Sir - There are parts of the world where it is impossible to install a precision approach (Flight International, 6-12 March, P5 and 20-26 March, P100) because it does not meet International Civil Aviation Organisation standards, so a non-precision approach is used, in most cases without terminal-approach radar at the airport.
Surely a precision approach is safer than a non-precision approach using an instrument landing system at a pre-calculated rate of descent which, in bad weather and reduced visibility, makes it difficult, if not impossible, to establish visual reference.
A non-precision approach should be flown as such. After passing the final-approach fix inbound, a descent to minimum descent altitude (MDA) should take place immediately. The sooner the aircraft reaches the MDA the better.
DIMITRIS VOURDOUBAS
Athens, Greece
Sir - It is clear from Capt Passmore's comments (Flight International, 20-26 March, P100) that he did not misuse the term minimum descent altitude. The term is used only for non-precision approaches.
The real issue concerns the dangers in flying any approach with the aircraft in an unstable situation. The non-precision approach will be with us for some time in those parts of the world where resources are limited. The fact that many long-haul flights are operated to destinations served by these approaches and that these procedures are often flown at night at the end of a maximum duty or flight time day highlights the hazards.
All pilots should be trained to fly a constant slope in a stabilised configuration. Many approach plates show a conversion table to calculate rate of descent against ground speed for a constant slope from the final-approach fix to the missed- approach point. If ground speed is not displayed or not able to be calculated, then the probability of a missed approach is increased. On a 3û path the difference between 100kt (185km/h) and 120kt is about 100ft/min (0.51m/s). A missed approach from a stabilised approach is just like a missed approach using an instrument-landing system - a procedure for which all instrument- rated pilots receive extensive training. The second attempt, will then be based on knowledge of the real effect of the prevailing wind, or other factors.
Capt John Raby
Vienna, Austria
Source: Flight International
