Dear TechOps-forum members,

I seem to hear a very distinct sound during the take-off roll in the cockpit of the 747-400 on YouTube videos. It's best heard on this video below starting at the 8:43 mark. Is it someone adjusting his seat?


https://www.youtube.com/watch?v=aXYszjLaE7s&t=

I've been noticing it for years but don't have any operational flying experience (just some maintenance). The sound reminds of of something in between automatic spoiler deployment and seat adjusting on the 747-400/777. Is it THS-related?

Thank you in advance and kind regards,

Vaux.

That'll be the nosewheel over the centreline lights I'm sure, just before rotation. Lots of bangs and rattles!

Leej wrote:

That'll be the nosewheel over the centreline lights I'm sure, just before rotation. Lots of bangs and rattles!

Hi Lee, that's definitely what you're mainly hearing. I'm referring to the more 'elec sound'.

Hmm, will listen again....

Ah, sort of like a radio being tuned in?

It's changing in pitch, yes. Wondering if any flight crew or type-rated would know the answer. I wouldn't be surprised if it's a very obvious eureka thing.

That does sound a lot like one of the electric seats moving but why one of the pilots would be adjusting their seat that late in the takeoff roll is beyond me. Maybe some airlines out there had an electric adjustment for the Jumpseat but all of the -400s that I flew had electric for the front two seats and manual adjustments for the 3rd seat. And from what I can remember and if anyone cares the 4th seat was not adjustable.

It definitely wasn’t either of the front seat pilots adjusting their seats

I thought it sounded like an electric seat being moved at first but it’s a bit of a different noise, definitely sounds electric in nature

The sound can be heard on different videos as well (always different registrations).

For example: https://www.youtube.com/watch?v=16XTAK-4Xbk

1.55 mark, not very distinct.

It sounds almost like the servo motor that moves the spoiler lever backwards on landing, but obviously this wouldn't be the case on takeoff. Very much like an electric motor of some sort. The only other thing I could think of is the autothrottle going into HOLD and disconnecting the throttle servos but the motors themselves continue moving or something..

Possibly a noisy avionics cooling fan, close to failure ?

Max Q wrote:

Possibly a noisy avionics cooling fan, close to failure ?

Nah, definitely something electromechanically driven. It seems to start just before V1 on most videos.

77west wrote:

It sounds almost like the servo motor that moves the spoiler lever backwards on landing, but obviously this wouldn't be the case on takeoff. Very much like an electric motor of some sort. The only other thing I could think of is the autothrottle going into HOLD and disconnecting the throttle servos but the motors themselves continue moving or something..

It’s been awhile since I’ve flown the 747 but I seem to remember the Autothrottle going into HOLD (or whatever the 747 term was) at 80 knots. If so the sound starts just prior to V1 so well past 80 knots.

On the 400, the outflow valves start to close at 65 knots. There are other switches on the nose gear strut and on the tilt switches on the maingear which control a multitude of devices. But I can't think of anything in particular that make a noise. on takeoff..

Some versions of the MD80 had a very loud avionics cooling fan that was deactivated when weight came off the nose gear

It was always disconcerting to hear that fan turn off with a sound exactly like a jet engine spooling down right after you rotated

No matter how many times it happened you always had a quick look at the engine instruments

Thanks for your answers guys. Interesting, no definitive answer yet! The search continues, maybe some seasoned mechanics could shed some light on this subject.

I’m not a 747 guy, but could it be the autospeedbrake actuator disarming (retracting)after T/O?

DL_Mech wrote:

I’m not a 747 guy, but could it be the autospeedbrake actuator disarming (retracting)after T/O?

Speedbrakes aren't armed for take-off on a Boeing jet.

AirKevin wrote:

DL_Mech wrote:

I’m not a 747 guy, but could it be the autospeedbrake actuator disarming (retracting)after T/O?

Speedbrakes aren't armed for take-off on a Boeing jet.

Yes and no…the speed brakes are not armed by the pilots however on all the Boeing aircraft I’ve flown the speed brakes will automatically deploy in the event of a Rejected Takeoff. So even though we take off with the lever in the DOWN position the actuator and speed brakes would still be armed to deploy in the event of a rejected takeoff.

That all having been said the noise of the actuator (after landing at least, I would think takeoff would be the same) is quite short, normally only a second or two. Outside of a mechanical irregularity the actuator shouldn’t be running for that extended of a time period.

SaveFerris wrote:

AirKevin wrote:

DL_Mech wrote:

I’m not a 747 guy, but could it be the autospeedbrake actuator disarming (retracting)after T/O?

Speedbrakes aren't armed for take-off on a Boeing jet.

