Safest approaches are those flown at 3¼ angle-

David Lonsdale's letter (Flight International, 23 December, 1998-5 January, 1999, P48) makes the very valid point that the safest approaches flown in a swept-wing transport are those that are close to a 3¼ angle, and the reply from SAS (Flight International, 6-12 January, P38) makes the point that SAS charts have a continuous descent with a 3¼path where possible.

Aerad developed, during its long time under British Airways' ownership, its own recommended altitude/heights tables which enabled the pilot to fly a 3¼ (or similar, if 3¼ was not possible) approach to the runway from the final approach fix or aid, on a non-precision approach chart where the state information shows a shallower approach. This does not conflict with the state information, but enables such state data to be flown in a 3¼ way.

During my 30 years flying with BA, I have flown many such non-precision approaches and can vouch for the improved safety provided when given this data. In Racal Aerad we intend to continue presenting this helpful information for all Aerad users. It is not just a case of a continuous descent, but of enabling a 3¼ descent to be flown in nearly all cases.

Brian Bristow

Marketing Manager - Airline Services

Racal Avionics


Surrey, UK

-rather than the stepped approach

I would like to endorse David Lonsdale's comments.

I have spent the last 18 months teaching US crews to fly "stepped" non-precision approaches during their conversion training on the British Aerospace Avro RJ simulator.

What should be a relatively straightforward procedure is made extraordinarily difficult by the large power changes required at each "step", especially when using asymmetric power. The approximated 3¼ descent makes for a much more stabilised approach. Even if there is no DME [distance measuring equipment] guidance, complying with published height checks makes the procedure easier and, in my opinion, safer.

But I would like to take the argument further. Why on earth do national authorities continue to approve missed approach points (MAPs) that are at, or adjacent to, the runway threshold?

Minimum descent altitudes are typically 400-600ft [120-180m] AAI and to go visual at the runway threshold is to invite the "dirty dart" for the runway, resulting in a totally unstabilised final approach and potential overrun. Using the 300ft/mile [57m/km] rule for a 3¼ slope, an aircraft on a stabilised approach will touch down about. 12,000ft past the threshold. There are not many runways that long.

I see no justification whatsoever for MAPs of more than 400m beyond the point at which MDA coincides with a 3¼ slope.

Having pilots fly level at MDA for 3km (1.6nm) or more, in a high drag, landing, configuration, looking for a gap in low-cloud visibility conditions, is as good as an invitation to the Board of Inquiry.

Terence Kenny

Wilmslow, Cheshire, UK

Heading goes in here

It is well known that fuel flowing from a nozzle can generate static electricity, so any that an aircraft dumps is likely to be ionised. Such a trail would provide a comparatively low resistance path for an electric discharge, thus predisposing the aircraft to a lightning strike.

Normally, lightning damage is confined to the entry and exit points. But what if the fuel/air mix in the emptying fuel tank was explosive? If the tank was a complete metal enclosure, then no electric charge could exist inside it, and thus no spark to act as an ignition source. On modern aircraft, though, fuel tanks form an integral part of the aircraft structure and must have apertures for wiring looms, etc. In this regard, dielectric cable, such as fibre-optic, could pose more of a hazard than copper cable.

Of course, no pilot would fly into a thunderstorm to dump fuel. But, if turbulence can exist in clear air, why not electrostatic potential? Readers may by now have guessed that this line of thought was prompted by the Halifax disaster. It could explain why the captain delayed putting the aircraft on the ground: in the course of appropriate action in response to a minor emergency, a totally separate mechanism caused a sudden catastrophe. There are also resonances to Nantucket. Here the aircraft was burning fuel on climb-out rather than dumping it, but the reports of "missiles" could have been St Elmo's fire, and the cause was an unexplained explosion in a fuel tank.

I would hope that the various investigations (in which I am not in any way involved) would have considered this line of enquiry. It may also be prudent to think through again the design and operation of modern aircraft, given the increasing use of glassfibre and composite materials.

R M Smith

Shefford, Bedfordshire, UK

Source: Flight International