Questions for the airline pilots on here
Discussion
I have a couple of daft questions for any pilots/plane lovers on here.
In a normal passenger jet eg. 737/757.....
1. How do you decide if you are going to do a standing or rolling takeoff when looking down the runway?
2. What would happen if from a standing start, you applied full power stright away (in car terms, putting the pedal to the floor).
In a normal passenger jet eg. 737/757.....
1. How do you decide if you are going to do a standing or rolling takeoff when looking down the runway?
2. What would happen if from a standing start, you applied full power stright away (in car terms, putting the pedal to the floor).
...you daft twaatCan't answer for the airline types but speaking for medium-large bizjets (same principles)...
Firstly you need to know that when the tower clear an aircraft to enter an active runway the clearance can either be to line up and wait or cleared for (immediate) take-off so the choice of whether to keep rolling or not doesn't always lie with the crew.
1. Take-off data (distance) is predicated upon a standing start. Rolling starts have infinite variables not least how fast the aircraft is moving when it enters the runway. If I have calculated a (say) 3000 foot take-off distance required and am operating from a 10000 foot runway then I would probably elect and brief for a rolling take-off (if cleared to do so) on the basis that there's no point in bringing the aircraft to a stop only to have to get it rolling again.
Most modern jets have some form of variable vane guidance system on the inlet side to prevent compressor stalls (air 'choking' in the intake) which was often a risk on older types of engine. It's still a good idea (airmanship) not to ram the levers forward.
If however the take-off distance required and take-off distance available are close to each other then it might be prudent to opt for a standing start.
2. Simply ramming the levers against the stops is rarely a good idea and never during take-off. Most pilots will stand the levers up to the vertical position, check the engine instruments for anomalies and then apply full power as they release the brakes. Fine tuning of the power setting will then be achieved either by the non-flying pilot or autothrottle system.
The principle is the same for all engines whether they be turbine or piston, some turbo-charged piston engines need careful handling. There is also the argument that applying immediate full power to propellor driven aircraft results in cavitation of the air around the propellors and (initially) reduce thrust.
Hope this makes sense.
Firstly you need to know that when the tower clear an aircraft to enter an active runway the clearance can either be to line up and wait or cleared for (immediate) take-off so the choice of whether to keep rolling or not doesn't always lie with the crew.
1. Take-off data (distance) is predicated upon a standing start. Rolling starts have infinite variables not least how fast the aircraft is moving when it enters the runway. If I have calculated a (say) 3000 foot take-off distance required and am operating from a 10000 foot runway then I would probably elect and brief for a rolling take-off (if cleared to do so) on the basis that there's no point in bringing the aircraft to a stop only to have to get it rolling again.
Most modern jets have some form of variable vane guidance system on the inlet side to prevent compressor stalls (air 'choking' in the intake) which was often a risk on older types of engine. It's still a good idea (airmanship) not to ram the levers forward.
If however the take-off distance required and take-off distance available are close to each other then it might be prudent to opt for a standing start.
2. Simply ramming the levers against the stops is rarely a good idea and never during take-off. Most pilots will stand the levers up to the vertical position, check the engine instruments for anomalies and then apply full power as they release the brakes. Fine tuning of the power setting will then be achieved either by the non-flying pilot or autothrottle system.
The principle is the same for all engines whether they be turbine or piston, some turbo-charged piston engines need careful handling. There is also the argument that applying immediate full power to propellor driven aircraft results in cavitation of the air around the propellors and (initially) reduce thrust.
Hope this makes sense.
Edited by Flintstone on Friday 17th October 19:20
ajcj said:
Is there such a thing in tight situations as a 'carrier take-off', where the aircraft is held on the brakes while the throttles are run up to maximum power to get the fastest start possible, or was the chap I heard describing this in the pub full of manure?
http://www.youtube.com/watch?v=MbQpCcQrsKwYep. But then Carrier-Based fighter planes are designed for STOL work. I don't think that your typical airliner could pull that off.
ajcj said:
Is there such a thing in tight situations as a 'carrier take-off', where the aircraft is held on the brakes while the throttles are run up to maximum power to get the fastest start possible, or was the chap I heard describing this in the pub full of manure?
yes indeed.. it wasn't pub talk in this case.edited to add:
Do you ski? A number of the smaller international airports with very high moutains surrounding them in close proxmity will mean that some jets use this technique. Its not ideal to run turbofan engines at high power for extended periods of time on the ground because the chances of foreign object debris ingestion are much higher.
Edited by Merritt on Friday 17th October 19:21
It can be done, but jet engines produce more thrust the faster they go. It's to do with airflow being rammed in rather than sucked in.
If you are that performance limited, then I wouldn't be going. If I was flying something little out of somewhere like a grass strip that was short, then I would even shut the aircraft down, push the tail into the hedge to get as much room as possible and then have full power set before releasing the brakes.
I would not take any risks like that at work and as there are big safety factors built in, then we are rarely limited by take off distance available in this country. Especially as we generally use derated thrust for the calculations and have a load more power available if we need it.
Somewhere hot and high would be very different matter though.
Flintstone has pretty much nailed it though.
If you are that performance limited, then I wouldn't be going. If I was flying something little out of somewhere like a grass strip that was short, then I would even shut the aircraft down, push the tail into the hedge to get as much room as possible and then have full power set before releasing the brakes.
I would not take any risks like that at work and as there are big safety factors built in, then we are rarely limited by take off distance available in this country. Especially as we generally use derated thrust for the calculations and have a load more power available if we need it.
Somewhere hot and high would be very different matter though.
Flintstone has pretty much nailed it though.
As per IforB, if the takeoff performance vs available distance and conditions was that tight then your best bet is to think about another way of getting home (and better still not to have gone into the strip in the first place). There a couple of reports of sadly fatal accidents involving take-off performance miscalculations in this month's AAIB bulletin:
http://www.aaib.gov.uk/publications/bulletins/octo...
The PA28 was marginal, a suspected worn and underperforming engine suspected of sapping enough performance to cause the accident, and the circumstances surrounding the Saratoga meant it was just an accident looking for a grid reference from the outset.
In tailwheel types, applying anything near maximum power with the brakes applied is asking for a new prop and engine, and a suitable bill to accompany.
http://www.aaib.gov.uk/publications/bulletins/octo...
The PA28 was marginal, a suspected worn and underperforming engine suspected of sapping enough performance to cause the accident, and the circumstances surrounding the Saratoga meant it was just an accident looking for a grid reference from the outset.
In tailwheel types, applying anything near maximum power with the brakes applied is asking for a new prop and engine, and a suitable bill to accompany.
IforB said:
Flintstone has pretty much nailed it though.
Seconded, with a couple of additional provisos: It's SOP (at my outfit) to do a rolling take-off where possible, subject to the timing of the Take-off clearance you receive from ATC with the aim of achieving what we call 'Minimum Runway Occupancy". The difference performance-wise between a rolling start and a standing start is difficult to measure and is said to be negligable - There are safety factors incorporated into the performance numbers anyhow. However, (on the 747-400) at certain combinations of aircraft weight and Centre of Gravity limitations we have to set a minimum level of thrust (1.2 EPR) at a standstill before commencing the take-off roll. A rolling take-off also reduces the risk of an engine 'surge' (a bit like a back-fire) with high power settings at low speed.Ref question 2: If you don't 'stand-up' the power and let the engines stabilise before setting Take-off thrust you run the risk of exiting the runway stage left (or right) in an embarassing and expensive fashion, because at low speeds (idle to approx 40%N1) the engines do not always accelerate at the same rate!
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