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Georgetown
Topic Author
Posts: 425
Joined: Wed Dec 28, 2005 12:50 pm

Increased thrust close to rotation

Thu Apr 19, 2018 6:12 pm

Hello all-

Back with one of my “I fly a ton so when I notice something that seems out of the ordinary I get curious” questions.

Was taking off from SAN this morning for a quick flight up to SFO on a UA 737-800. The flight was pretty full, I was in row 4.

Our takeoff roll seemed completely normal to me - sounds, expected perceptible forces, etc. About five to ten seconds before rotation the thrust was increased in a way that seemed meaningful: the pitch of the engine sound (the “buzz”) became much higher pitched and louder and the forces pushing me back in my seat increased, etc. We took off as normal and then, also as normal, the thrust came back a bit after we became airborne.

I have experienced plenty of slow spool up takeoffs before, but never (at least that I noticed) a noticeable thrust change right before rotation. How common is this? What are some of the reasons it would occur (wind change/gust/ etc?).

Any insights from you pilots out there would be much appreciated - I’m simply curious and like learning new stuff.
Let's go Hoyas!
 
paullam
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Re: Increased thrust close to rotation

Thu Apr 19, 2018 7:38 pm

I has a similar experience not too long ago on a Fokker 100 in TSE. We took off with tailwind so they might’ve done it to prevent the engine from flaming out due to the wind coming from behind.
This is just a guess though.
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GalaxyFlyer
Posts: 6252
Joined: Fri Jan 01, 2016 4:44 am

Re: Increased thrust close to rotation

Thu Apr 19, 2018 8:49 pm

Considering the jet exhaust out the fan section is about 500 mph and the fire about 20% faster than that, a tailwind ain’t gonna blow out the fire.

Take-off thrust, once established, doesn’t appreciably change during the roll and rotation unless there was an engine failure.

GF

GF
 
Georgetown
Topic Author
Posts: 425
Joined: Wed Dec 28, 2005 12:50 pm

Re: Increased thrust close to rotation

Thu Apr 19, 2018 9:14 pm

GalaxyFlyer wrote:
Take-off thrust, once established, doesn’t appreciably change during the roll and rotation unless there was an engine failure.


That’s why I was caught off guard this morning. Not used to it changing at all once set.
Let's go Hoyas!
 
BoeingGuy
Posts: 6313
Joined: Fri Dec 10, 2010 6:01 pm

Re: Increased thrust close to rotation

Thu Apr 19, 2018 9:21 pm

Georgetown wrote:
GalaxyFlyer wrote:
Take-off thrust, once established, doesn’t appreciably change during the roll and rotation unless there was an engine failure.


That’s why I was caught off guard this morning. Not used to it changing at all once set.


Also, they almost always use the Autothrottle for takeoff. On Boeing airplanes, the Autothrottle goes into HOLD mode at 80 knots (the speed varies on a few models, but it's somewhere around there). It doesn't come out of HOLD until you transition to Climb Thrust. (On the newer models, if you push TO/GA after lift-off, it will come out of HOLD and the autothrottle will remove the derate).

Power is removed from the autothrottle servo motor when in HOLD. This is by design to prevent any kind of system failure from moving the thrust levers at a critical phase of takeoff.

Thus, there is no physical way the Autothrottle can increase thrust close to rotation. If the flight crew felt they needed to remove the takeoff derate and advance thrust prior to rotation, they would have to do so manually.

Outside of an emergency, I know of no reason why the crew would manually advance the thrust close to rotation.
 
Andre3K
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Re: Increased thrust close to rotation

Thu Apr 19, 2018 10:13 pm

The last responses are all good and well, but the man heard what he heard. If it were not possible to increase power after power is set then there would have been tons of crashes due to runway contamination at this point (like maybe that Emirates flight that took off on a shortened runway and didn't see the vehicles on the runway until just a few seconds before rotation speed, yet they still powered up even further to clear the obstacles).
 
BoeingGuy
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Re: Increased thrust close to rotation

Thu Apr 19, 2018 10:44 pm

Andre3K wrote:
The last responses are all good and well, but the man heard what he heard. If it were not possible to increase power after power is set then there would have been tons of crashes due to runway contamination at this point (like maybe that Emirates flight that took off on a shortened runway and didn't see the vehicles on the runway until just a few seconds before rotation speed, yet they still powered up even further to clear the obstacles).


Where did anyone say it is not possible to increased thrust? You can do but it has to be done manually, and you need a very good reason for it.

BTW, if you are doing a Fixed Derate, you are prohibited from increasing thrust past that unless ground contact is imminent. If an engine fails you could have Vmca problems.
 
GalaxyFlyer
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Re: Increased thrust close to rotation

Thu Apr 19, 2018 11:06 pm

Just because he thinks he heard a thrust increase doesn’t mean there was a thrust increase. I’ve had passengers say or ask all kinds of things that were just wrong.

