Use of autobrakes with full reverse. Who is fooling who?
Many 737 operators require autobrakes to be used for all landings as a matter of company policy. This is in addition to the commonsense need for autobrakes for operational reasons such as short runways, strong crosswinds and other situations recommended in the FCTM. One operator for example directs pilots to use auto brake 2 for all landings regardless of runway length.
Typical rate of deceleration figures for the 737-300 with autobrakes operating are given as:
Autobrake 1 1250 psi and 4 feet per second/per second.
Autobrake 2 1500psi and 5ft per second/per second.
Autobrake 3 2000psi and 7.2ft per second/per second.
Autobrake MAX 3000psi 12ft per second/per second below 80 knots and 14ft per second/ per second above 80 knots.
I have no guarantee the above numbers are correct (in fact I have a suspicion that autobrake MAX numbers are a bit out) - but near enough for the purpose of my question below, and that is:
Assume touch down on dry surface using Flap 40, max landing weight (737-300) and full reverse applied immediately. Assume autobrakes are not selected and manual braking not commenced yet. What would be the expected rate of deceleration caused by full reverse initially until down to (say) 80 knots? This includes auto-speed brakes operating.
For example, could the initial rate of deceleration from speed brakes and full reverse on touch down, be as high as the equivalent of (say) autobrake 2? I haven't got a clue which is why I ask the question.
The question arose in the classroom when discussing the autobrake systems where the amount of brake pressure applied is reduced automatically as reverse thrust comes in, in order to maintain a set deceleration rate.
Recently, the captain of a 737 decided to see how quickly the aircraft would pull up with full manual braking in conjunction with full reverse. It proved an unwise and costly exercise. As expected, the aircraft stopped in a remarkably short space. On arrival at the terminal the brakes were very hot and all tyres showed sign of heavy wear and tear including clearly defined flat spots.
Word quickly got around via the QAR and soon there was some spirited discussion over tea and bikkies with the captain pleading the anti-skid operation must have been defective to allow such obvious wear on the tyres. However a check of the anti-skid system showed no such problems and the offender was told he was a naughty boy.. Others would have taken a stronger view but there we are.. Imagine the cost of replacement tyres.
Where company policy mandates use of autobrakes at all times regardless of the circumstances, would it be correct to say that by using full reverse immediately on touch down, the initial rate of deceleration is so high that it far exceeds that of autobrakes 1,2, and even 3? Any use of autobrakes is bound to increase wear on the tyres since they are applied at high speed and thus high energy. So what is the logic of using the autobrakes on normal no sweat landings if the intention is to use full reverse on touch down and leaving manual braking until needed?
The FCTM clearly states the occasions when autobrakes are recommended. In many companies that seems carte blanche to use autobrakes for ALL landings, which is not what the manufacturer intended, I'm sure?
Last edited by Tee Emm; 8th October 2011 at 10:41.