Most demanding loads on aircraft landing gear

[JammedStab]

From an article I am reading,

"Landing gear is designed to last the lifetime of an aircraft and operate flawlessly every time. All components are designed to endure the most demanding structural loads, which for a wide-body aircraft is a high-speed turn off an active runway, not a landing or takeoff."

[DaveReidUK]

Sideways loads are always going to be more of a challenge for a landing gear to absorb than vertical ones.

[TURIN]

"Landing gear is designed to last the lifetime of an aircraft and operate flawlessly every time.

Those gear changes I did whilst in the hangars must have been a dream then.

[Check Airman]

Why would it be limited to widebodies?

[JammedStab]

Those gear changes I did whilst in the hangars must have been a dream then.
Maybe that statement includes overhauls.

[pulse1]

I once sold a control system to BA Engineering for use in their huge hard chromium plating shop. All they did in there was refurbish undercarriages. I don't know if it is still there.
On one of my early visits I was being shown around and spotted a complete u/c leg, wheels and all. All the metal parts of the lower u/c leg were blue from excess heat. I was told that this was caused by a single heavy landing and I still find it unbelievable that one single shock load could cause sufficient heating to raise the temperature of such a mass of metal to a temperature of about 500 degrees C. I was assured that it was so but they also pointed out that it wasn't off a BA aircraft.

While the undercarriages may last the life of the aircraft they do seem to require sufficient refurbishment to justify such a large dedicated engineering establishment.

[Goldenrivett]

I still find it unbelievable that one single shock load could cause sufficient heating to raise the temperature of such a mass of metal to a temperature of about 500 degrees C.
I agree.
I suspect the heavy landing was reference to heavy braking. (maybe a rejected take off from high speed).

[pulse1]

I suspect the heavy landing was reference to heavy braking. (maybe a rejected take off from high speed).

GR, that is exactly what I thought at the time but the engineer who was my guide assured me that it was from a single heavy landing and that they get quite a lot of similar u/c units for repair (not BA of course).

[Goldenrivett]

Hi pulse1,

Even a heavy landing of say 1,000 feet per min at touch down is only 10 kts vertically. That amount of energy is tiny compared to full heavy braking from say 150 kts (or more with some flap problem.)

The overheating signs (blue due oxidation interference pattern) must have come from the heat transfer from very hot brakes.

[Capt Quentin McHale]

" spotted a complete u/c leg, wheels and all. All the metal parts of the lower u/c leg were blue from excess heat. I was told that this was caused by a single heavy landing and I still find it unbelievable that one single shock load could cause sufficient heating to raise the temperature of such a mass of metal to a temperature of about 500 degrees C."


Don't think so boys and girls. Yes pulse1 totally agree. Where was this 500C heat generated from? Brakes or from inside the shock strut, or both?


Brake heat at that temp would result in the fusible plugs melting on the wheel rims and resulting in flat tyres and possible wheel rim magnesium fire. Then heat transfer through wheel assemblies to landing gear trucks and associated structure AND melting the protective paint to expose "blue metal".... don't think so.


Heat generated from within the shock strut? Definately not. The strut assembly is an overgrown shockstrut just like on our cars. It is filled with a combination of oil and nitrogen and on compression through an orifice will produce next to no temp rise at all. If temps did rise to 500C (which it won't) the strut O-rings would melt and you would have a deflated strut and vast pools of oil on the ground around the gear. In my 40yr career I have never seen or heard of such a thing. Keep smiling....


McHale.

[zlin77]

From what I can recall, all transport category A/C are required to withstand a vertical load of 600 feet/minute at maximum landing weight and 360 feet/minute at maximum take-off weight, so there is a lot of margin to cover firm/hard landings, I know of one B777 operator with a 40 kt. crosswind limit and the normal procedure is to land crab on.....the torsional loads on the main gear attachment points must be incredible but the airframe obviously can take it, even at Max LDG Wt. of about 251,000 kg......

[Chu Chu]

As a hobby blacksmith, I know that pounding on a piece of cold metal will make it hot. Some smith even show off by lighting cigarettes that way, though I've never seen that in person. But you have to deform the metal a lot to get it hot -- for example forging the square end of a bar into a point.