GF
 
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Starlionblue
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Re: Increased thrust close to rotation

Thu Apr 19, 2018 11:52 pm

I can think of one scenario. On the 330/350 (and I suspect on other types as well), if you get a windshear warning or otherwise suspect windshear during a FLEX thrust take-off ("assumed temperature"), thrust must be increased to TOGA.

Thrust during a FLEX take-off may be increased to TOGA at any point if the pilots feel this is warranted. On the other hand, as BoeingGuy mentions, if the take-off is with fixed derate, thrust may not be increased due to the risk of exceeding rudder control authority in case of an engine out.

What is the windshear during take-off procedure on the 737?
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Mutt
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Re: Increased thrust close to rotation

Thu Apr 19, 2018 11:58 pm

In 737NG's 84 KIAS is where the autothrottles "Hold" and won't fight a pilot trying to input more power. Someone I know very well regularly waits until the hold, and then increases power just a little on reduced thrust takeoffs, if there is an airplane taking off in front of him and he wants to rotate to avoid the first airplane's wake. He is mindful of VMCG and VMCA, and the fact that the reduction is there for a reason, but sometimes he sees the safer course of action to split the difference between the two power settings.
 
BoeingGuy
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Re: Increased thrust close to rotation

Fri Apr 20, 2018 12:06 am

Starlionblue wrote:
I can think of one scenario. On the 330/350 (and I suspect on other types as well), if you get a windshear warning or otherwise suspect windshear during a FLEX thrust take-off ("assumed temperature"), thrust must be increased to TOGA.

Thrust during a FLEX take-off may be increased to TOGA at any point if the pilots feel this is warranted. On the other hand, as BoeingGuy mentions, if the take-off is with fixed derate, thrust may not be increased due to the risk of exceeding rudder control authority in case of an engine out.

What is the windshear during take-off procedure on the 737?


It's the same for all Boeing models. The FCOM says to RTO up to V1 for the Predictive Windshear warning. However, the PWS warning is inhibited above 100 knots so in reality you'd only RTO prior to 100 knots.


Windshear Warning

Predictive windshear warning during takeoff roll: (“WINDSHEAR
AHEAD, WINDSHEAR AHEAD” aural)
• before V1, reject takeoff
• after V1, perform the Windshear Escape Maneuver.

Windshear encountered during takeoff roll:

• If windshear is encountered before V1, there may not be sufficient
runway remaining to stop if an RTO is initiated at V1. At VR,
rotate at a normal rate toward a 15 degree pitch attitude. Once
airborne, perform the Windshear Escape Maneuver.

• If windshear is encountered near the normal rotation speed and
airspeed suddenly decreases, there may not be sufficient runway
left to accelerate back to normal takeoff speed. If there is
insufficient runway left to stop, initiate a normal rotation at least
2,000 feet before the end of the runway, even if airspeed is low.
Higher than normal attitudes may be needed to lift off in the
remaining runway. Ensure maximum thrust is set.


Windshear Escape Maneuver

MANUAL FLIGHT
• Disengage autopilot.
• Push either TO/GA switch.
• Aggressively apply maximum thrust*
• Disengage autothrottle.
• Simultaneously roll wings level and
rotate toward an initial pitch attitude
of 15 °.
• Retract speedbrakes.
• Follow flight director TO/GA
guidance (if available)**

• Do not change flap or gear
configuration until windshear is no
longer a factor.
• Monitor vertical speed and altitude.
• Do not attempt to regain lost airspeed
until windshear is no longer a factor
 
Georgetown
Topic Author
Posts: 425
Joined: Wed Dec 28, 2005 12:50 pm

Re: Increased thrust close to rotation

Fri Apr 20, 2018 12:55 am

GalaxyFlyer wrote:
Just because he thinks he heard a thrust increase doesn’t mean there was a thrust increase. I’ve had passengers say or ask all kinds of things that were just wrong.

GF


This is a fair point, but it was pretty pronounced in the context of every other take off I’ve had, enough so that I came on here to ask. Is there a chance it was my imagination, sure I suppose, but I would be surprised.

Mutt wrote:
In 737NG's 84 KIAS is where the autothrottles "Hold" and won't fight a pilot trying to input more power. Someone I know very well regularly waits until the hold, and then increases power just a little on reduced thrust takeoffs, if there is an airplane taking off in front of him and he wants to rotate to avoid the first airplane's wake. He is mindful of VMCG and VMCA, and the fact that the reduction is there for a reason, but sometimes he sees the safer course of action to split the difference between the two power settings.


Don’t know if this has to do with it at all, but we took off right after a UPS heavy. Who knows. Could be what I experienced.
Let's go Hoyas!
 
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Starlionblue
Posts: 20092
Joined: Fri Feb 27, 2004 9:54 pm

Re: Increased thrust close to rotation

Fri Apr 20, 2018 1:41 am

BoeingGuy wrote:
Starlionblue wrote:
I can think of one scenario. On the 330/350 (and I suspect on other types as well), if you get a windshear warning or otherwise suspect windshear during a FLEX thrust take-off ("assumed temperature"), thrust must be increased to TOGA.

