Crash of SR111

Date:         09 Sep 98 04:12:08 
From:         James Matthew Weber <m.weber@duc.com.kw>
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As is my custom in these posts, the usual dislcaimers apply. This post
involves a good deal of speculation that at least is not yet supported by
factual material.

Based upon the track of the aircraft,I can't help noticing some
similarities to the EL-AL Freighter crash at Amsterdam, and some wisdom
passed on to me by a very senior American Airlines DC10 driver shortly
before his retirement.

At this stage it isn't clear the initial event was. I suspect some kind of
power plant problem on one of the under wing engines, probably the right
engine.

Observation number 1. If it is bad enough that you want to make an
emergency landing, you probably should make As soon as possible. While the
efforts to protect the airframe and not do an overweight landing, or
overstress the engines (as in Air Florida 90) may be laudable, all you may
be doing is protecting the equipment for the post accident investigation. A
thumbnail calculation says the MD11 was unlikely to be more than 100,000
pounds over MLW. A heavy landing that might damage the aircraft, however if
you don't get it on the ground in a hurry, as my friend the captain put it,
once things go wrong, they tend to get worse, and they often do so very
suddenly, and in a very unpleasant fashion. Ergo, SR111 should have made
the dive for the runway, and not bothered to try to dump fuel and carry out
any sort of maneuvers. The high speed descent would probably have also
prevented the sequence of events that probably resulted in the crash.

Exactly what goes wrong isn't clear, at this stage, but things obviously
did go from bad to worse (pilot's call goes from level 2 emergency to level
1), and my guess is that is the point at which one engine really does pack
it in, and probably takes one of the hydraulic systems with it.

Observation 2: All jetliners since the 707 have incorporated a feature
called a rudder ratio computer. The purpose of this device is to maintain
an almost identical feel to the rudder control throughout the flight
envelope. It is sort of analogous to variable ratio power steering. Its a
feature that very few pilots think about very much, however I suspect this
is the third accident in the last decade in which this device lead the
pilot to a fatal mistake.

The Rudder ratio computer masks the loss of rudder efficiency with reduced
airspeed until the rudder authority is GONE. In theory if you keep track of
minimum control speed this doesn't happen, however it is obvious in the
heat of battle, this very important issue seems to have been lost several
times. If you look at the crash of the El-AL 747 freighter, it is clear the
crew losses control of the aircraft, and in fact I believe the FDR showed
that at the time of the crash, the aricraft was in fact below minimum
control speed. A similar accident occurs to an RAAF C135 a few years back
simulating engine and hydraulic failure (the guy decided to do so at about
1500 feet, so when the airplane got away, it was in the water in a flash).
The RAAF pilot flew right down the edge of rudder authority, and when he
needed a lot of rudder assistance, there was nothing left.

My suspicion on SR111 is in the descent with the loss of the engine, the
crew lost track of minimum control speed, and lost the required rudder
authority. As I said, a dive for the runway would have maintained a very
high air speed, and while it might well have damaged the aircraft on
landing, the crew and passengers might well have lived to discuss their
'night to remember'..

As I mentioned previously, this analysis is highly speculative, and at
least at the moment there is not a lot of data to support the hypothesis.
James Matthew Weber
Service Delivery Manager

Diyar United Trading and Contracting Co.
P.O. Box 44240
Hawalli 32057
State of Kuwait
PH +965 434 0560 x 230  FAX +965 431 5107 Mobile +965 971 2069