Plane incredible

Justin Gudgeon observes in his correspondence 'You can't be serious' (

) that letters are often unchallenging. But sometimes challenges can drop out of the blue.

A small incident years ago, while descending into Heathrow, has intrigued me ever since the 777 crash landing in January.

Our rock-steady plane made a small unaccountable movement about four miles out. The pilot announced that so accurate is the wireless-controlled flightpath, that vortices from the aircraft in front, are often intercepted by the following plane, causing such movement.

Since numerous planes are now so similar, being of the large twin jet type, it is possible with today's congested landing frequency, that lingering, hot, low-density, vitiated exhaust gases from preceding aircraft could be sucked into the giant engine inlet fans.

This could compromise engine thrust, combustion performance and confuse the electronic engine controls, whatever the throttle position, at the critical moments.

The predominant thrust of the 777's engine is from the fan, and for fans it is inlet conditions that are critical.

With convoys of planes only 70-80 seconds apart, all locked into the single descent path, and with all aircraft increasing thrust (and exhaust gases) at exactly the same position (for final levelling off, including combatting enormous drag from deployed flaps and landing gear) it is possible that one day the above scenario, epitomised by the symptoms of the actual 777 crash landing in January, could occur.

Hopefully a definite cause of the 777 mishap will be found, but, in any case, the above possible silent, invisible, deadly menace could be lurking at any busy airport approach.

The sooner the planned mixed-mode operation is adopted at Heathrow (alternate take-offs and landings from both runways to increase capacity by 10 per cent) the sooner the above risk will be eliminated.