Thrust during a FLEX take-off may be increased to TOGA at any point if the pilots feel this is warranted. On the other hand, as BoeingGuy mentions, if the take-off is with fixed derate, thrust may not be increased due to the risk of exceeding rudder control authority in case of an engine out.

What is the windshear during take-off procedure on the 737?


It's the same for all Boeing models. The FCOM says to RTO up to V1 for the Predictive Windshear warning. However, the PWS warning is inhibited above 100 knots so in reality you'd only RTO prior to 100 knots.


Windshear Warning

Predictive windshear warning during takeoff roll: (“WINDSHEAR
AHEAD, WINDSHEAR AHEAD” aural)
• before V1, reject takeoff
• after V1, perform the Windshear Escape Maneuver.

Windshear encountered during takeoff roll:

• If windshear is encountered before V1, there may not be sufficient
runway remaining to stop if an RTO is initiated at V1. At VR,
rotate at a normal rate toward a 15 degree pitch attitude. Once
airborne, perform the Windshear Escape Maneuver.

• If windshear is encountered near the normal rotation speed and
airspeed suddenly decreases, there may not be sufficient runway
left to accelerate back to normal takeoff speed. If there is
insufficient runway left to stop, initiate a normal rotation at least
2,000 feet before the end of the runway, even if airspeed is low.
Higher than normal attitudes may be needed to lift off in the
remaining runway. Ensure maximum thrust is set.


Windshear Escape Maneuver

MANUAL FLIGHT
• Disengage autopilot.
• Push either TO/GA switch.
• Aggressively apply maximum thrust*
• Disengage autothrottle.
• Simultaneously roll wings level and
rotate toward an initial pitch attitude
of 15 °.
• Retract speedbrakes.
• Follow flight director TO/GA
guidance (if available)**

• Do not change flap or gear
configuration until windshear is no
longer a factor.
• Monitor vertical speed and altitude.
• Do not attempt to regain lost airspeed
until windshear is no longer a factor


Seems very similar to Airbus conceptually. The procedure is to reject take-off if windshear is detected before V1, but Predictive Windshear warnings are inhibited above 100 knots and until a height of 50 feet. However, "windshear detection" doesn't just mean the predictive system, but also awareness through instrument scan. Reactive windshear detection starts 3 seconds after liftoff.

Differences in details. No TOGA buttons; just forward to TOGA. Autopilot use is recommended (and if engaged, keep on) and we can pull the stick to full back and hold it there if needed.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
User avatar
Faro
Posts: 2003
Joined: Sun Aug 12, 2007 1:08 am

Re: Increased thrust close to rotation

Fri Apr 20, 2018 5:31 am

Starlionblue wrote:
BoeingGuy wrote:
Starlionblue wrote:
I can think of one scenario. On the 330/350 (and I suspect on other types as well), if you get a windshear warning or otherwise suspect windshear during a FLEX thrust take-off ("assumed temperature"), thrust must be increased to TOGA.

Thrust during a FLEX take-off may be increased to TOGA at any point if the pilots feel this is warranted. On the other hand, as BoeingGuy mentions, if the take-off is with fixed derate, thrust may not be increased due to the risk of exceeding rudder control authority in case of an engine out.

What is the windshear during take-off procedure on the 737?


It's the same for all Boeing models. The FCOM says to RTO up to V1 for the Predictive Windshear warning. However, the PWS warning is inhibited above 100 knots so in reality you'd only RTO prior to 100 knots.


Windshear Warning

Predictive windshear warning during takeoff roll: (“WINDSHEAR
AHEAD, WINDSHEAR AHEAD” aural)
• before V1, reject takeoff
• after V1, perform the Windshear Escape Maneuver.

Windshear encountered during takeoff roll:

• If windshear is encountered before V1, there may not be sufficient
runway remaining to stop if an RTO is initiated at V1. At VR,
rotate at a normal rate toward a 15 degree pitch attitude. Once
airborne, perform the Windshear Escape Maneuver.

• If windshear is encountered near the normal rotation speed and
airspeed suddenly decreases, there may not be sufficient runway
left to accelerate back to normal takeoff speed. If there is
insufficient runway left to stop, initiate a normal rotation at least
2,000 feet before the end of the runway, even if airspeed is low.
Higher than normal attitudes may be needed to lift off in the
remaining runway. Ensure maximum thrust is set.


