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View Full Version : Slippery runway crosswind. MAX autobrake setting not recommended by Boeing


Judd
21st Apr 2018, 14:09
Reading the magazine "Boeing Airliner" dated April 1974, there were several pages devoted to the Auto Braking system of the Boeing 747.
One paragraph caught my eye and it stated:
Q. Can early brake application cause difficulty in controlling the airplane at touchdown under adverse conditions of crosswind and slippery runways?

Answer by Boeing: "Considerable flight test experience under such conditions has shown that when testing autobrakes on the MIN and MED settings the airplane is actually more stable and controllable at touchdown than when no braking is applied.

It is true, of course, that tyre cornering is reduced during braking. However, the smooth brake application rate combined with the stabilizing effect of the brakes and the additional weight pitched forward onto the nose gear combined to more than offset any small loss in cornering.

On the MAX setting, however, the sudden hard brake application may cause a significant loss in tire cornering. Therefore, selection of the MAX setting is not recommended under adverse runway conditions."
............................................................ ........................................

While that was the Boeing position on the B747 in 1974, is the statement "Therefore, selection of the MAX setting is not recommended under adverse runway conditions," still valid now to other Boeing types? Certainly in the various B737 series FCTM, there is no mention of this recommendation that the MAX setting is not recommended under adverse runway conditions.

On the other hand, the FCTM does state if landing in a crosswind on a slippery runway and if the aircraft starts to weathercock into wind, the brakes should be released and reverse set to idle reverse until directional control is regained. But it doesn't say the MAX setting should not be selected for the landing.

Comments requested?

wiggy
21st Apr 2018, 15:39
Therefore, selection of the MAX setting is not recommended under adverse runway conditions," still valid now to other Boeing types? Certainly in the various B737 series FCTM, there is no mention of this recommendation that the MAX setting is not recommended under adverse runway conditions.

Just had a v quick scan and can see no mention of this in the 777 FCTM.

Cloudtopper
21st Apr 2018, 15:57
Judd,

You do realise there has being technological advancements since the 70s.


Currently there are no procedures or recommendations in any current Heavy Boeing jet manuals .

mustangsally
21st Apr 2018, 18:32
Forty year old comments, may still have some merit, but technology has maybe gone through three major renovations. The most advanced carbon fiber brake designs and braking computers have very little in common with the early seventies. The 747 had few restriction on the application of reverse thrust application at touch down. Most twin engine jets want weight on the nose wheel prior to reverse thrust being increased.


Another factor that I'm sure will be commented on is the desire for a firm touch down with a wet and/or slippery runway. The loss of five or ten knots at touch down help with lateral control and reduce landing distance. Back in the old, (I really mean old) Saber Jet days we need to demonstrate a landing with the loss of fifteen knots at touch down, anti-skid was not installed. It was a solid plant but far from hard touch down.


Now Google, Boeing demonstrated cross wind touch downs and note how the aircraft is far from aligned with the runway. It appears to be at least 15 to 20 degrees from alignment with the runway. OCH, but I'm sure a good number of knots were lost at wheel spin up.


The Boeing article mentions corning, which I assume it the tire/wheel assembly friction with the runway. A skidding wheel has little to no friction with the runway surface. Recall a few close calls when braking your car on ice, left/right turn with a locked wheel did nothing, this of course is prior to anti-skid systems becoming standard.


Another observation, jet jockeys from certain back grounds want to load up the nose shortly after touch down, (yoke or stick fully forward). Knock, knock, dude where are the breaks? So lets load up the nose with no brakes and unload the mains with brakes? Had many a discussion trying to break this habit.
Live long and prosper...