Aircraft Accidents and the Forgotten Engines

On January 15, 2009, US Air flight 1549, an Airbus A320, took off out of LaGuardia Airport and intercepted a flock of Canadian Geese going in the opposite direction.  Both CFM56-5B4/P engines were FOD’d out, meaning they received Foreign Object Damage and stopped putting out power, or thrust.  The airliner was ditched in the Hudson River.

While I was researching this accident for a class I’m teaching, I came across an AMT Magazine article (Foreign Object Damage, Chandler, 5/15/2009), which reported that the two accident aircraft engines: CFM56-5B4/P series, manufactured by CFM International – a joint effort by General Electric (GE) and Société Nationale d’Étude et de Construction de Moteurs d’Aviation (SNECMA) – were sent to the manufacturer’s Cincinnati facility for a teardown within weeks of the National Transportation Safety Board (NTSB) onsite investigation.  This is normal procedure, because the manufacturer has available to it all the necessary overhaul tooling, specifications, and full access to the design engineers.

The NTSB accomplished their due diligence, e.g. pictures, analysis of bird remains, etc. to the recovered engines before shipping it out for teardown, which is removing components and ducting, exposing the engine’s inaccessible parts for better analysis.  The teardown allows the manufacturer to determine exactly what components failed and the extent of damage incurred.

NTSB report AAR 10/03 has the initial information the NTSB engine/powerplant team found.  It is unclear what information comes from the teardown and what comes from the initial on-site inspection.  There are no references to the Cincinnati facility’s teardown, beyond biological findings.  Furthermore, Search Engine queries cannot find any evidence of a published CFM International report concerning the two accident aircraft engines’ teardown.

This is unusual.  The NTSB is meticulous; they investigate every technical anomaly.  Even general aviation single engine aircraft investigations result in an engine teardown.  When a Cessna 150 has an inflight engine failure or a Beech Bonanza suffers a propeller strike, the engine case is split, the cylinders are checked for scoring; pistons for bearing wear; the crankshaft is inspected for sudden stoppage damage.

However, US Air 1549 was a high-profile accident, its recognition tied directly to both engines, damaged by bird ingestion, that the two engines could not provide adequate power to reach an airport.  How does an investigation of this caliber stop here?  Where are the CFM studies?

Reading through NTSB Report AAR 10/03 can be confusing; facts are cited, but sources are unclear.  The only reference to the teardown comes under Section 1.16 Tests and Research, subsection 1.16.2 Biological Material Sampling and Analysis.  Inside this subsection, the NTSB describes the different samples of biological material procured, e.g. bird tissue, bone and how they damaged the engines’ Outlet Guide Vanes.  Later in this Section the Engine Dual Failure checklist and Airplane Performance Studies are discussed.

In the Analysis Section, subsection 2.1 General, AAR 10/03 states, “Although both engines experienced an almost total loss of thrust after the bird encounter, the flight crew was able to ditch the airplane in the Hudson …”  This statement is ambiguous; ‘Almost’ is imprecise.  Does ‘almost’ mean 1% thrust was available? 5% thrust? 10% thrust?  Does ‘almost’ apply to one or both engines equally?  Does ‘almost’ imply the engines were turning and producing adequate power?  Too much information is left to interpretation.  Furthermore, it contradicts information found later in the report, including conversations captured by the cockpit voice recorder (CVR) transcripts.

As one moves forward through NTSB report AAR 10/03, it is stated that the #2 (right) engine suffered a compressor stall days earlier, but that the stall was corrected in maintenance.  The flight data recorder (FDR) parameters suggest that a compressor stall was not a culprit in the accident, that a compressor stall did not reoccur.  This works because it defuses any speculation that the compressor stall was a possible cause.

In Section 2.2 Engine Analysis, subsection 2.2.2 Identification of Ingested Birds, the report says each engine ingested two Canadian Geese weighing between 7.3 through 9.2 pounds each.  This exceeds the size of bird the engines were certificated to ingest and still provide power to sustain flight.  Why?  What damage would larger birds impose on the Fan?  What Turbine damage?  Would one overwhelm the Burner section?  The report doesn’t say what the consequences would be.  What would CFM do to fix these problems in the future?

