Sir - I know little of the Boeing 737 accident at Coventry Airport on 21 December 1994, accident, but alarm bells rang in my mind when I read that a surveillance radar approach (SRA) was used.
At Hamburg in 1991, I flew such an approach in instrument meteorological conditions. It was a short-notice approach because of the disablement of a Boeing 727 on the instrument-landing system runway. Contrary to what I had expected, air traffic control (ATC) only advised of distance to go - and not the expected altitude at each mile to go. I had the appropriate Jepps SRA chart available, and so could study the required profile.
I had expected ATC advice such as, "four miles - you should be 1,200ft ". Instead we were told: "Four miles - turn left 5°," and nothing about the altitude.
The approach seemed disjointed and uncomfortable because we did not know what increments would be used in the next call. This meant constant references to the distance/altitude scale on the chart by both pilots.
A problem with an SRA occurs if the aircraft goes high on profile. For example, a call of "...four miles to go, you should be at 1,200ft" is a fine, but suppose the aircraft is actually at 1,400ft. The natural tendency is to increase the rate of decent in an attempt to "catch up" by the time the next increment is called. On a ground-controlled approach, ATC would say, for example, "...slightly high on glide slope, increase your rate of descent". There is no such luxury on the SRA.
Add anxious piloting because of poor visibility, and the picture emerges of an aircraft high on slope, power reducing to minimise power build-up, and the inevitable increase in sink rate possibly going unnoticed.
Most medium/heavy airliners have maximum recommended sink rates of 1,000ft/min (5m/s) when below 1,000ft. Perhaps a cautionary call by ATC would be appropriate on all SRAs - eg, "caution - sink rate", which could be inserted at appropriate points down the approach profile.
John Laming
Tullamarine
Victoria
Australia
Source: Flight International
