Saturday, August 17, 2013

Dropping like flies

There was another high-profile jet crash last week, a UPS cargo plane this time, flown by American (or at least non-Korean) pilots.  Details are very sketchy (no civilian casualties means light news coverage in today's world) but superficially the crash looks an awful lot like Asiana 214.  Both planes crashed short of the runway with (apparently) no mechanical problems.  Both times pilots received warnings of impending doom seconds before the crash.

Nonetheless, I contend (tentatively, subject to new information coming to light of course) that the crashes are not comparable.  The Asiana crash happened under ideal conditions: a visual approach in perfect weather to a nice long runway.  The Birmingham crash, by contrast, happened (apparently) in bad weather in the dark.  I say "apparently" because the reporting on this crash has been absolutely abysmal.  Whoever is writing these stories clearly doesn't have the first clue about flying.  For example, here's an excerpt from the Christian Science Monitor (normally a very good source of news):
... a combination of weather (low clouds and raining), time of day (before dawn), and a tricky visual approach [emphasis added] over hills to the airport’s shorter runway because the much longer, more familiar runway – the one that provided glide slope as well as direction information to approaching pilots – was closed for maintenance.
A visual approach by definition is one where you can see the runway.  If there were low clouds and rain then the pilots were not doing a visual approach.

Here's another example, from ABC News:
National Transportation Safety Board member Robert Sumwalt ... said the aircraft went down during its first landing attempt. Sumwalt said investigators have not found any problems with the runway's lights or navigation system, which typically provides pilots with information about their lateral position but not about their altitude, unlike those on runways where pilots can land using only instruments [emphasis added].
This reporter is confused in so many ways it's hard to know where to begin to deconstruct this.  What the reporter (almost certainly) meant to say was that the UPS plane was landing on a runway that only had a so-called non-precision instrument approach procedure, which provides no vertical guidance.  But not having vertical guidance provided as part of the approach procedure is not even remotely the same thing as not having information about the plane's altitude.  Pilots always know their altitude, at least if they're doing their job right.  The difference is that in a non-precision approach the pilot has to manually cross-reference the plane's location against the chart to figure out at what altitude the plane should be, whereas in a precision approach there's a little needle on the panel constantly telling you whether you are too high or too low.

No matter what kind of approach you are flying, at some point you must be able to see the runway in order to land (with one very rare exception called a Category III ILS, which was not available at Birmingham on any runway).  The difference between a precision and non-precision approach is not that one lets you land using only instruments, but simply that a precision approach lets you descend lower before you have to either be able to see the runway or abort the approach.

Non-precision approaches are harder to fly.  On a precision approach you just "fly the needle" (or have the autopilot do it for you).  On a non-precision approach you must constantly cross-reference your position against a chart, and manually control the plane's altitude so that it is close to -- but never lower than -- the minimum altitude allowed for the particular segment of the approach you are flying.  It is one of the most challenging operations a pilot is ever called on to perform, and it is what the pilots of the UPS plane were doing when they crashed.

The approach to runway 18 at BHM is particularly tricky because the approach procedure has unusually low margins for error.  The last waypoint on the approach has a minimum altitude of 1380 feet. The runway altitude is 650 feet.  Between the final waypoint and the runway there is a hill that is 915 feet high.  The minimum descent altitude before you must be able to see the runway is 1200 feet, less than 300 feet above the hill.  The official weather report said there were "few" (which is FAA-speak for scattered) clouds at 1100 feet.  And it was dark.

The "normal" margin of error in maintaining altitude when flying under instruments is 200 feet.  Deviating by more than that is considered a serious mistake, but it is actually not that uncommon, especially among pilots who don't fly very often (ahem).  That was obviously not the case here, but the point is that even under normal circumstances any pilot who flies this approach is one mistake away from death.  That is unusual.  Normally you have to make two or three pretty serious mistakes in a row to actually kill yourself in an airplane.  Not here.

So this accident, like Asiana 214, looks like it was very likely pilot error.  But I would say the two situations are not comparable.  Asiana was pilot error under ideal circumstances, a situation that any pilot should have been able to handle with ease.  The UPS crash was pilot error under some of the most demanding circumstances possible short of an actual emergency.  No less tragic, and no less of an error, but not nearly as indicative of a systemic failure as Asiana 214.

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