Windshear Escape Maneuver

MANUAL FLIGHT
• Disengage autopilot.
• Push either TO/GA switch.
• Aggressively apply maximum thrust*
• Disengage autothrottle.
• Simultaneously roll wings level and
rotate toward an initial pitch attitude
of 15 °.
• Retract speedbrakes.
• Follow flight director TO/GA
guidance (if available)**

• Do not change flap or gear
configuration until windshear is no
longer a factor.
• Monitor vertical speed and altitude.
• Do not attempt to regain lost airspeed
until windshear is no longer a factor


Seems very similar to Airbus conceptually. The procedure is to reject take-off if windshear is detected before V1, but Predictive Windshear warnings are inhibited above 100 knots and until a height of 50 feet. However, "windshear detection" doesn't just mean the predictive system, but also awareness through instrument scan. Reactive windshear detection starts 3 seconds after liftoff.

Differences in details. No TOGA buttons; just forward to TOGA. Autopilot use is recommended (and if engaged, keep on) and we can pull the stick to full back and hold it there if needed.



Wonder why SOP’s require taking hands off throttles above V1 if one may still need further throttle adjustments in the above cases...


Faro
The chalice not my son
 
User avatar
Starlionblue
Posts: 20092
Joined: Fri Feb 27, 2004 9:54 pm

Re: Increased thrust close to rotation

Fri Apr 20, 2018 6:47 am

Faro wrote:
Starlionblue wrote:
BoeingGuy wrote:

It's the same for all Boeing models. The FCOM says to RTO up to V1 for the Predictive Windshear warning. However, the PWS warning is inhibited above 100 knots so in reality you'd only RTO prior to 100 knots.


Windshear Warning

Predictive windshear warning during takeoff roll: (“WINDSHEAR
AHEAD, WINDSHEAR AHEAD” aural)
• before V1, reject takeoff
• after V1, perform the Windshear Escape Maneuver.

Windshear encountered during takeoff roll:

• If windshear is encountered before V1, there may not be sufficient
runway remaining to stop if an RTO is initiated at V1. At VR,
rotate at a normal rate toward a 15 degree pitch attitude. Once
airborne, perform the Windshear Escape Maneuver.

• If windshear is encountered near the normal rotation speed and
airspeed suddenly decreases, there may not be sufficient runway
left to accelerate back to normal takeoff speed. If there is
insufficient runway left to stop, initiate a normal rotation at least
2,000 feet before the end of the runway, even if airspeed is low.
Higher than normal attitudes may be needed to lift off in the
remaining runway. Ensure maximum thrust is set.


Windshear Escape Maneuver

MANUAL FLIGHT
• Disengage autopilot.
• Push either TO/GA switch.
• Aggressively apply maximum thrust*
• Disengage autothrottle.
• Simultaneously roll wings level and
rotate toward an initial pitch attitude
of 15 °.
• Retract speedbrakes.
• Follow flight director TO/GA
guidance (if available)**

• Do not change flap or gear
configuration until windshear is no
longer a factor.
• Monitor vertical speed and altitude.
• Do not attempt to regain lost airspeed
until windshear is no longer a factor


Seems very similar to Airbus conceptually. The procedure is to reject take-off if windshear is detected before V1, but Predictive Windshear warnings are inhibited above 100 knots and until a height of 50 feet. However, "windshear detection" doesn't just mean the predictive system, but also awareness through instrument scan. Reactive windshear detection starts 3 seconds after liftoff.

Differences in details. No TOGA buttons; just forward to TOGA. Autopilot use is recommended (and if engaged, keep on) and we can pull the stick to full back and hold it there if needed.



Wonder why SOP’s require taking hands off throttles above V1 if one may still need further throttle adjustments in the above cases...


Faro


Because you don't want to pull the thrust levers back after V1 for any reason beyond a few astronomically unlikely events. So the hand goes off. However if you, after V1, decide that actually TOGA sounds like a really comforting idea right now, putting the hand back on thrust levers will be a deliberate action as opposed to an ill-advised reflex.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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Balerit
Posts: 624
Joined: Sun Aug 23, 2015 9:14 am

Re: Increased thrust close to rotation

Fri Apr 20, 2018 11:40 am

Listen to the sounds here especially the RR first up and tell us what you think - listen for the note changes and watch the inlet.

https://www.youtube.com/watch?v=vVD2NQbthgE
Licensed Aircraft Maintenance Engineer (retired).
 
Georgetown
Topic Author
Posts: 425
Joined: Wed Dec 28, 2005 12:50 pm

Re: Increased thrust close to rotation

Fri Apr 20, 2018 2:46 pm

Balerit wrote:
Listen to the sounds here especially the RR first up and tell us what you think - listen for the note changes and watch the inlet.

https://www.youtube.com/watch?v=vVD2NQbthgE


Both seemed pretty normal to me (had seen the US 757 out of SAN video before, it’s s good one). Yesterday was markedly different from those, with the increase a couple seconds before rotation.
Let's go Hoyas!
 
