A Takeoff Question
Discussion
Every time I've flown in a commercial jet, I've always wondered why, upon takeoff, the engines seem to be powered up a little to get the aircraft rolling and then really opened up to fire it down the runway. This not only sounds cool, but also adds to the feeling of unrelenting acceleration! What is the reason behind this and not simply 'flooring it' right from the off? I am assuming here, of course, that this is the case and it isn't just how jet engines power up! This video demonstrates it (the sound part at least!) nicely between 1:19 for the spool-up bit and 1:22 onwards for the high-power bit. It also really makes me want to go somewhere in a Boeing 777!
http://www.youtube.com/watch?v=Ii0nUcm24Jw&fea...
http://www.youtube.com/watch?v=Ii0nUcm24Jw&fea...
Takeoff depends on several things, but if you are talking about why it's not a full-snap spool-up? In fact modern only occasionally use full thrust, most take-offs are derated in power terms, reducing engine wear/thermal shock but more importantly maintenance, - the rolling take-offs are generally down to operator and airport requirements.
Morning.
Asymmetric spool up of the engines.
Despite the technology and digital control over modern engines, they can (and do) spool up at uneven rates. Imagine on a low friction runway surface (wet/snow covered etc) or even a dry runway, If one engine spools up faster then the other...hello grass.
The engine manufacturer will guarantee symmetrical spool up of the engines at a certain point. So, it's standard procedure to 'stand the engines up' to that point. This lets them stabilise (the thrust is not enough to push you off the runway in the event of asym. spool up), then go for take off thrust. The engines on my chariot are considered stable at 50% and it's right enough, I've never yet seen asymmetric spool up above this figure. Below, yes quite a few times.
Asymmetric spool up of the engines.
Despite the technology and digital control over modern engines, they can (and do) spool up at uneven rates. Imagine on a low friction runway surface (wet/snow covered etc) or even a dry runway, If one engine spools up faster then the other...hello grass.
The engine manufacturer will guarantee symmetrical spool up of the engines at a certain point. So, it's standard procedure to 'stand the engines up' to that point. This lets them stabilise (the thrust is not enough to push you off the runway in the event of asym. spool up), then go for take off thrust. The engines on my chariot are considered stable at 50% and it's right enough, I've never yet seen asymmetric spool up above this figure. Below, yes quite a few times.
Brother D said:
Takeoff depends on several things, but if you are talking about why it's not a full-snap spool-up? In fact modern only occasionally use full thrust, most take-offs are derated in power terms, reducing engine wear/thermal shock but more importantly maintenance, - the rolling take-offs are generally down to operator and airport requirements.
Flex/Derate has nowt to do with the OP's question.You are correct though, full power is rarely used.
Chuck328 said:
Morning.
Asymmetric spool up of the engines.
Despite the technology and digital control over modern engines, they can (and do) spool up at uneven rates. Imagine on a low friction runway surface (wet/snow covered etc) or even a dry runway, If one engine spools up faster then the other...hello grass.
The engine manufacturer will guarantee symmetrical spool up of the engines at a certain point. So, it's standard procedure to 'stand the engines up' to that point. This lets them stabilise (the thrust is not enough to push you off the runway in the event of asym. spool up), then go for take off thrust. The engines on my chariot are considered stable at 50% and it's right enough, I've never yet seen asymmetric spool up above this figure. Below, yes quite a few times.
That makes perfect sense! Thank you.Asymmetric spool up of the engines.
Despite the technology and digital control over modern engines, they can (and do) spool up at uneven rates. Imagine on a low friction runway surface (wet/snow covered etc) or even a dry runway, If one engine spools up faster then the other...hello grass.
The engine manufacturer will guarantee symmetrical spool up of the engines at a certain point. So, it's standard procedure to 'stand the engines up' to that point. This lets them stabilise (the thrust is not enough to push you off the runway in the event of asym. spool up), then go for take off thrust. The engines on my chariot are considered stable at 50% and it's right enough, I've never yet seen asymmetric spool up above this figure. Below, yes quite a few times.