In subsection 2.2.3.1 Engine Spinner, Fan Blade and Fan Inlet Case Damage, harm to each engine Fan is documented; as reported, “Although the fan blades of both engines showed evidence of bird ingestion and subsequent mechanical damage, as noted, no significant fan blade damage or fractures were found.”  This means both Fans were physically capable of providing thrust; their integrity was intact.  In subsection 2.2.3.2 Engine Core Damage, the report reveals damage to the Low and High Pressure Compressors, Inlet Guide Vanes, Variable Guide Vanes (VGV), which, as reported, is significant.  The core’s damage is enough to prevent sufficient thrust to sustain flight.

Here this reader is confused; this makes little sense.  No matter the size of the bird, for the meat and bone to damage the Compressor Core, the carcasses would have to first pass through the Fan, which, per the previous paragraph, did not incur significant damage (no Fan blades were missing or broken).  The Fan, at takeoff power, is turning at over 5000 rotations per minute.  A bird’s flesh and bone, no matter the size, passing through the Fan, would be reduced to the consistency of a liquid, a thick milk shake and/or an atomized mist; the Fan would be chewing up the meat and hollow bones at an incredible rate.  In addition, the birds strike the engines at close to two hundred miles per hour; at this point they begin the ‘Cuisinart Effect’, reducing the carcasses integrity from solid to liquefied sludge.  Result: if the carcasses did not destroy the Fan, then any subsequent damage had nothing to do with the size of the birds.  Past the Fan, bird size is irrelevant.

So why were the Compressor blades damaged?  Per AAR 10/03, the Compressor Core blades and Guide Vanes were significantly damaged; this is factual information.

NOTE: As air passes through the Compressor, it is compressed to incredible pressures; this is because a gas (air) is compressible.  Liquids and solids, however, are not compressible.  As the carcasses passed through the Compressor, the pureed carcasses resisted compression; pressure feedback damaged the blades.  This raises an important point: If the engines were certified to ‘provide sufficient thrust to sustain flight’ after a bird ingestion, why did both engines fail?  Did they fail?  Were they capable of operating sufficiently and awaiting the command to throttle up?  What did CFM International find out about this?

The CVR shows the bird impact happened at 15:27:07 (15:00 hours, 27 minutes, 7 seconds).  At 15:27:54, the crew attempts to relight an (?) engine; 15:28:25, number one engine comes back up ‘a little bit’; at 15:29:00, an (?) engine relight fails; 15:29:21, something about power on number one followed by 15:29:26, “go ahead, try number one.”  At 15:30:09, “Got no power on either one?  Try the other one,” (are they discussing an engine or a power bus?).  The question here is: At 15:28:25, did they have power on Number One engine and, if so, did it shut down and when?  If they did have sustainable power, why does the NTSB report say there was no power output while simultaneously saying, ‘an almost total loss of thrust’?  Could number one engine have ‘sustained flight’?

Why is a report from CFM International important?  Because no organization understands the limits of the CFM56-5B4/P engine better than the manufacturer.  The NTSB has to be forgiven for not being able to ‘fix’ the issues that caused the accident aircraft’s CFM56-5B4/P engines to fail.  In the investigation of all major accidents, the NTSB is a ‘jack of all trades, master of none’; they cannot adequately discover problems, analyze problems and fix problems better than the engine manufacturer.  And the NTSB cannot adequately document those ‘fixes’ better than the manufacturer.

I have not been able to locate the CFM Industries report.  It would provide information about what needed to be fixed, why the engines failed to sustain power, etc.  If the engines were unable to perform as certificated, what could the crew have done to save the plane?  Was there something they could have done?  And … what if the plane was savable to begin with?  That is why, the CFM International report is so important.

2 thoughts on “Aircraft Accidents and the Forgotten Engines”

  1. Very interesting article. You raised honest questions.
    Starting with Mohawk and the other airlines to be acquired; I was aware of the mechanics and the continuous schooling they went through.
    They were all basically conscientious about their work and it extended beyond aircraft.
    I think this is one of the most honorable professions.
    I enjoy these articles. Just wanted you to know…..

    1. Carol, Thank you for writing in, you are right, we are in an honorable profession. I sincerely hope there is a report that I may have missed, but I haven’t been able to find it. Thank you, also, for reading my articles; your attention is high praise.

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