Toyota UA-LAC

[Scruggs Family]

Well, Brett... let's have a look at the situation.  Although my first degree
was in music my second is in aeronautical engineering.  I have a private
pilot license and am an inactive member of Mensa.  

I recently retired from 24 years at the Office of Naval Intelligence as a
Scientific and Technical Intelligence Analyst of... you guessed it...
aircraft platforms.  I've been to the Paris Airshow and to the Farnborough
Airshow many times.  I've been to the production facilities of every major
military and commercial airframe and engine manufacturer in the West.  I've
seen platforms and technologies that I still can't talk about.  

Prior to ONI I worked at the Arnold Engineering Development Center (AEDC) in
Tullahoma TN in the area called Engine Test Facility.  I've written many
reports on the early application of Digital Electronic Engine Control
systems for the Pratt & Whitney F110 Component Improvement Program.  I was
on engine incident investigation teams where we had to find why an engine
was damaged during test from ingestion of a foreign object.  (Did you know
that when a 'soft' object strikes a rotating turbine blade the leading edge
of the blade bends forward toward the front of the engine but a 'hard'
object will bend the blade leading edge backward?)  

Low on any list of credible observers in an airline incident is the typical
passenger.  They're usually panicked, not trained observers and ignorant of
aviation systems.  But going with media accounts of the passengers'
descriptions... 'bangs' are usually the result of what is called turbine
engine 'surge.' Turbine engines achieve their overall pressure ratio
(analogous to compression ratio) through incremental pressure increase
across each stage of the compressor section.  If this flow is interrupted or
degraded then often the path of least resistance is back out the inlet end
and is usually accompanied by a percussive 'bang.'  Flames are common during
a surge event.  'Odor of unburned fuel' is typical as now that flow has
reversed through the engine you get fuel into the compressor section's
'bleed air' manifold which provides the compressed air for the passenger
compartment.

The engines were not 'completely destroyed' they simply spooled down when
their fuel was shut off by the computer.  A turbine engine typically turns
in the vicinity of 12krpm, called 'N1,' and if anything enters the engine
with sufficient disruptive force to break or deflect a compressor blade it
will likely come in contact with adjacent stationary blades called
'stators.'  This causes a cascade of compressor blades breaking and causing
more damage.  (In extreme cases the engine will be said to have 'corn
cobbed' where the rotor will have shed all the blades.) These unrestrained
blades don't just flush out the hot end they travel radially through the
engine casing and then through the engine nacelle and into the fuselage and
through any unfortunate passengers.  A casual examination of the imagery of
the aircraft in question will show that neither the nacelles nor the
fuselage were punctured and there were no passenger deaths.  Certainly those
engines are junk but mostly because they have been cold cycled... running at
normal temperature then plunged into cold water.  

Turbine engines don't 'throttle down' as their power is not a function of
the amount of air passing through it but the amount of fuel that the
combustion section is given... just like a diesel engine.  Spark ignition
engines do have throttles because the power produced there is a function of
how much air is passed through the engine.

When reading an FAA report you must understand the dual responsibilities of
the FAA.  On one hand they are required to promote safety in aviation but on
the other they are tasked with encouraging commercial aviation.  After
you've read a few hundred of these reports you begin to see that what is not
said is the most important part.  Although charged with aviation accident
investigation FAA is also charged with not stampeding the traveling public
away from commercial aviation.  Thus the ironic softness with which the FAA
does its accident reporting.  Unless there is a very large incident they
scold in private not in the media. When there is a very large incident they
of course use the media face time to justify budget requests.

Regarding "... designed to tolerate ingestion of. (sic)"... this phrase
means that the engine will ingest a bird up to a certain size and continue
to perform normally.  Any larger bird and normal operation may be impaired.
Birds can shut down engines but I've not heard of a turbine engine being
destroyed by a bird regardless of its size.  Given these SNECMA engines are
of the high bypass ratio turbofan design a further consideration is whether
or not the birds struck in the compressor core or at the bypass section
where the first stage blades direct the air around the engine core... one
will compromise engine efficiency and the other will just make a mess.  

Media coverage of technical events should generally be dismissed out of
hand.  Reporters have little or no technical background and gravitate toward
the sensational at the expense of accuracy.  Only industry publications like
Aviation Week are to be believed or quoted for instances like this.
Wikipedia is a good place to start an investigation but as peer review is
only voluntary it is not a good place to conclude an investigation.  

So then, Brett, what I've offered the list is not a regurgitation of the
ignorance found in the media but a bit of insight into a possible cause of
the 1549 incident from the standpoint of someone who does indeed have both
knowledge of the physical systems involved and of the culture of those
entities charged with the investigation of those incidents.  The offer of
this theory was intended to stimulate thought regarding the relationship
between automotive and aviation digital control systems and the great
potential for difficulties when trying to program a system for all of those
'not likely to be encountered' four sigma situations.  

Two suggestions... don't bring a knife to a gunfight... and try decaf.  

Regards,
Gross Scruggs
Annapolis MD