Yes and no…the speed brakes are not armed by the pilots however on all the Boeing aircraft I’ve flown the speed brakes will automatically deploy in the event of a Rejected Takeoff. So even though we take off with the lever in the DOWN position the actuator and speed brakes would still be armed to deploy in the event of a rejected takeoff.

That all having been said the noise of the actuator (after landing at least, I would think takeoff would be the same) is quite short, normally only a second or two. Outside of a mechanical irregularity the actuator shouldn’t be running for that extended of a time period.

Correct, in the 757/67 the speed brakes will deploy automatically when reverse thrust is deployed on a rejected take off or if they are not placed in the arm position for landing when they would normally extend with weight on wheels


This is a very good design feature, in the event of a rejected take off when the Captain reduces thrust to idle, initially the RTO function of the autobrakes activates automatically providing maximum braking and selecting reverse deploys the speed brakes saving crucial time and eliminating delay by eliminating the necessity of taking those two actions separately

Max Q wrote:

SaveFerris wrote:

AirKevin wrote:

Speedbrakes aren't armed for take-off on a Boeing jet.

Yes and no…the speed brakes are not armed by the pilots however on all the Boeing aircraft I’ve flown the speed brakes will automatically deploy in the event of a Rejected Takeoff. So even though we take off with the lever in the DOWN position the actuator and speed brakes would still be armed to deploy in the event of a rejected takeoff.

That all having been said the noise of the actuator (after landing at least, I would think takeoff would be the same) is quite short, normally only a second or two. Outside of a mechanical irregularity the actuator shouldn’t be running for that extended of a time period.

Correct, in the 757/67 the speed brakes will deploy automatically when reverse thrust is deployed on a rejected take off or if they are not placed in the arm position for landing when they would normally extend with weight on wheels


This is a very good design feature, in the event of a rejected take off when the Captain reduces thrust to idle, initially the RTO function of the autobrakes activates automatically providing maximum braking and selecting reverse deploys the speed brakes saving crucial time and eliminating delay by eliminating the necessity of taking those two actions separately

I believe all current Boeing aircraft do this. Cant speak for the earlier models but certainly 744/75/76/77/78 and 73NG/Max all do. It will throw a takeoff config warning if you try to arm them before takeoff like you would in an Airbii

77west wrote:

Max Q wrote:

SaveFerris wrote:

Yes and no…the speed brakes are not armed by the pilots however on all the Boeing aircraft I’ve flown the speed brakes will automatically deploy in the event of a Rejected Takeoff. So even though we take off with the lever in the DOWN position the actuator and speed brakes would still be armed to deploy in the event of a rejected takeoff.

That all having been said the noise of the actuator (after landing at least, I would think takeoff would be the same) is quite short, normally only a second or two. Outside of a mechanical irregularity the actuator shouldn’t be running for that extended of a time period.

Correct, in the 757/67 the speed brakes will deploy automatically when reverse thrust is deployed on a rejected take off or if they are not placed in the arm position for landing when they would normally extend with weight on wheels


This is a very good design feature, in the event of a rejected take off when the Captain reduces thrust to idle, initially the RTO function of the autobrakes activates automatically providing maximum braking and selecting reverse deploys the speed brakes saving crucial time and eliminating delay by eliminating the necessity of taking those two actions separately

I believe all current Boeing aircraft do this. Cant speak for the earlier models but certainly 744/75/76/77/78 and 73NG/Max all do. It will throw a takeoff config warning if you try to arm them before takeoff like you would in an Airbii

That makes sense, as you know the speed brakes will also automatically retract if you increase power after landing allowing you to go around after touchdown in case you forget them ( only if you haven’t selected reverse)


If I remember correctly however and even after the AA 757 crash in Cali they still won’t auto retract when airborne even with full power selected

Max Q wrote:

77west wrote:

Max Q wrote:

Correct, in the 757/67 the speed brakes will deploy automatically when reverse thrust is deployed on a rejected take off or if they are not placed in the arm position for landing when they would normally extend with weight on wheels


This is a very good design feature, in the event of a rejected take off when the Captain reduces thrust to idle, initially the RTO function of the autobrakes activates automatically providing maximum braking and selecting reverse deploys the speed brakes saving crucial time and eliminating delay by eliminating the necessity of taking those two actions separately

I believe all current Boeing aircraft do this. Cant speak for the earlier models but certainly 744/75/76/77/78 and 73NG/Max all do. It will throw a takeoff config warning if you try to arm them before takeoff like you would in an Airbii

That makes sense, as you know the speed brakes will also automatically retract if you increase power after landing allowing you to go around after touchdown in case you forget them ( only if you haven’t selected reverse)


If I remember correctly however and even after the AA 757 crash in Cali they still won’t auto retract when airborne even with full power selected

You would think even without auto-retract there would at least be a master caution or something, but no, nothing.