I'm pretty sure that steel absorbs very little energy simply by being stressed; it only gets hot through plastic deformation. Otherwise, think what would happen to the connecting rods and valve springs in piston engines, for example.

[Mach Stall]

Pulse1, I can't support the 500 deg C temperature, but what some seem to be overlooking is that shock absorbers function precisely by generating heat. Kinetic energy is absorbed by internal fluid friction forces within the shock, resulting in heat, which heat is what dissipates the descent rate.

Basic physics -- unless the shocks are converting their work to electricity or powering some device or storing energy, then if they did not generate heat, they would necessarily function only as springs, and the airplane would go bouncing back up into the air. This is our friend, the First Law of Thermodynamics (aka conservation of energy) at play.

The fact that the descent of the massive tonnage of a large commercial aircraft is able to be arrested by dissipating all that energy (in the form of heat) with such a small device as a shock absorber, and in such a short time as it takes to compress the strut, does lend credibility to the idea that they could get noticeably hotter with a hard landing.

The question is how much hotter.

We all know the heat generated from the brakes is sufficient to cause a fire, so that would be one's first thought as to the cause when looking at heat damage on landing gear. And as someone pointed out, the brakes have a lot more kinetic energy to dissipate because the forward speed is so much faster than the vertical descent rate (about 2 orders of magnitude more energy).

The temperature limits of hydraulic fluid and seals tell us the shocks are not intended to get very hot (nothing remotely approaching 500 deg C). But there will be some temperature increase of the strut from the shock absorption during a very hard landing (though I would guess less than a 50 deg C increase, rather than 500 deg C). If someone has actual data, it would be fascinating to find out.

[Volume]

Hevily depends on the type of aircraft. Short range aircraft have their take of of weight close to the landing weight and get beaten several flights a day. Long range aircraft may do just one flight per day, but they taxi out significantly more heavy than they land. Fast turns on taxi out may indeed produce the highest sideloads. Depending on the design of the gear again, this may create the highest stress in the most critical locations, while the stress distributes much more evenly on a straight landing.

[pulse1]

Mach Stall,

Very nice to have a contribution from someone who understands the thermodynamics. My estimate of 500 degrees was based on my experience that tells me that this is the minimum sort of temperature which will blue steel, particularly over such a short time scale.

For those who are equating this with normal heavy landings, it is obvious that we are talking here of landings which required the undercarriage to be replaced. What other damage may have occurred, I have no idea.

[Chu Chu]

No matter how hard the landing, the struts can only compress so far -- beyond that the energy has to go somewhere else. I guess other things, for example the wings, would then flex, and to some extent they would flex back and cycle the struts again. But it seems hard to imagine a vastly greater amount of energy going into the struts on a hard landing than on just a firm one.

Oh, and on plain steel it takes 575 degrees F (about 300 C) to get a blue temper. But it might well be significantly higher on chrome plate.

[riff_raff]

pulse1-

I don't think the chrome plated surfaces you saw were heated to 500degC. The MLG components (hydraulic struts, axles, etc) base metal thin dense chrome plating is applied to is usually heat treated to very high strength levels. Exposure to temperatures of 500degC would significantly alter the base metal properties, and I don't think the component would be salvageable.

There are only a few surfaces of a commercial aircraft MLG that are plated with thin dense chrome. These are typically surfaces subject to sliding contact with seals or bushings, to reduce friction, minimize wear, and prevent corrosion. Often shaft surfaces in contact with bearing races are plated with thin dense chrome to inhibit fretting. And sometimes the plating is used to refurbish shaft surfaces that have been scratched or abraded.

The blue hue you saw on the MLG component plated surfaces may have been due to one of the processes involved in the refurbishment procedure. One of the first things done after dis-assembly is to strip/clean the part surfaces, and then perform NDI to check for structural damage. If any chrome plated surfaces require repair, the first step is to electro-chemically strip the old plating.

There is high demand for refurbished commercial aircraft landing gear assemblies. So as long as the components are safe/serviceable/salvageable, they will be refurbished and put back into service.

Here is a good article describing the landing gear overhaul process:
http://www.avitrader.com/wp-content/uploads/2011/09/AviTrader_MRO_2011-091.pdf

[Blink182]

Pulse 1 .............