I've spend the last hour on YouTube watching various aircraft-related videos and the hour before that on Wikipedia and I must say I find the whole thing fascinating. This is probably my favourite find so far: http://en.wikipedia.org/wiki/Non-stop_flight#Curre... The fact you can board an aircraft that will then takeoff and fly for 18.5 hours continuously is, frankly, amazing. I suppose with a reasonably light load something like a 777-200LR or A340-500 could pretty much fly from any point to any point on Earth without stopping.
Mr Sparkle said:
Does how much air the engine can suck in have much effect on power? I heard that while stationary a jet engine can get enough air to reach full power as opposed to when it is being blasted by a 300 mph head wind. How much difference does this make?
I think that's 2 separate questions...Tha amount of air an engine can suck in has an effect on power, just like it does on a car engine. Twice the air = twice the power. But; the spped of the aircraft doesn't have much effect on how much air it can take in. The inlet is designed to always give the front of the engine a fairly constant airflow.
However, the speed of the aircraft has a significant effect on thrust. Thrust comes from change in momentum of the air through the engine. The speed of the air coming out of the back of a turbofan is fixed (at a given power setting), lets say 600 mph. If the aircraft is stationary, the engine is accelerating the air from 0 to 600 mph. If the aircraft is already doing 300 mph, then the engine is only accelerating the air from 300 to 600 mph, so you only get half the momentum change, and half the thrust
I assumed it was that the pilot flying advances the throttle to ~ 40% of maximum thrust, to ensure both engines spool up to the same level (40% N1 is it?). Then the pilot flying hits the Take-Off/Go-Around (TOGA) button. This engages the autothrottle, which will advance the thrust levers to the correct level as required at the airfield (adjusting for payload, altitude and temperature). This is obviously done manually if AT isn't being used.
Incidentally, co-pilots who fly for Cathay Pacific (and a few other Far Eastern airlines) often place their glove-bound hand behind the throttle quadrant as the pilot flying advances the throttle levers, to ensure all throttle levers are being advanced in unison.
Incidentally, co-pilots who fly for Cathay Pacific (and a few other Far Eastern airlines) often place their glove-bound hand behind the throttle quadrant as the pilot flying advances the throttle levers, to ensure all throttle levers are being advanced in unison.
Edited by TheDiplomat on Tuesday 5th June 15:33
No jet engine can make peak thrust at zero forward speed, even a high bypass ratio turbofan, because the fan and compressor efficiency is reduced by the intake cowl being choked. As the plabe starts to move, the upstream pressure improves, and so for the same downstream pressure the pressure ratio has fallen across the compressor, and it gets more efficient.
However, on a comercial jet this is a fairly small effect, but the bypass fan intake cowl is sized to optimise "drag on cruise", rather than max thrust at zero speed. On military turbojets the effect is much more pronounced as the engine is generally buried in the middle of the airframe, hence you often see a lot of inward pivoting "pressure relief" doors that help the engines "gulp factor" at low speeds (or high AOA's)
However, on a comercial jet this is a fairly small effect, but the bypass fan intake cowl is sized to optimise "drag on cruise", rather than max thrust at zero speed. On military turbojets the effect is much more pronounced as the engine is generally buried in the middle of the airframe, hence you often see a lot of inward pivoting "pressure relief" doors that help the engines "gulp factor" at low speeds (or high AOA's)
TheDiplomat said:
Incidentally, co-pilots who fly for Cathay Pacific (and a few other Far Eastern airlines) often place their glove-bound hand behind the throttle quadrant as the pilot flying advances the throttle levers, to ensure all throttle levers are being advanced in unison.
We do not Edited by TheDiplomat on Tuesday 5th June 15:33
Some gloves would be glorious, but I believe this is a predominately Japanese thing. You may see thrust levers being advanced from behind the base of the levers by old-time hooray Henries during taxy, particularly on 4 holers. They pretend it makes it easier to apply smaller amounts of thrust.
Nowadays on take off, there is an SOP in all airlines that one pilot will have his hand on the thrust lever until V1. Most airlines I've seen, it is the captain. On Boeings, due to the Autothrottle take off profile, prior to 80kts, you need your hand in a position to disconnect the A/T before closing the levers. Above 80, you don't need to disconnect manually if you abort, so somewhere sensible where you have effortless finger reach of the revsersers would be sensible.