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Balerit
Posts: 624
Joined: Sun Aug 23, 2015 9:14 am

Re: Increased thrust close to rotation

Fri Apr 20, 2018 3:09 pm

If you notice as they hit pockets of damp air, the engine pitch changes due to the less dense air, especially after liftoff and the first retarding of the throttles but I suppose it could be a number of other things as well. The fact that you noticed means something did occur. A flight engineer once told me that they were taking off and suddenly the end of the runway got closer and closer and they hadn't reached V1 so he rammed the throttles to the wall and they managed to lift off but had a tail strike while doing so. He checked later on and found that he had miscalculated the take off weight by 10K kgs.
Licensed Aircraft Maintenance Engineer (retired).
 
BoeingGuy
Posts: 6313
Joined: Fri Dec 10, 2010 6:01 pm

Re: Increased thrust close to rotation

Fri Apr 20, 2018 3:10 pm

Starlionblue wrote:
BoeingGuy wrote:
Starlionblue wrote:
I can think of one scenario. On the 330/350 (and I suspect on other types as well), if you get a windshear warning or otherwise suspect windshear during a FLEX thrust take-off ("assumed temperature"), thrust must be increased to TOGA.

Thrust during a FLEX take-off may be increased to TOGA at any point if the pilots feel this is warranted. On the other hand, as BoeingGuy mentions, if the take-off is with fixed derate, thrust may not be increased due to the risk of exceeding rudder control authority in case of an engine out.

What is the windshear during take-off procedure on the 737?


It's the same for all Boeing models. The FCOM says to RTO up to V1 for the Predictive Windshear warning. However, the PWS warning is inhibited above 100 knots so in reality you'd only RTO prior to 100 knots.


Windshear Warning

Predictive windshear warning during takeoff roll: (“WINDSHEAR
AHEAD, WINDSHEAR AHEAD” aural)
• before V1, reject takeoff
• after V1, perform the Windshear Escape Maneuver.

Windshear encountered during takeoff roll:

• If windshear is encountered before V1, there may not be sufficient
runway remaining to stop if an RTO is initiated at V1. At VR,
rotate at a normal rate toward a 15 degree pitch attitude. Once
airborne, perform the Windshear Escape Maneuver.

• If windshear is encountered near the normal rotation speed and
airspeed suddenly decreases, there may not be sufficient runway
left to accelerate back to normal takeoff speed. If there is
insufficient runway left to stop, initiate a normal rotation at least
2,000 feet before the end of the runway, even if airspeed is low.
Higher than normal attitudes may be needed to lift off in the
remaining runway. Ensure maximum thrust is set.


Windshear Escape Maneuver

MANUAL FLIGHT
• Disengage autopilot.
• Push either TO/GA switch.
• Aggressively apply maximum thrust*
• Disengage autothrottle.
• Simultaneously roll wings level and
rotate toward an initial pitch attitude
of 15 °.
• Retract speedbrakes.
• Follow flight director TO/GA
guidance (if available)**

• Do not change flap or gear
configuration until windshear is no
longer a factor.
• Monitor vertical speed and altitude.
• Do not attempt to regain lost airspeed
until windshear is no longer a factor


Seems very similar to Airbus conceptually. The procedure is to reject take-off if windshear is detected before V1, but Predictive Windshear warnings are inhibited above 100 knots and until a height of 50 feet. However, "windshear detection" doesn't just mean the predictive system, but also awareness through instrument scan. Reactive windshear detection starts 3 seconds after liftoff.

Differences in details. No TOGA buttons; just forward to TOGA. Autopilot use is recommended (and if engaged, keep on) and we can pull the stick to full back and hold it there if needed.


While I understand that some people like Airbus' method of selecting TO/GA better because it very unambiguous - just push the thrust levers forward to the TO/GA detent - most Boeing airplanes have a design feature that I don't think Airbus has.

First push of TO/GA gets you a thrust setting to give about a 2000 fpm climb. Thus preventing the airplane from pitching up too rapidly. The mode is programmed that the thrust levers move forward only, for windshear protection. If you overshoot 2000 fpm then so be it.

Second push of TO/GA advances to the Go-around thrust limit. That's maximum available go-around thrust.

I believe Airbus only has one Go-Around thrust setting - full Go-around thrust, which may not be needed in other go-around scenarios (e.g. traffic on the runway)

On takeoff on the 777 and 787, if you push TO/GA after lift-off, it will remove the derate and come out of HOLD and advance the thrust levers (if you had a derate). If you were in a fixed or assumed temperature Takeoff derate, the thrust levers will advance to full TO thrust. If you were in Climb thrust, it will advance to full Go-around thrust.

If you push TO/GA after 80 knots and before lift-off, it will disarm LNAV and VNAV. That would be handy in suspected windshear conditions. VNAV SPD doesn't have windshear protection logic like TO/GA does. Thus disarming VNAV prevents it from engaging automatically at 400 feet.

However, as mentioned earlier, nothing will automatically advance thrust levers on takeoff roll prior to lift-off on any Boeing model.
 
