Speedbrakes at FL350 ????

[BA@LHR]

Hi

I just came back from backpacking around China and an airline i flew out there was China SouthWest Airlines in a Boeing 757-200.

We took off from Lhasa in Tibet and did not start our take off role on the centre line of the runway which was a bit alarming. Then in the cruise at 35000ft, speedbrakes or spoilers, (whats the difference please?), were selected on both wings for about 5 mins until the B757 started to shudder.

Can anyone tell me please what anyone thinks the pilot might have been trying to achieve apart from decreasing his speed? I am a serious wannabe and have asked a few pilot friends about this already and they don't understand why so i thought about asking a wider audience.

Thank you in advance

Oli

[expedite_climb]

Shudder is normal when speedbrakes are deployed...

Its impossible to say what he was doing, but how do you profess to know any better ???? How do you know it wasnt from the centre of the runway ?

[Hand Solo]

Probably decreasing speed and nothing else. Done it myself at high level when entering a jetstream which caused a rapid increase in airspeed towards the limit. Judicious use of speedbrake will recover the speed margin faster than just waiting for the engines to come back to idle, especially if you fly an Airbus.

[Spitoon]

Whilst I would agree that the layman can offer little comment about the deployment of speedbrakes, if you can see the centeline markings out of the cabin windows you don't need to be an expert to know that the aircraft's not on the centreline! :)

[Edited to ensure that it's obvious that this was intended as a humerous comment!]

[Speedbird252]

Hey, Expedite Climb, it aint that daft a question!

If you can see the runway centreline as the power goes on it aint rocket science is it? If the PH turned early onto the active with the clearance to take off he may have wound the power up to the usual 60% (or whatever) N1 for the power checks during the last part of the turn onto the runway, which could have given the impression that he wasnt on the CL. (very early I agree, but from a PX point of view)

And as for the speedbrakes, sure a 752 can shudder when deployed, but how often do they get used at FL35? Bearing in mind that the Vref / lack of air density at that flight level isnt where you would normally associate the need to use them?

BA@LHR - What exactly did you see?, the verticle lift dumpers on the upper surface being deployed?

Any 752/763 drivers care to shed any light?

Hmmmm....curious.

Regards all,

Speedy:p

[max_cont]

It’s no big deal. Speed brakes are used as sop’s during an over speed or a condition that is approaching that condition. All you get is a rumble and a pitch up as they deploy. The pitch up is countered by deploying the speed brake slowly.

The C2 powered 762’s are so overpowered that the a/c will exceed the barbers’ pole with the autopilot engaged during the level off if you don’t watch it.

[BlueEagle]

Anyone care to confirm or deny?

I was told that the speed brakes first appeared on the Comet as it was so over powered it was quite easy for it to exceed Mne in the cruise, hence the name, "Speedbrake"!

Have used them at altitude myself, (when suddenly entering turbulence and speed excursions occur), on both the B767, C2 engines and the B744 with P & W engines.

[HPSOV]

When entering a jetstream it is very common for the speed to "run away".
I've had full speedbrake, idle thrust and 4000fpm rate of climb just to avoid the overspeed while climbing through a jetstream (763).
5 minutes is a long time thought, but you're still alive so it must have been fine!

[411A]

OTOH....watched a new Captain deploy speedbrakes at FL390 just south of GVA on a TriStar, and got sooo slow that max thrust was needed just to keep the speed UP after descending to 310...needless to say, he did NOT pass his line check.:rolleyes:
Was sent back to the sim for "retraining". :eek:

[expedite_climb]

Speedbird252 (and others),

I am a 752 driver. Yes it is uncommon to use speedbrakes in the cruise, but they may well be used, and for the reasons above. These days they are described by many as a primary flight control. Do pax complain when I use the elevator or rudder??

As for lined up, of course the pax can see a white line, but without knowing the airfield markings how do they know that it is the centre line ? We quite commonly set 1.2 epr for TO in the turn on, to make a quicker departure.

[DrSyn]

5 minutes does seem quite a long time, but we obviously don't know the conditions at the time. The following may not be applicable to BA@LHR's observation and I won't indulge in speculation here.

I (and Boeing/Airbus) am with those above who use speedbrake to check rapidly increasing air speed, in the cruise . A jet is designed to be very slippery at high altitude and the first thing that happens when you retard the thrust levers is . . . . . nothing . . . . . . followed occasionally by the overspeed clackers :)

Eventually, as the air speed begins to decrease, alpha increases, the drag comes on, the power goes back on and . . . . . nothing happens, because there isn't too much power available at Max Con, high up, and you're probably near the wrong side of the drag curve. It can take a long time to get back to original cruise speed!

When the speed starts to increase, it is usually sufficient to "crack" the speedbrakes open (very gently) and then slowly adjust the lever until the speed increase is checked. By that time the autothrottle (if applic) will have got the measure of it and the speedbrake can be gently re-stowed. 9 times out of 10 no one would even notice it (unless someone was on board and watching the spoiler panels with an eagle-eye!).

Only in extreme cases is it necessary to touch the thrust levers and, if so done, to ensure that power is restored as soon as the speed increase has been checked. This is only likely in the vicinity of Cb activity or an exceptionally steep-gradient jetstream.

This may or may not be relevant to the original question but is to some of the subsequent discussion!

[john_tullamarine]

.. speculation only in regard to the first post .. but, in respect of the lineup consideration ...

One really doesn't want to line up and waste a bunch of runway prior to commencing the roll ...... this is especially the case with a shortish runway in hot and high conditions with a limiting RTOW .... there is no difficulty or concern with regaining the centreline during the initial takeoff roll ... but beware of an early engine failure at low speed if one is near the edge of the runway ...

[BA@LHR]

Thank you all very much for the replies.

What i meant by the aircraft rolling when not on the centre line was that we were stationary, perpendicular to the runway, so say the runway is 360 degrees, then the taxiway leading to the runway was at 270 degrees. We then were given clearance for T/O which was staright away, (as only 4 flights land there a day), and we then taxied a little further then turned probably about 45 degrees to 315 degrees and then the pilot applied some thrust and then applied full thrust and then we drastically had to correct our position on the runway and we could easily feel and see that we were never anywhere near the centre line.

But anyway, thank you all for the replies

Best wishes

Oli

[expedite_climb]

BA@LHR.

This sounds about as normal as putting milk in your cup of tea. You will have been held at the edge of the runway, waiting for clearence. Even if there are only 4 flights a day, the cabin may not have been ready, or you may have been waiting for ATC enroute clearence.

As you turn onto the runway, about 45 degrees away from lined up, it is quite normal to begin setting takeoff thrust, as the engines are normally spooled up to approx 1.2 epr, to stabilise, prior to setting take off thrust.

The other options are wait until fully lined up without getting as tight into the corner as possible, which would waste runway, or to turn as tight as possible into the corner, which will take time, and if runway length is not limiting is not always necessary.