Mexican Unions Reacts to the Crash

The pilots and controllers unions here in México have been quite active in their refutation that wake turbulence could have caused the Lear jet to crash last Tuesday.  I wanted to take the time to respond to some of their statements.  While I respect their expertise, I am quite confused by their responses… and I wanted to present this discussion as a complement to earlier discussions of the reaction to the public.


On Wednesday, the day after the crash, ASPA (Asociación Sindical de Pilotos Aviadores), which is the Union for Mexican Pilots, discounted the possibility that wake turbulence could have been an issue:

“Se pudo haber movido un poco, pero eso no es ninguno motivo de peligro o de riesgo o de intuir que eso pudo haber tirado al avión”

Basically: “(The plane) could have moved a little (due to wake vortices), but there’s no reason to believe this could have caused a risk or danger to the plane.” 

— ASPA spokesperson Leonardo Sánchez Herrera — El Universal, 5 Nov. 2008

True, wake vortices (which are caused by all aircraft, all the time!) often do not result in any harm, ASPA’s conclusion is completely incorrect.  First, as I originally posted (“Why things fly…”), wake vortices have caused accidents before, such as American Airlines flight 587… but there are others, too.  While I am not a pilot (nor an expert on flying), I’ve had personal experience with wake turbulence as a passenger and can assert that “move a little” is a vast understatement (we rolled 40° in a Boeing 757).  In both my experience and in that of AA 587, these were large aircraft, which means that they were much more able to handle turbulence than a smaller Learjet.  Perhaps the best analog to this Lear jet in the wikipedia list is the accident which killed Rich Snyder (of In-N-Out fame), or this article about a Cessna that was flipped as if it was made of “balsa wood”: small jet + wake turbulence = uncontrolled roll + crash.  I’m not sure where ASPA is coming from when they say that the wake turbulence of a big jet could not have had an effect.


Then, on Saturday, SNCTA (Sindicato Nacional de Controladores de Tránsito Aéreo), the union of air traffic controllers, asks:

“¿Cómo es que no afectó al helicóptero que venía volando cerca de él?”

Or: “How is it that the wake vortices didn’t affect a nearby helicopter?”

— SNCTA spokesperson Leonardo Sánchez Herrera — El Universal, 9 Nov. 2008

I am absolutely baffled by this comment.  To understand why, requires a little background on wake vortices.  Simply put, as aircraft fly through the air, they create very turbulent, but highly localized, vortices in their wake.  It’s not as if the dangerous wake vortices affect everything — the helicopter simply did not fly through them.  Frankly, air traffic controllers know this.


Furthermore, SNCTA also reported on Saturday that radar says the plane was 4-5 miles away from the aircraft in front and not 2.8 miles as previously reported.  First, I can’t verify whether or not this is true, though I do wonder 1) why did aircraft controllers initially report a distance of 2.8 miles and 2) why did aircraft controllers ask the Learjet to reduce to the minimum speed possible (which would increase this aircraft separation)?  Second, how do they know that 4 miles is sufficient?  El Universal cites:

“…el manual de Jeppesen indica que un Learjet 45 debe estar a cuatro millas de un Boeing 767.”  

Basically: “The Jeppesen manual says that a Learjet 45 should trail a Boeing 767 by 4 miles”

— Miguel Ángel Valero, president of the Mexican Pilots College — El Universal, 8 Nov. 2008

However, the ICAO standard miles-in-trail would be 6 nautical miles.  (I mention ICAO because the nice folk at the airport threw the ICAO books at us when we pointed out that the design of terminal 2 could have been better… a story for another time!)  And I can attest that, in my own personal experience (read above!), we were five miles behind the plane in front of us… at least according to the air traffic controller who I was listening to at the time.  

So that’s the story today.  I maintain that wake vortices could have been a factor in the crash of the plane.  And, for now, I’ll refrain from drawing any conclusions as to the unions’ comments.


México, D.F.


Sunday update: EL Universal, 9 Nov 2008: The distance between the jets was 3.9 miles; El Universal’s experts disagree about whether this was sufficient to avoid problems with wake turbulence.  They also discuss the possibility of failure of the Learjet’s air brakes (a mechanical part of the plane) that the pilots could have tried to deploy to slow the plane just before the accident.


2 Responses

  1. […] turbulence be to blame?  Wake turbulence creates a highly localized but very severe vortex (see post on the unions) that, in theory, could have trailed the 767 by several miles.  The Learjet, which was on approach […]

  2. […] mechanism by which pilots gain and maintain their certification.  Recall the strange comments (see this past post on Mexican unions) made by the ASPA and SNCTA spokesmen just days after the accident… I wonder what ASPA has to […]

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