T1a
Posts: 57
Joined: Tue Apr 24, 2012 5:41 pm

Re: Increased thrust close to rotation

Fri Apr 20, 2018 4:28 pm

Well, just throwing it out there, but a little story of something that happened to me once:

We were accelerating down the runway, when going through around 100kt the captain noted (I was pilot flying) that the left hand engine for some reason suddenly developed more than the previously calculated T/O power setting. The RH engine was at the bug (which indicates the target power setting) while the LH engine was about 10% over. Since that was still below the max rated setting we continued the T/O. After airborne and the gear up the captain then fine tuned the LH engine back to the desired setting. We needed a power lever split of about 3cm (1.2in) to have an equal power output on both engines. After clean up and the aircraft positioned in a safe and steady climb, we tried to troubleshoot the problem, but couldn't find a solution other then flying with split power levers. Since we were flying away from base to an airport where we would not have had maintenance, we decided to turn around. If it had happened on the flight back to base we probably would have continued since with the power levers split both engines were developing power just fine. But since this is not normal in modern FADEC controlled engines, this definitely was a "maintenance action required prior next flight" thing.

I don't know if something similar with one engine developing more thrust all by itself happened to you here, just wanted to share it to give you a picture of the kind of things that might happen.

Cheers,
T1a
All views expressed under this username are mine as a private person and don't necessarily reflect the views of my employer.
 
Georgetown
Topic Author
Posts: 425
Joined: Wed Dec 28, 2005 12:50 pm

Re: Increased thrust close to rotation

Fri Apr 20, 2018 5:48 pm

Thanks for all the replies thus far! Seems like it’s one of those “who knows?” things. It is interesting to see that it is, in fact, uncommon. Should have asked the guys up front on the way out, but they seemed busy so didn’t bother them.
Let's go Hoyas!
 
BoeingGuy
Posts: 6313
Joined: Fri Dec 10, 2010 6:01 pm

Re: Increased thrust close to rotation

Fri Apr 20, 2018 5:55 pm

Georgetown wrote:
Thanks for all the replies thus far! Seems like it’s one of those “who knows?” things. It is interesting to see that it is, in fact, uncommon. Should have asked the guys up front on the way out, but they seemed busy so didn’t bother them.


It’s been an interesting discussion. I’m wondering if just the buzz saw sound changed so it gave you the perception that thrust increased when it didn’t. I’m not an expert on sound physics but I’ve heard the engine sound appear to change on climbout, for example, with no change in thrust. Like you’ll said, we’ll never know exactly what occurred.
 
Georgetown
Topic Author
Posts: 425
Joined: Wed Dec 28, 2005 12:50 pm

Re: Increased thrust close to rotation

Fri Apr 20, 2018 7:39 pm

BoeingGuy wrote:

It’s been an interesting discussion. I’m wondering if just the buzz saw sound changed so it gave you the perception that thrust increased when it didn’t. I’m not an expert on sound physics but I’ve heard the engine sound appear to change on climbout, for example, with no change in thrust. Like you’ll said, we’ll never know exactly what occurred.


The sound change is an interesting point (as brought up by Balerit as well). Certainly can’t rule that out. As much as I could swear I could feel it as well, who knows - it’s was 6:15 in the morning and I’ve certainly had my senses fool me. This seemed pretty pronounced though. Who knows.
Let's go Hoyas!
 
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Starlionblue
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Re: Increased thrust close to rotation

Fri Apr 20, 2018 11:01 pm

BoeingGuy wrote:
Starlionblue wrote:
BoeingGuy wrote:

It's the same for all Boeing models. The FCOM says to RTO up to V1 for the Predictive Windshear warning. However, the PWS warning is inhibited above 100 knots so in reality you'd only RTO prior to 100 knots.


Windshear Warning

Predictive windshear warning during takeoff roll: (“WINDSHEAR
AHEAD, WINDSHEAR AHEAD” aural)
• before V1, reject takeoff
• after V1, perform the Windshear Escape Maneuver.

Windshear encountered during takeoff roll:

• If windshear is encountered before V1, there may not be sufficient
runway remaining to stop if an RTO is initiated at V1. At VR,
rotate at a normal rate toward a 15 degree pitch attitude. Once
airborne, perform the Windshear Escape Maneuver.

• If windshear is encountered near the normal rotation speed and
airspeed suddenly decreases, there may not be sufficient runway
left to accelerate back to normal takeoff speed. If there is
insufficient runway left to stop, initiate a normal rotation at least
2,000 feet before the end of the runway, even if airspeed is low.
Higher than normal attitudes may be needed to lift off in the
remaining runway. Ensure maximum thrust is set.