And yes, Diplomats description was pretty clear on all bar the gloving!
Spoof said:
Or, for bus drivers, you advance to ~50% then unless it's TOGA Tuesday, you just move the thrust levers to flex/mct for a derated takeoff.
Off topic, a quick question has sprung to mind. I know you have your TOGA detent, FLX/MCT gate and CLB gate. Now, I understand to smoothly transition to manual thrust, you line up TLA with N1 before disconnecting.But, what if you were to move your levers to a position below the CLB gate, say in th cruise? Does this have any impact on auto thrust operation? Would it continue in CLB mode, and step climb etc in the same way?
Just curious. I like tractors!
ETOPS said:
Off topic, a quick question has sprung to mind. I know you have your TOGA detent, FLX/MCT gate and CLB gate. Now, I understand to smoothly transition to manual thrust, you line up TLA with N1 before disconnecting.
But, what if you were to move your levers to a position below the CLB gate, say in th cruise? Does this have any impact on auto thrust operation? Would it continue in CLB mode, and step climb etc in the same way?
Just curious. I like tractors!
You get a flashing FMA on the PFD advising you to return the levers to the Clb detent. The autothrust system will stay engaged but limit the thrust available to the position on the levers.But, what if you were to move your levers to a position below the CLB gate, say in th cruise? Does this have any impact on auto thrust operation? Would it continue in CLB mode, and step climb etc in the same way?
Just curious. I like tractors!
Chuck328 said:
Spoof said:
Or, for bus drivers, you advance to ~50% then unless it's TOGA Tuesday, you just move the thrust levers to flex/mct for a derated takeoff.
Pedant mode: It's a flexible temp not a derate, there is a difference. 
ETOPS said:
We do not Some gloves would be glorious, but I believe this is a predominately Japanese thing.
You may see thrust levers being advanced from behind the base of the levers by old-time hooray Henries during taxy, particularly on 4 holers. They pretend it makes it easier to apply smaller amounts of thrust.
Nowadays on take off, there is an SOP in all airlines that one pilot will have his hand on the thrust lever until V1. Most airlines I've seen, it is the captain. On Boeings, due to the Autothrottle take off profile, prior to 80kts, you need your hand in a position to disconnect the A/T before closing the levers. Above 80, you don't need to disconnect manually if you abort, so somewhere sensible where you have effortless finger reach of the revsersers would be sensible.
And yes, Diplomats description was pretty clear on all bar the gloving!
It is helpful information like this that destroys the illusion of what we SLF down the back actually think is happening up front! Some gloves would be glorious, but I believe this is a predominately Japanese thing. You may see thrust levers being advanced from behind the base of the levers by old-time hooray Henries during taxy, particularly on 4 holers. They pretend it makes it easier to apply smaller amounts of thrust.
Nowadays on take off, there is an SOP in all airlines that one pilot will have his hand on the thrust lever until V1. Most airlines I've seen, it is the captain. On Boeings, due to the Autothrottle take off profile, prior to 80kts, you need your hand in a position to disconnect the A/T before closing the levers. Above 80, you don't need to disconnect manually if you abort, so somewhere sensible where you have effortless finger reach of the revsersers would be sensible.
And yes, Diplomats description was pretty clear on all bar the gloving!
Having pulled out onto the runway, our crew see Bob and Jane from Air Something line up on an adjacent runway in their new A380. Looks are exchanged, last night's fierce debate of Airbus/Boeing at the local watering hole still fresh in the minds. The A380 squats on the nosewheel, rocking back and forward as the engines are brought up to power. On the 777, the F/O is braced against the overhead panel pressing down on the brakes with all his might, the captain adopting a relaxed pose of elbow on window, one hand on control column and the other hand slamming the throttle controls foward. A howl fills the air as the plane vibrates against the smoking brakes when Air Something, displaying the international hand symbol for "see ya suckers" nails it and takes off down the runway.
