficient heat to discolour the metal, > 500C?.

I found this very hard to believe at the time and asked if they meant that it was caused by heavy braking. They were emphatic that it was caused by the actual impact and were quick to assure me that it was not BA gear that we were looking at.

Was I being told a tale or is it really possible for a single heavy impact to generate sufficient heat to discolour metal?

[Mark 1]

No specific knowledge here, but a few engineers fag-packet calcs:

Heavy landing say 300T landing weight at 1000 ft/min (very heavy landing).

Energy absorbed in vertical deceleration = 0.5 * 300000 * 5(m/s)^2
= 3.75 MJ
Specific heat of Hydraulic fluid ~ 2 Kj/Kg/K

Not sure how much fluid in average bogey oleos.

As a guess 50 litres would rise ~40 degrees , but to get a 200 degree rise in temperature suggests that the energy is absorbed in only about 10 litres of fluid.

[Lu Zuckerman]

If in fact the chrome shock strut turned blue by being heated to in excess of 500 degrees C then there are other problems to consider. The rise in heat was instantaneous and effected the hydraulic fluid and the elastomeric elements in the shock strut. The rapid increase in temperature effected the chrome plate turning it blue and more than likely effected the heat-treat of the parent metal in the shock strut.

The structural loads had to be reacted by the attach fittings for the landing gear and the fuselage attach points. Aside from rejecting the landing gear because of the change in heat treat there was most likely some damage to the attach fittings and the airframe.

If the BA engineers made the required repairs (re-plating the shock strut and replacing the fluids and the elastomeric elements) then they have created a time bomb that will blow up some time in the future when the pilot makes a normal landing. This landing gear will most likely not be installed on the original aircraft so there are two time bombs. The landing gear and the airframe from which it was removed.


:eek:

[pulse1]

Thanks for your answers.

Mark1, I'm always impressed by those who attempt to quantify these things, especially on the back of a fag packet - sign of a real engineer.

Lu, I certainly do not want to leave an impression that this L/G was destined for re-use. I have no idea except that it was standing, partly assembled, in the doorway to the plating department. For all I know it may have been on the way out, after being rejected. They did say that they saw quite a few of these sent in by other airlines who do not have replating facilities.

My question was more to do with me getting my head round the possibility of this amount of heat being generated instantaneously, so that such a large mass of metal can see a temperature rise which will lead to discolouration.

When I read reports which include heavy landing checks I have never noticed any reference to this condition. It would certainly seem that the rest of the aeroplane must be in a very poor state if this has happened.

Surely someone out there must have seen it actually happen if it is true.