The BA Landing Gear shop was offloaded ( outsourced ) to a company called Hawker Pacific quite a few years back.... they in turn have been taken over by Lufthansa Technik.

Not much left to sell off......... next on the list is the whole Engineering operation at LGW


have carried out quite a few Overweight / Heavy Landing Inspections, never found any of the reported "Blueing" on external visible parts... Suspect that it was as previously suggested , a RTO and or Brake sieze.

[glum]

Not forgetting that if the aircraft had been in the sky for several hours at the usual cruising altitudes, the undercarriage would be pretty chilly at the point of touchdown.

I've had a complete covering of ice on the gear (and wing lower surfaces) having descended through cloud on a winter approach.

[JamesBird]

Back to the OP, few years ago an engineer who'd spent his working life on Vulcans said the critical case with undercarage was the turning torque on the 'static' leg as the aircraft 'screws' around tightly as it manovers.

[A Squared]

The question is how much hotter.

Not that hard to calculate. Lets say we have an airplane landing at 250,000 Kg in an unchecked 600 ft/minute impact. That's a pretty good thump. In fact it's the FAR 25 design limits for hard landing. The kinetic energy would be 250,000 kg X ( 3.05 m/s )**2 = 2,325,625 joules or 2,326 Kilojoules. Lets say all kinetic energy was converted with perfect 100% efficiency into heat in the landing gear struts. And lets say that the collective main landing gear struts is 200 Kg of steel. Obviously the landing gear struts of a 250,000 kg airplane is going to be a *lot* more than 200 kg, but for illustrative purposes it's useful to use a ridiculously small mass. Steel has a specific heat of 0.49 kilojoules/kg degree C so, that 2326 kjoules will heat our 200 kg of landing gear about 23 degrees celcius. Not quite enough to turn steel blue. What sort of impact would it take to turn steel blue? My information is that steel turns blue at about 300 decrees C, Using the same airplane mass, and same landing gear mass, and working backwards, we find that a 250,000 kg airplane would have to touch down at a vertical speed of 2125 ft/min in order to raise the temperature of 200 kg of steel 300 degrees C. Remember we're assuming that the entire kinetic energy will be transformed perfectly efficiently into heat energy and concentrated exclusively into 200 kg of steel, which is absurd on several levels. Bottom line is there is just no way a hard landing could generate enough temperature rise in the metal portions of the landing gear to turn steel blue. Someone was having pulse1 on.

[riff_raff]

A Squared-

Good post. The only thing I would add is that the heat transfer mechanism to the strut is via compression work applied to the hydraulic fluid mass within the cylinder. Obviously, the hydraulic fluid mass absorbing the energy would get hotter than the strut body it is transferring heat to.

[twochai]

I recall being told by a landing gear designer that the critical L/G loads were those induced by wheel spin-up on touchdown!

[Volume]

which of course is only relevant if you have more than one axle per leg or more than two main landing gear legs. The Vulcan has 4 tires on 2 axle per leg.
Even the 4 tires on one axle design of the Trident prevents most of this type of load.
This (http://forums.pelicanparts.com/uploads5/RWLG_211123074867.jpg) is how it looks if you turn an A380 around tight corners...

[Mach Stall]

The sole concern with A Squared's calculation is that it assumes thermal equilibrium is reached for the assumed gear strut mass (i.e., plenty of time for the thermal energy to diffuse through that mass) -- whereas in reality, we're talking about on the order of a half-second for strut compression. During this short time, the fluid friction would transfer heat to the comparatively tiny local mass of steel surfaces adjacent to the heat source. Bottom line, the uppermost piston and cylinder steel surfaces would see a drastically higher peak temperature rise than would the bulk mass of the strut.

As I said in my previous post (having run the same sort of back-of-the-envelope calc), I thought the temperature rise of the whole strut would be under 50 deg C, and I remain in agreement with A Squared on that front. And since pulse1 described the entire lower leg being blue, I still think there was probably some other cause at play in that large-scale thermal damage.

But I'm leaving the door just a little cracked in my mind that during a violently hard landing, some small, localized surface (e.g., upper piston) could possibly get hot enough to blue steel (over 275 deg C). I just can't quite rule it out intuitively without seeing actual data or a dynamic thermal analysis.

You generated an interesting sub-discussion, pulse1.