The 777 is fast off the mark in response, the F/O calling out the speed in between goading and abuse over the radio to the Airbus.."yeah, try and run fatty, we gotcha...we gotcha YESSSS" as the 777 pulls past and off it goes.
"Re-match in Singapore next week!" calls the A380 in response to the 777 victory 'wing waggle' as it clears the end of the runway.
As I sit in economy, questioning my choice of career, I can only conclude that this is what happens :-)
We won't tell anyone!
XB70 said:
ETOPS said:
We do not Some gloves would be glorious, but I believe this is a predominately Japanese thing.
You may see thrust levers being advanced from behind the base of the levers by old-time hooray Henries during taxy, particularly on 4 holers. They pretend it makes it easier to apply smaller amounts of thrust.
Nowadays on take off, there is an SOP in all airlines that one pilot will have his hand on the thrust lever until V1. Most airlines I've seen, it is the captain. On Boeings, due to the Autothrottle take off profile, prior to 80kts, you need your hand in a position to disconnect the A/T before closing the levers. Above 80, you don't need to disconnect manually if you abort, so somewhere sensible where you have effortless finger reach of the revsersers would be sensible.
And yes, Diplomats description was pretty clear on all bar the gloving!
It is helpful information like this that destroys the illusion of what we SLF down the back actually think is happening up front! Some gloves would be glorious, but I believe this is a predominately Japanese thing. You may see thrust levers being advanced from behind the base of the levers by old-time hooray Henries during taxy, particularly on 4 holers. They pretend it makes it easier to apply smaller amounts of thrust.
Nowadays on take off, there is an SOP in all airlines that one pilot will have his hand on the thrust lever until V1. Most airlines I've seen, it is the captain. On Boeings, due to the Autothrottle take off profile, prior to 80kts, you need your hand in a position to disconnect the A/T before closing the levers. Above 80, you don't need to disconnect manually if you abort, so somewhere sensible where you have effortless finger reach of the revsersers would be sensible.
And yes, Diplomats description was pretty clear on all bar the gloving!
Having pulled out onto the runway, our crew see Bob and Jane from Air Something line up on an adjacent runway in their new A380. Looks are exchanged, last night's fierce debate of Airbus/Boeing at the local watering hole still fresh in the minds. The A380 squats on the nosewheel, rocking back and forward as the engines are brought up to power. On the 777, the F/O is braced against the overhead panel pressing down on the brakes with all his might, the captain adopting a relaxed pose of elbow on window, one hand on control column and the other hand slamming the throttle controls foward. A howl fills the air as the plane vibrates against the smoking brakes when Air Something, displaying the international hand symbol for "see ya suckers" nails it and takes off down the runway.
The 777 is fast off the mark in response, the F/O calling out the speed in between goading and abuse over the radio to the Airbus.."yeah, try and run fatty, we gotcha...we gotcha YESSSS" as the 777 pulls past and off it goes.
"Re-match in Singapore next week!" calls the A380 in response to the 777 victory 'wing waggle' as it clears the end of the runway.
As I sit in economy, questioning my choice of career, I can only conclude that this is what happens :-)
We won't tell anyone!
May it please the court, that at no point did I declare, implicitly or otherwise, that drag-racing/chest beating doesn't occur.
Chuck328 said:
ETOPS said:
Off topic, a quick question has sprung to mind. I know you have your TOGA detent, FLX/MCT gate and CLB gate. Now, I understand to smoothly transition to manual thrust, you line up TLA with N1 before disconnecting.
But, what if you were to move your levers to a position below the CLB gate, say in th cruise? Does this have any impact on auto thrust operation? Would it continue in CLB mode, and step climb etc in the same way?
Just curious. I like tractors!
You get a flashing FMA on the PFD advising you to return the levers to the Clb detent. The autothrust system will stay engaged but limit the thrust available to the position on the levers.But, what if you were to move your levers to a position below the CLB gate, say in th cruise? Does this have any impact on auto thrust operation? Would it continue in CLB mode, and step climb etc in the same way?
Just curious. I like tractors!
Must learn more about Airbii. Very clever box of tools.
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