Page:CAB Accident Report, American Airlines Flight 383.pdf/23

– 22 – All events leading up to impact, including the final radio transmissions from the flight, indicate that the crew was not concerned with and totally unaware of a dangerously low altitude situation. In examining the many potential ramifications involved in the aircraft's deviation from the desired altitude profile one obvious possibility is a misinterpretation or misreading of the information being presented on the altimeters. It is noted that the drum altimeter has been in operation for many years and is considered by all segments of the industry to be an accurate, highly reliable instrument. While the Board has no reason to believe the drum altimeter is deficient in any way whatsoever, it does believe that improper monitoring of the instrument could possibly result in misinterpretation, and this in turn could help to explain the inappropriate descent involved in this accident.

It can be seen that in reading this altimeter the pilot must first look at the number below the index on the "drum" to identify the thousand-foot level, and then to the radial pointer to determine the hundred feet indication. Two separate readings are required to assess altitude. At stabilized altitudes, or at low rates of climb/descent where the drum is moving slowly or not at all, the pilot must be certain to associate the proper thousand feet indication in relationship to the hundred feet reading. For example, an altimeter presentation of 900 feet would show the "1" (thousand feet) slightly above the thousand feet (drum) index and "zero" below the index. The radial pointer would be pointing to the nine on the outside dial. A misinterpretation could occur if a pilot mistakenly associates the radial pointer reading with the "1" slightly above the index rather than the "zero" below the index for the thousand feet indication. The result would be a reading of 1,900 feet rather than the 900 feet actually portrayed–or, and error of 1,000 feet. It was also noted that AA altimeters have a crosshatched band adjacent to the numbers from plus 1,000 feet to minus 1,500 feet as a means of emphasizing these lower altitude values. In a descending situation to below zero elevation the radial pointer, rotating counter-clockwise, does not point to the actual hundred of feet below zero. For instance, 100 feet below zero elevation would be portrayed with the radial pointer on the outside dial nine and with the zero on the drum slightly above the index. This requires the pilot to interpret the nine hundred feet indicated as actually meaning 100 feet below zero. Additionally, when reading negative values the number above the drum index rather than the number below the index gives the correct thousand feet determination. In other words the drum presentation and reading reverses at below zero altitudes.

It is realized that in these cases or in any altitude presentation a pilot knowledgeable in the altimeter should have no problem whatsoever in quickly determining the correct altitude reading. However, it can also be seen that under certain conditions compounded by infrequent, fragmented, or distracted monitoring, a misreading/misinterpretation of altitude could occur.

Cockpit Workload

Another factor which must be considered in completely evaluating the apparent improper monitoring of altitude is the cockpit workload and crew coordination involved in this approach. It was shown that higher than normal airspeeds were maintained by the aircraft throughout most of the approach and that flap extension sequencing was concentrated on the base leg. As late as the turn to the final approach course, 2-1/2 miles from the runway, only 25 degrees (approach) flaps had been extended. In accordance with recommended operating