FAA History Lesson -- July 31 (08)

From the FAA Historical Chronology, 1926-1996...

”Jul 31, 1974: A Delta Air Lines DC-9 crashed against a sea wall while making an instrument approach to Logan International Airport in Boston, Mass., with the loss of 89 lives. The National Transportation Safety Board attributed the accident to flight crew error. Although the Board also named "nonstandard" air traffic control service as a contributory factor, a U.S. district court cleared FAA of liability. “

In a rare mistake, the FAA’s historians got the date wrong. This crash occurred in 1973. It’s evident when you start to search for it on a site like PlaneCrashInfo.com.

This crash, like so many others, is a chain of mistakes. Everyone takes away something different when they read these reports. In that air traffic control was cited as a contributing factor, I concentrated on that portion.

”As Delta Flight 723 was descending, the approach clearance was given by the controller after a delay, because the controller was preoccupied with a potential conflict between two other aircraft. This caused the flight to be poorly positioned for approach. The aircraft passed the Outer Marker at a speed of 385km/h (80km/h too fast) and was 60m above the glide slope. “

We refer to this as “hot and high.” I don’t know if the accident was the one that started talk of “stabilized” approaches but this is the type of accident the safety folks teach.

Many pilots out there still aren’t believers in stabilized approaches. Controllers are taught to vector aircraft far enough out on the localizer that they can be descended to an altitude below the glideslope. This allows the aircraft to be level and join the localizer (the horizontal guidance to the runway) and then capture and follow the glideslope (the vertical guidance to the runway.) That is, unless the pilot requests a vector in closer.

5-9-1. VECTORS TO FINAL APPROACH COURSE

”2. If specifically requested by the pilot, aircraft may be vectored to intercept the final approach course inside the approach gate but no closer than the final approach fix.

b. For a precision approach, at an altitude not above the glideslope/glidepath or below the minimum glideslope intercept altitude specified on the approach procedure chart.“

Pilot after pilot will ask to be vectored “in tight” until some controllers start to believe that all pilots want to be handled this way. If the controller gets distracted (as was the case in this accident) the airplane can wind up too high and in too close to the airport. It makes the pilot’s job difficult as he tries to get down, slow down and capture the localizer and glideslope in rapid succession.

Everyone -- pilots and controllers -- is taught that it’s a very bad idea to “chase” the glideslope. It puts the pilot in the position of descending rapidly (close to the ground) to try and get down to the localizer and then descend gradually on the glideslope beam. It’s just too easy to get below the glideslope at the wrong moment.

On a final note, if you’re interested in this sort of thing, you can usually find a pre-crash picture of an airliner if you have the tail number. Just do a search on it -- N975NE in this case -- and something will usually pop up. Here’s a picture of N975NE in Northeast Airlines’ colors -- before it was sold to Delta.

Don Brown
July 31, 2008