Windshear Escape Maneuver

MANUAL FLIGHT
• Disengage autopilot.
• Push either TO/GA switch.
• Aggressively apply maximum thrust*
• Disengage autothrottle.
• Simultaneously roll wings level and
rotate toward an initial pitch attitude
of 15 °.
• Retract speedbrakes.
• Follow flight director TO/GA
guidance (if available)**

• Do not change flap or gear
configuration until windshear is no
longer a factor.
• Monitor vertical speed and altitude.
• Do not attempt to regain lost airspeed
until windshear is no longer a factor


Seems very similar to Airbus conceptually. The procedure is to reject take-off if windshear is detected before V1, but Predictive Windshear warnings are inhibited above 100 knots and until a height of 50 feet. However, "windshear detection" doesn't just mean the predictive system, but also awareness through instrument scan. Reactive windshear detection starts 3 seconds after liftoff.

Differences in details. No TOGA buttons; just forward to TOGA. Autopilot use is recommended (and if engaged, keep on) and we can pull the stick to full back and hold it there if needed.


While I understand that some people like Airbus' method of selecting TO/GA better because it very unambiguous - just push the thrust levers forward to the TO/GA detent - most Boeing airplanes have a design feature that I don't think Airbus has.

First push of TO/GA gets you a thrust setting to give about a 2000 fpm climb. Thus preventing the airplane from pitching up too rapidly. The mode is programmed that the thrust levers move forward only, for windshear protection. If you overshoot 2000 fpm then so be it.

Second push of TO/GA advances to the Go-around thrust limit. That's maximum available go-around thrust.

I believe Airbus only has one Go-Around thrust setting - full Go-around thrust, which may not be needed in other go-around scenarios (e.g. traffic on the runway)

On takeoff on the 777 and 787, if you push TO/GA after lift-off, it will remove the derate and come out of HOLD and advance the thrust levers (if you had a derate). If you were in a fixed or assumed temperature Takeoff derate, the thrust levers will advance to full TO thrust. If you were in Climb thrust, it will advance to full Go-around thrust.

If you push TO/GA after 80 knots and before lift-off, it will disarm LNAV and VNAV. That would be handy in suspected windshear conditions. VNAV SPD doesn't have windshear protection logic like TO/GA does. Thus disarming VNAV prevents it from engaging automatically at 400 feet.

However, as mentioned earlier, nothing will automatically advance thrust levers on takeoff roll prior to lift-off on any Boeing model.


The A350 has a new thrust setting for go-around, "go-around soft". Thrust levers to TOGA, then back to MCT. This gives a less aggressive go-around for situations when full thrust is not needed. I don't know if the 380 has GA SOFT.

Pitch up in Airbus is mitigated by the flight control logic. The aircraft will stabilise on the commanded flight path.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
barney captain
Posts: 2355
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Re: Increased thrust close to rotation

Sun Apr 22, 2018 2:51 am

This might be getting over-analyzed. It may have simply been the crew realizing - for whatever reason - that the AT's failed to set the correct thrust. I've seent this very thing. They then advanced the trust manually to the correct N1. Maybe it was set correctly and the crew wanted a little extra - for any number of reasons.
Southeast Of Disorder
 
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longhauler
Posts: 6488
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Re: Increased thrust close to rotation

Sun Apr 22, 2018 1:38 pm

I echo the comments of Barney Captain above. Sometimes, a gut feeling tells you something is up. Simply go from flex thrust to max thrust. If you're wrong, the worst that can happen is you burned more fuel.

I recall many years ago, taking off from YYZ in a full A321 for SFO, we used 33L, one of YYZ's shorter runways.

Just past half way down the runway, it didn't "look right". When the F/O said something like "this doesn't look right", it was all I needed. I went to TOGA thrust. In spite of what most on here think, the A321 is a beast on the ground and the extra thrust was quite a kick. We gave the gents in the FedEx terminal quite an airshow as we roared overhead at about a hundred feet!

When safely away and in cruise, we went over our calculations and entries. In my opinion, we had to have done something wrong. We could find nothing wrong. We told Maintenance Control and they did a scan on the engine parameters for take-off, saw the increase, but nothing wrong. I wrote several safety reports, saved my calculations, but the answer was always the same. They could see everything, including wind on the FDA chip, (over 2000 parameters) ... but everyone was left scratching their heads.

I am not sure if what I did was necessary, but I am sure glad I did. Maybe the same thing happened to the OP. But as I say to the new guys coming on line now as Captains ... trust to your gut instinct, your senses are assessing more than you think!
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!
 
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Balerit
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Re: Increased thrust close to rotation

Sun Apr 22, 2018 1:42 pm

longhauler wrote:
I echo the comments of Barney Captain above. Sometimes, a gut feeling tells you something is up. Simply go from flex thrust to max thrust. If you're wrong, the worst that can happen is you burned more fuel.

I recall many years ago, taking off from YYZ in a full A321 for SFO, we used 33L, one of YYZ's shorter runways.

Just past half way down the runway, it didn't "look right". When the F/O said something like "this doesn't look right", it was all I needed. I went to TOGA thrust. In spite of what most on here think, the A321 is a beast on the ground and the extra thrust was quite a kick. We gave the gents in the FedEx terminal quite an airshow as we roared overhead at about a hundred feet!

