View Full Version : How tough are the landing gear?

21st Jun 2007, 20:39
I was reading through the report into the crash of an MD-11 at HKG in 1999 which was caused by the plane impacting the runway with a sink rate of 18fps (1080fpm). The report stated that this was beyond the design limits of the plane. I'd also heard of a Tristar which hit the runway at Brize going down at 3600fpm and was still able to execute a go-around and land again safely.

What is the maximum rate of descent that most heavy jets were designed to take (assume MLW) and what is the most they will realistically take? I have to say, I'd have thought the MD-11 would have survived (although obviously very badly damaged) - 1000fpm isn't that much surely?

21st Jun 2007, 20:50
My Airbus is certified to touchdown with 360 f/min at MaxLDGW.

21st Jun 2007, 20:59
The design sink rate of the MD-11 was 600fpm on landing - although obviously designed to land at much less than that usually. I was just curious, found it rather suprising that the gear actually collapsed - I remember watching a video (part of a documentary about the building of the A380) of a Korean Air 747 landing a Kai Tak on what looked like one wheel.

21st Jun 2007, 21:26
I think you will find the 360 ft/min is applicable when above MLW. At or below MLW it's 540 ft/min. Also expressed as 1.7G or 2.0G at CofG. Above these limits inspection is required i.e. damage is possible.

This is all from memory, so I might be wrong.

21st Jun 2007, 21:49
Just to add another figure....

Concorde was certified to 10 fps = 600 fpm.

F-BVFD suffered a heavy landing in Dakar in Nov 1977 at 14 fpm = 840 fpm, where the tail wheel was crushed. Flew again after repairs, though.

Mad (Flt) Scientist
21st Jun 2007, 22:08
Well, per FAR 25...

Landing load conditions and assumptions.
(a) For the landing conditions specified in Sec. 25.479 to Sec. 25.485 the
airplane is assumed to contact the ground--
(1) In the attitudes defined in Sec. 25.479 and Sec. 25.481;
(2) With a limit descent velocity of 10 fps at the design landing weight
(the maximum weight for landing conditions at maximum descent velocity); and
(3) With a limit descent velocity of 6 fps at the design take-off weight
(the maximum weight for landing conditions at a reduced descent velocity).
(4) The prescribed descent velocities may be modified if it is shown that
the airplane has design features that make it impossible to develop these

Yes, 1000fpm "isn't that much". But it is 67% more than the maximum you're required to design for. Since the key to a good design is to design to meet the requirements, not to vastly exceed them (especially in an application, like an aircraft, where overdesign = weight = penalty), no-one is going to provide a landing gear system that can easily take 167% of the design requirement. Typical factors of safety would be of the order of 1.5, so 1000fpm is getting to the point where you'd expect to sustain structural damage.

A somewhat sobering fact for you to consider: the maximum design descent velocity of 10fps is achieved by simply dropping (like a stone) from about 18 inches. That's not very high, is it?

22nd Jun 2007, 01:06
Regarding the TriStar L1011 at Brize, I think you will find that it was a cock up as the crew thought they were doing an autoland and the aircraft was not properly configured for it, I believe the aircraft sustained very serious damage. Any RAF L1011 people out there?

22nd Jun 2007, 02:16
A significant safety issue in several MD11 landing accidents was that the ‘aircraft limits’ were exceeded, not the landing gear. Whereas in other ‘Boeing’ aircraft, a beyond limits landing may result in gear failure or more likely that the gear assembly leaving the aircraft, in the MD 11 there have been several instances where the wing structure failed before the gear. In at least one of these accidents (HKG?) the wing failure contributed to the aircraft flipping over.

22nd Jun 2007, 08:57
PE21 is correct in HKG the wing failed first in HKG...and Brize.
In the Brize L1011 incident, the a/c established on the ILS too late for the automatics to go into fine capture mode and thus, although the indications were initially correct for the ILS, the autoland was never annunciated and was never going to work! The a/c flew into the runway at 700'/min and because it was light, bounced and then 'fell' on one main leg first. Not sure about the 3,600'/min though. Main spar failed with a 9inch gap through which the fuel began to pour. After removal of gears and engines the gap closed up and a plate was fixed over the split and she flew again! Whilst doing the repair the engineers found that the other rear main spar had a similar repair....but wasn't in the tech records anywhere! Scuttlebutt had it that the wing had was not drilled properly during construction and that a repair was used at this stage!! Never confirmed tho.

22nd Jun 2007, 10:07
There was also a VC10 that was damaged at Gatwick years ago by the spoilers being pulled while still above the runway. It landed heavily and needed repairs but was far from collaspe. The 3600fpm came from a thread about the 1999 HKG crash that drifted onto talking about the Tristar.

Capt. Slow
22nd Jun 2007, 11:07
Wasnt there an air canada DC8 that crashed many moons ago after a heavy landing caused by premature extension of the ground spoilers? Captain was able to perform a go around (despite losing an engine and part of the wing during the "impact") but eventually ran out of time. believe that impacted at 18fps (1080fpm) as well...any slower and it would have made the GA without issue, any faster and it wouldnt have been able to T/O again...

