MD-11 Landing Instability

Trip7

Well-Known Member
From a co-workers Father who worked for Douglas..................





What happened here (in Narita) is the same as what happened in Hong Kong
(to a China Airlines MD-11) and Newark NJ (to another FedEx MD-11) some
years ago. The hard landings, which resulted in ALL cases from failure of the
aircraft to respond appropriately to pilot control inputs, resulted in
rupture of the wing spar by the main gear oleo strut, breaking the wing; the
lift from the wing not yet broken then caused the aircraft to roll, and turn
upside down. In the case this morning, both wings broke - first the left,
and then as the roll angle reached about 80° LWD, the right wing also broke.
this can be seen clearly on the video presented on the BBC News website.


The failure of the aircraft to respond appropriately to pilot control
inputs is the result of certification of the aircraft despite the omission of a
vital part of the aircraft flight control system: namely the rate command
function of the LSAS system. This omission leads, under certain
circumstances, to what can be described as a "cliff-like" APC* as a result of the
extremely slow response of the airplane to elevator control inputs at high
gross landing weights. Despite protestations by the Douglas Aircraft Co. that
the aircraft flies "just like a DC-10" (which are blatant untruths) the
airplane is inherently dangerous and extremely difficult to handle in gusty
wind conditions, especially at high landing weights.


I have had many discussions with FedEx ( and Delta) pilots and instructors
about the MD-11, and have invariably heard horror stories about landings.
I feel a great sense of sadness for the families of the airmen who were
victims of this tragedy, and I know that sadness will turn into anger when
once again the NTSB/Boeing clique start to insist that this was all caused by
pilot error.


I am once again reminded of a statement by the chief investigator of the
Hong Kong MD-11 accident, Mr. Robert Benzon, who, in the presence of several
Boeing engineers, the Asst. VP of flight safety at China Airlines, and
myself, said: "...sometimes we have to burn a couple of pilots to protect the
local industry..." Not this time, Mr. B.


* "cliff-like" APC
Airplane-Pilot-Coupling - the phenomenon used to be called PIO, but the
name was changed so to avoid the implication of pilot causality.
A more complete description of this aircraft design/response problem can
be found in Aviation Safety and Pilot Control published by the National
Research Council. ISBN-10: 0309056888

The terms stability, stable, and unstable have specific meanings with respect to the flying qualities of an airplane. Those meanings are confined to "static stability" and "maneuvering stability" and refer to control force gradients vs. speed, and vs. normal acceleration ('g') respectively. Unfortunately, in common practice the terms are used indiscriminately by "aviation experts".



The software changes (implementation of the -908 FCS load, and some subsequent set mandated by the FAA to allow certification of the MD-10 on the same type cert) improved the predictability of the MD-11 in the landing phase; they did nothing to correct the omission of a simple lead-lag pitch SCAS (à la F-4) which was designed into the Rate Command CWS of the LSAS (which is there, but set to zero gain, making it non-functional), which would have compensated for the extremely low short period natural frequency of the airplane in the landing configuration (making the airplane prone to APC). The change in geometry from DC-10 to MD-11 made this item a required addition from a handling qualities standpoint, yet the FAA still certified the airplane. They also did nothing to correct the gross elevator load feel (force vs. displacement) gradients in the landing configuration, which make it virtually impossible for a normal pilot (and in particular a soft-handed airline pilot) to access the necessary deflections for control of the airplane. Nor did they compensate for the inadequate bandwidth of the elevator hydraulic actuators, which incur significant lags when rapid elevator control inputs are required (as in turbulence).



The DC-10, and MD-10, does not suffer from the above problems for several reasons: it has a lower max landing weight, and thus much reduced pitch inertia (Iyy), the elevator throws required for adequate control are significantly reduced as a result of greater elevator control power (area) along with correspondingly lower required control forces. Additionally, the DC-10 (as employed at most airlines) does not employ RCWS, a ludicrous, parallel roll control "enhancement" on most MD-11s; FedEx, incidentally, ordered its airplanes to have RCWS turned off; a wise decision.
 
He doesn't appear to be making any assumptions or decisions. Looks to me like he's just copying and pasting the comments of another...
 
LSAS = Longitudinal Stability Augmentation System - enhances longitudinal stability and provides:
  • Pitch attitude hold
  • Pitch attitude limiting
  • Pitch rate dampening
  • Automatic pitch trim
  • LSAS speed protection
  • LSAS stall protection
LSAS is off when autopilot is engaged.​
SCAS = Stability Control Augmentation System
RCWS = Roll Control Wheel Steering

--------

So, what I read from this is that he is claiming the MD-11's longitudinal stability assistance does not provide compensation for instabilities present when the aircraft is in a typical flight condition near landing (flare?). There is provision for compensation in the software, but it is not employed on the MD-11.

And, the elevator actuators don't have sufficient oomph to move the surfaces at the rates needed to counter those oscillations (so even if the LSAS, the autopilot, or a pilot were providing appropriate commands, the surface couldn't respond as needed?). The inability to respond can lead to what he calls '"cliff like" APC'.

All of which will lead to a higher likelyhood (than in a DC-10/MD-10) of landing with sufficient force being transmitted through the landing gear to cause damage.
 
So, while it is not okay to produce your own speculation, it IS okay to post someone else's?

Looks like it.

Also, with PMDG offering a kick ass MD-11 for FSX now every wannabe airline pilot can experience what it was like! How Cool!!! :sarcasm:
 
While it does have a definite air of "a friend-of-a-friend told me...", the information in that the OP posted does seem to at least meander down the same path as a lot of other information that's been thrown up on the interwebs regarding MD-11 handling in the landing flare. I dunno though. Time (and the NTSB) will tell.
 
Went out for beers, came home to log onto JC and thought I logged onto a different forum. Wait, let me check the URL address again? Nope, not flightinfo. I'm confused. And out of beer.
 
Do you fly/instruct on the -11?

The person who is my source is an MD-11 check airman. The factual events of the crash are a bit more than just the airplane.

Seeing as it's not appropriate for me to say what I have been told, on top of the fact that my source could easily be figured out, I will just say that there is more to this crash than a peviously known handling "quirk". Not my place to comment though.
 
It would be fair to say that I have more knowledge than most on this topic, and, as expected, I cannot comment at this time.

I will, however, comment on something else that was said there. Bob Benzon is one of the sharpest IIC's that NTSB has (currently the manager for Major investigations). It appears he was either misquoted or the quote attributed to him was taken out badly of context. In any event, anyone who has been around this industry, particularly on the accident investigation side knows that Bob is one of the best out there.

Also, one minor note, the NTSB is only the accredited rep to a non-U.S. accident. The investigation is conducted by the State in which it occurs, JTSB in this case.
 
The person who is my source is an MD-11 check airman. The factual events of the crash are a bit more than just the airplane.

Seeing as it's not appropriate for me to say what I have been told, on top of the fact that my source could easily be figured out, I will just say that there is more to this crash than a peviously known handling "quirk". Not my place to comment though.

Fair enough. But the all accidents are unique and involve more than just one factor.

FWIW, I have spoken to two friends. One flew the -11 and the other instructed on it. I asked specifically about handling during landing and both said essentially the same thing.. it could be a real handful. One remarked, "I always felt like it was getting ready to bite me in the *ss if I relaxed just a little." Both are high time aviators with a broad range of experience so their remarks were not the casual observations of some plane spotter who flew MSFS. (last comment made not to insult anyone)
 
There's a difference between uninformed speculation and informed speculation, and neither of the two are right 100% of the time.

Even accident investigators must sometimes speculate.
 
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