Speedbrake on the B757

[eeper]

In the event of the auto-speedbrake being inoperative, our company procedures call for manual deployment of the speedbrake on touchdown only after the nose has started to lower.

Why should this be, given that, when the auto-speedbrake is functioning, it will deploy as soon as the main wheels touch the ground, irrespective of the attitude of the nose.

Also, should I be deploying the reversers before or after I select the speedbrake? Sounds very anal, I know, but neither the Captain or I knew the correct answer on a recent trip.

[Diesel8]

I would say speedbrake (spoilers at this point?) first to make the AC stay on the ground, then T/R's to make it stop.

[askcv]

A long time since I flew the type, but rapid manual selection can cause the nose to pitch up and this can catch the unawares out. A definite chance of tail strike. The reason, I was told, is that the spoiler panels will come up in sequence normally, with the outboard ones last, to avoid this problem but if manually selected they all come up together. But I could be wrong! I've noticed a similar thing with the 747 Classic, but not the -400.

[MetroDog]

The reason is most definitely to reduce the possiblity of pitch-up and the resultant tailstrike. Rapid application of spoilers with one hand on the yoke and the other pulling back on the speedbrake lever could result in a tailstrike.

[shlittlenellie]

For further info on ground spoilers and reversers - try searching the AAIB site with respect to the Airtours 757 runway excursion at Gatwick.

One main finding was that specifically in the case of single reverser deployment the ground spoilers should be deployed first; ensures continued directional control on the ground even though 757 reversers are not particularly effective.

[Expert]

To expand the topic slightly, Boeing state that with a L and L&C HYD SYSTEM(S) PRESSURE problem, that the speedbrakes have extended prior to using the R reverser.

The reason for this is that an incident occurred where a 757 lost the left hydraulic system, including the PTU, resulting in loss of nosewheel steering. After landing the crew selected the right reverser first. The yaw caused by a single engine reverse thrust was countered with the rudder. The 757 rudder is tall enough to cause roll which unloaded the wheels on the left side. The anti-skid sensed this and reduced brake pressure. The result was that the airplane departed the side of the runway at high speed. Selecting the speedbrakes first ensures that the maximum weight is on the wheels. This increases the braking force which is more effective than the reverse thrust as well as helps keep the wheels more equally loaded.

[QAVION]

"A long time since I flew the type, but rapid manual selection can cause the nose to pitch up and this can catch the unawares out. A definite chance of tail strike. The reason, I was told, is that the spoiler panels will come up in sequence normally, with the outboard ones last, to avoid this problem but if manually selected they all come up together. But I could be wrong! I've noticed a similar thing with the 747 Classic, but not the -400."

I can't envisage how the spoiler deployment sequence would vary depending on whether it's initiated manually or automatically. All the "auto" does on a 747 is drive a motor which drives the speedbrake lever mechanism in the cockpit .... from then on, it's basically the same mechanism.

Rgds.
Q

[GMS]

The official Boeing Flight Crew Training Manual has a nice diagram with the increase/decrease in stopping distance, from an RTO at V1, for various cases such as not selecting spoilers, or using one thrust reverser during deceleration.

As we know the normal RTO distance is based on full brakes and spoilers only, no reverse. In the same situation, adding the use of one thrust reverser only decreases the stopping distance by 120 ft. Therefore why bother with more than reverse idle when only one reverser is available? I am, of course, refering to the landing case where there is a big safety margin built into the figures.

[This message has been edited by GMS (edited 09 September 2000).]

[This message has been edited by GMS (edited 09 September 2000).]

[askcv]

For Qavion. You are right I guess, as i said its been a long time. I am confusing the problem with one where the hydraulics are not fully operational, which causes the outboard spoilers to come up ahead of the inboards, and so the pitch up.. It can be very pronounced.
Sometimes it did it even with auto spoiler and everything normal, which leads me to believe there is a sequence and if that sequence is upset, the nose will pitch up on landing.

[av8er]

I am not a trainer, but following the Airtours runway excursion made a firm decision in my mind to deploy the speedbrake before using the reverser.

I understood it to be that the reverse thrust dumped a load of lift off the wing and if reverse was applied on one side without any speedbrakes (spoilers for the technocrats) then the weight would be unevenly loaded and cause a harsh swing - but I like the rudder lift theory mentioned above and it does help explain something else I have seen recently.