Speed Brakes In Flight
Discussion
I am quite a regular flyer, but on a flight back from Ireland I noticed something I’d never seen before. Whilst at cruising altitude I noticed the speed brakes deploy, I have never noticed them raised in flight before other than on approach or landing. Is this normal? And if not why would they have been deployed? Thanks for any answers
At high altitude on the 737 it is preferable to use a little speedbrake if there is a risk of an over speed. It is better to lower the airspeed with a small amount of drag than to reduce the thrust. This is because of the increased spool up times of the engines at high altitudes and how small the margin is at high altitude to both a high speed and low speed limit. If you were to close the throttles due to increasing airspeed you could then find them yourself watching the speed drop off and willing the engines to spool up again to avoid a low speed warning. That’s the only reason I can think of to deploy speed brakes in the cruise.
kapiteinlangzaam said:
Disclaimer: ATC, not pilot.
I can imagine it will be to prevent an airframe overspeed whilst maintaing a specific rate of descent that has been assigned by ATC.
Or specific speed control whilst making a descent restriction. Pretty much everything into Heathrow these days descends at 250kts to eat up the standard 10-15 min delay. They need to hurry up and build that bloody runway.I can imagine it will be to prevent an airframe overspeed whilst maintaing a specific rate of descent that has been assigned by ATC.
kapiteinlangzaam said:
We already slow em down at German boundary for Heathrow these days... Bloody useless!! 
The overspeeding is something we see quite often with Lufthansa A320s in to Frankfurt. No idea why, as we often give a discretionary clearance, yet we see M0.81/0.82 and a very sudden levelling off.
I wonder if there is a DLH SoP to prevent or limit the use of the airbrakes...?; Or something specific to the A320/321.
There isn't, other than Airbus speedbrakes being sThe overspeeding is something we see quite often with Lufthansa A320s in to Frankfurt. No idea why, as we often give a discretionary clearance, yet we see M0.81/0.82 and a very sudden levelling off.
I wonder if there is a DLH SoP to prevent or limit the use of the airbrakes...?; Or something specific to the A320/321.
t. Sir Benny has it exactly right.
kapiteinlangzaam said:
We already slow em down at German boundary for Heathrow these days... Bloody useless!!
The overspeeding is something we see quite often with Lufthansa A320s in to Frankfurt. No idea why, as we often give a discretionary clearance, yet we see M0.81/0.82 and a very sudden levelling off.
I wonder if there is a DLH SoP to prevent or limit the use of the airbrakes...?; Or something specific to the A320/321.
I think DLH SOP is to fly their A320’s like they stole them. 340kts IAS everywhere.The overspeeding is something we see quite often with Lufthansa A320s in to Frankfurt. No idea why, as we often give a discretionary clearance, yet we see M0.81/0.82 and a very sudden levelling off.
I wonder if there is a DLH SoP to prevent or limit the use of the airbrakes...?; Or something specific to the A320/321.
dvs_dave said:
I’m not a pilot, but have also noticed this before. It almost always seems to coincidence with the very start of the descent, so presumably it’s just the initiation of a standard the descent routine for certain load configurations or weather conditions.
From the OP, it sounds like the question was regarding spoiler deployment while in the cruise. The only time it's standard procedure to deploy these in the cruise is when you have a potential overspeed (ie - speed is racing towards the max operating limit and showing no trend away from it!).Otherwise, flight spoilers aren't used to initiate a descent, but rather help increase descent rate. Main standout reasons for this would be to regain your calculated descent profile (if above it), complying with ATC request to get down faster (crossing traffic, for example), or for an emergency descent.
Pull the speedbrake on me at cruise and I’ll likely break your arm. Had one idiot try years ago when there was no margin.
On a flight from Ireland cruise altitude is usually low (giving wider margins) and of short period. Delays often mean ATC want minimum speed whilst still meeting descent profile. Slow speed means lower drag which in turn makes it more difficult to achieve descent at or below limits specified by ATC for separation. Hence speedbrakes may be deployed to achieve these as they increase drag. There is no valid reason to use them in cruise that I have seen in 30 years flying.
On a flight from Ireland cruise altitude is usually low (giving wider margins) and of short period. Delays often mean ATC want minimum speed whilst still meeting descent profile. Slow speed means lower drag which in turn makes it more difficult to achieve descent at or below limits specified by ATC for separation. Hence speedbrakes may be deployed to achieve these as they increase drag. There is no valid reason to use them in cruise that I have seen in 30 years flying.
I’m guessing you’ve not flown the 737 800 then? The preferred method of slowing the aircraft at high altitude to avoid a high speed buffet alert is partial speedbrake deployment. There is no point in retarding the thrust levers to avoid a high speed buffet alert to then initiate a low speed event. You said you’d break the other pilot’s arm if he pulled the speed brake on you in level flight. What would you do if he pulled back the throttles at high altitude? I know what I’d do. I see you have 30 years of flight experience but to say that speed brake deployment in the cruise is never required is plainly wrong.
Thanks for all of the answers! I thought it might have been a speed issue as we arrived pretty early, may have been a strong tail wind causing the aircraft to travel too fast or mess up air traffic control arrival time? To answer another question we weren’t turning and didn’t appear to be starting out decent. If it makes any difference we were flying from Shannon to Heathrow.
Edited by LamboDave on Monday 2nd April 12:31
I assume it's going to depend on load carried at that particular time? If you've got a light aircraft, then are there times when you need more drag (more than the light, clean trims provide) to meet the descent profile, even at flight idle? (perhaps not in day-to-day routing, but perhaps if ATC have ordered a bit of an "unexpected" swap around to go around other traffic etc?)
[quote=Thee Postman Pat]Pull the speedbrake on me at cruise and I’ll likely break your arm. Had one idiot try years ago when there was no margin.
Totally and fundamentally wrong to the point where newer aircraft like the 380 have automatic spoiler deployment to prevent overspeed regardless of the performance margin available.
You don't chop the thrust levers on a high bypass turbo fan and especially not at altitude.
Totally and fundamentally wrong to the point where newer aircraft like the 380 have automatic spoiler deployment to prevent overspeed regardless of the performance margin available.
You don't chop the thrust levers on a high bypass turbo fan and especially not at altitude.
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