22nd Jun 2007, 17:27
There's a video around on the web of the Hong Kong MD-11 accident showing the final approach, shot I think by a pilot just outside the perimeter. The aircraft really doesn't look to be deviating from normal at all or suffering unduly from the storm, it is quite a surprise to see it turn over on touchdown.

A FedEx MD-11 overturned on touchdown at Newark in what appeared to be very similar circumstances. I can't really think of any other accidents to other types like these MD-11 ones. Did all the passenger airline users of the MD-11 dump them prematurely (which they almost all did) only because of efficiency matters, or did they just feel it was too much of a risk to keep going ?

22nd Jun 2007, 17:53
Pretty rugged, especially those on the 'ole B707.
Long ago, a PanAmerican 707 was landed at KIDL (now KJFK) with the left main gear drag brace completely missing, yet the gear did not collapse on landing.

The L1011 incident at BZZ was mishandled by the flying pilot, who attempted an autoland close in and low...far too close in and low for the airplane to acquire proper automatic approach/land track, as required.

The 'ole Lockheed tri-motor is tough, as is its landing gear, no doubt about it.

The Bartender
22nd Jun 2007, 18:34
Here's a hard one: http://aviation-safety.net/database/record.php?id=19870223-0

DC-9-41 with a premature deployment of groundspoilers, followed by go-around, landing, and write-off...

22nd Jun 2007, 18:39
The disappearance of the MD-11 from the major carriers has always been a bit of mystery to me - certainly the captain of the Hong Kong MD-11 tried to blame the plane and attributed its lack of sales to its poor safety record. While it was clear that the landing gear's limits had been exceeded in the crash one can't help thinking that perhaps if that had been a 747 in the same circumstances it would have taken the punishment rather than snapping a wing off.

Is there an argument for mounting the gear on the body (like the AN-124 for example) because you'd be less likely to break a spar in a heavy landing? Certainly the AN-225's gear looks like it could take almost anything.

The Bartender
22nd Jun 2007, 18:39
Wasnt there an air canada DC8 that crashed many moons ago after a heavy landing caused by premature extension of the ground spoilers?


Doors to Automatic
24th Jun 2007, 22:05
If memory serves correctly the HKG MD-11 crash was caused by a failure of the right MLG following a high sink rate just prior to landing. Were not maintenance shortfalls of the MLG a contributory factor?

24th Jun 2007, 23:11
....perhaps if that had been a 747 in the same circumstances it would have taken the punishment rather than snapping a wing off.It might not have snapped the wing off, but would still be overstressed enough to be ready for the scrapman just the same.
Is there an argument for mounting the gear on the body (like the AN-124 for example) because you'd be less likely to break a spar in a heavy landing? Certainly the AN-225's gear looks like it could take almost anything.There are such things as "design limits".
As an engineer, I find myself staring at your question.
"...you'd be less likely to break a spar in a heavy landing..."
Either it's a "heavy" landing and you break something, or it's not a heavy landing.
Does one really design for "heavy" landings, or does one design for weight, in which case everything breaks up more or less at the same time... gear, wing spars, tail falling off, etc.
Not trying to be facetious in the slightest, just trying to think it out.

25th Jun 2007, 04:02
I've got a video of the 747 being certified, and it's landed without any flaring (so about 700 - 800 ft/min) with no flaring, at max landing weight.
The wings & engines flap around a bit, but there is zero damage.
From other experiences, they'll take hard landings with very little problems anyway, they're extremely tough.

25th Jun 2007, 11:07
Either it's a "heavy" landing and you break something, or it's not a heavy landing

Yes but surely some designs will take more punishment than others, I used the example of the 747 because I've always understood that due to how big it was compared to other airliners at the time it was over-engineered somewhat just to be on the safe side, 800fpm and no damage is surely quite impressive and probably better than some designs today could manage. Also it is interesting to note that Boeing says that when landing in a crosswind it's ok to decrab after the main gear have touched down, whereas McDonald Douglas underlined the importance of touching down already having lined up with the runway.

Of course one designs for weight, plus an a safety margin, but I was just wondering just how much a plane would really take.

Bolty McBolt
26th Jun 2007, 07:57
If the landing gear is fitted to a Boeing...


Sorry just could not help myself :ok:

26th Jun 2007, 10:15
Slightly OT, but for our sim model I just calculated the
B757 main gear spring constant to about kx = 120.000 lbs/feet/gear

If someone knows the vertical compression/travel length of the strut,
I guess we could calculate the energy needed to make it hit "bottom"
and break something - which would be a combination of ROD and landing weight.
The heavier your landing weight, the less ROD the gear can take.


Pugilistic Animus
25th Jul 2007, 04:23
It seems with the Boeing Corporation that if the regs say 150% they go 154% with many things;)

Dixons Cider
25th Jul 2007, 11:31
Have a look at this (http://www.youtube.com/watch?v=k8YkBOi3Y6A) footage...and in particular the landing gear.

It completely compresses (maybe to the point of failure?) but then extends again, the aircraft then continues on its roll out, albeit with a catastrophic airframe failure.

Quite amazing.

Dixons Cider
25th Jul 2007, 11:33
Actually just had a look at the above footage again, looks like the nose gear failed, the mains however are still functioning