When safely away and in cruise, we went over our calculations and entries. In my opinion, we had to have done something wrong. We could find nothing wrong. We told Maintenance Control and they did a scan on the engine parameters for take-off, saw the increase, but nothing wrong. I wrote several safety reports, saved my calculations, but the answer was always the same. They could see everything, including wind on the FDA chip, (over 2000 parameters) ... but everyone was left scratching their heads.

I am not sure if what I did was necessary, but I am sure glad I did. Maybe the same thing happened to the OP. But as I say to the new guys coming on line now as Captains ... trust to your gut instinct, your senses are assessing more than you think!


Good advice Cappy, better to be safe than sorry. We had a saying in maintenance: If in doubt, change it.
Licensed Aircraft Maintenance Engineer (retired).
 
Georgetown
Topic Author
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Re: Increased thrust close to rotation

Sun Apr 22, 2018 7:09 pm

Been a cool discussion. Thanks for all those in the know for chiming in. One of the great things about flying so much (it’s now up to about 150k - 200k butt in seat miles per year) is that you get a “base” sense of what is perceived as normal, and then a couple times per year something grabs my attention as as “different” and I’m able to come on here get some great perspective on what it may have been. It’s where I’ve learned about howling seals on cargo doors, why the slats on E170/190s sometimes have issues in cold weather (making for an exciting landing in BOS one time) and why we flew for what seemed like an eternity with the gear down once (hot brakes!).

Gives you an appreciation for what you guys up front and in maintenance do!
Let's go Hoyas!
 
Andre3K
Posts: 361
Joined: Tue May 30, 2017 10:11 pm

Re: Increased thrust close to rotation

Sun Apr 22, 2018 9:17 pm

longhauler wrote:
I echo the comments of Barney Captain above. Sometimes, a gut feeling tells you something is up. Simply go from flex thrust to max thrust. If you're wrong, the worst that can happen is you burned more fuel.

I recall many years ago, taking off from YYZ in a full A321 for SFO, we used 33L, one of YYZ's shorter runways.

Just past half way down the runway, it didn't "look right". When the F/O said something like "this doesn't look right", it was all I needed. I went to TOGA thrust. In spite of what most on here think, the A321 is a beast on the ground and the extra thrust was quite a kick. We gave the gents in the FedEx terminal quite an airshow as we roared overhead at about a hundred feet!

When safely away and in cruise, we went over our calculations and entries. In my opinion, we had to have done something wrong. We could find nothing wrong. We told Maintenance Control and they did a scan on the engine parameters for take-off, saw the increase, but nothing wrong. I wrote several safety reports, saved my calculations, but the answer was always the same. They could see everything, including wind on the FDA chip, (over 2000 parameters) ... but everyone was left scratching their heads.

I am not sure if what I did was necessary, but I am sure glad I did. Maybe the same thing happened to the OP. But as I say to the new guys coming on line now as Captains ... trust to your gut instinct, your senses are assessing more than you think!


What an excellent post. I really enjoy reading about the times where everything didn't go "according to plan". I just hope some of the other pilots and naysayers read this because these are the kinds of validation that maybe the passengers aren't all crazy and maybe this guy did hear the thrust increase. And it sounds like a sound decision by you to increase the thrust after it was already set and lo an behold you still have your job.

Point is metaphorically I think it's kind of like driving, you try your best to do the speed limit but sometimes you have to speed.
 
slcguy
Posts: 420
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Re: Increased thrust close to rotation

Tue Apr 24, 2018 12:09 pm

First thought is power setting for take off was set manually, with FMC/auto throttle engaged late in take off roll resulting in a power increase. A small increase in power may sound or feel significant but have little effect late in the take off roll. Also on a similar topic, flex power takeoffs often result in power increases after lift off. Been on a number of flights where when you expect the power to come back to climb power once airborne instead the engines power up to more than they were at takeoff for the en route climb.
 
BoeingGuy
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Re: Increased thrust close to rotation

Tue Apr 24, 2018 7:53 pm

slcguy wrote:
First thought is power setting for take off was set manually, with FMC/auto throttle engaged late in take off roll resulting in a power increase. A small increase in power may sound or feel significant but have little effect late in the take off roll. Also on a similar topic, flex power takeoffs often result in power increases after lift off. Been on a number of flights where when you expect the power to come back to climb power once airborne instead the engines power up to more than they were at takeoff for the en route climb.


On most Boeing airplanes, you can't engage the Autothrottle for takeoff above 50 knots. I don't know the 737 so well, so not sure, but the FCOM suggests it can't be engaged above 60 knots.

It is possible to have a thrust increase at climb thrust reduction but not very common. Might be more likely on a 757 or 767, in which you can't mix and match takeoff and climb derates. On those models if you do an Assumed Temperature derate takeoff, you'll get full Climb thrust. On other models you could select the CLB 1 or CLB 2 derates with an Assumed Temp takeoff.

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