Rushed take-off?
 

I want to open a thread about something I've noticed on the last few short-haul flights I've taken. I'm not sure if this is an issue or not (I'm sure I'll soon be told if it isn't!).

I've noticed that some pilots on short-haul flights "rev" (sorry - don't know the technical term) the engines to check they're stable literally as they are still turning onto the runway and before they are lined up.

Is this a common and accepted practice? Do any professional pilots frown upon it? (For example, if it caused you to mess up your alignment.) Is it at all a sympton of a rushed take-off?

I'd appreciate your comments.

I've been on the Flight Deck, and on approaching the Holding Point, have heard XXX123 are you ready Immediate?. If yes, then a/c is cleared for an "Immediate Take Off". A/c "spools up the engines" [advances the throttles] and enters the Runway...and departs. :ok:
IF not ready, then hold until they are.
Also in 35 years of [Civil] Aviation, I've lost count of the number of "rushed" departures :E
watp,iktch

Nothing to do with being rushed. It's a "rolling takeoff" - happens when all preflight checks are completed, the cabin is secure, and clearance to take off has already been given prior to entering the runway or whilst entering it. Of course you check with the guy in the seat next to you that he is happy to continue and off you go. Useful to keep the flow up on a busy strip.

I Think we have seen more 'rushed' T/Offs since the low cost has boomed, I can't remember the last time I saw an Aircraft doing an 'on the brakes roll off'

I still miss the DC9'S at the end of LBA'S R14, Doing a full thrust roll off the brakes!!:D

I remember an FR 737-200 in a rush going off R14 years ago, He was in a rush and had to be told to wait!!

The tower said wait, And he replied I can't wait!!:ugh:

G'day Nicholas,

The reason sometimes that jet aircraft are stopped on the runway and part power is applied before takeoff thrust is set is because sometimes the engines initially accelerate unevenly from idle. So rather than setting takeoff thrust straight from idle and getting a large thrust imbalance, part power is set, the engines stabilised and then takeoff thrust is set.

When doing a rolling takeoff you can advance the throttles as you enter the runway and if it all looks good then you can set takeoff thrust without getting a thrust imbalance.

It's not a rushed takeoff but an expedient way of entering the runway and taking off without stopping.

I'd be surprised if any jet operators set takeoff thrust on the brakes.

Regards,
BH.

Hey Mods, Has this thread been merged with another one?

Answer removed as question is rude!
 

Hopefully thread ended- I find the manner of querying it slightly offensive- it was more a burrowing exercise than an outright question. Where ever did 'rushed'/'messing up alignment' come into it?

Rainboe:

I raised the question because I wanted to know the answer, not because I was "burrowing". What a ridiculous assumption. If you check the name on the forum, you'll see that it's designed for questions. If you don't want to answer questions, then don't.

Bullethead, Caudillo and chiglet:

Thank you very much guys for your helpful, informative explanations.

On Concorde, you couldn't even do a "run-up on the brakes"... the aircraft would have literally started sliding forward.
So you lined up, did your last checks, released the brakes and slammed the trhottles forward, then very carefully monitored spool-up, reheat lighting correctly, etc., during the first few seconds of the take-off roll.

Nicholas49, I did take the trouble to answer your question, so i assume you were still thankful to me for doing so, even though I disliked your reference to
<<Is it at all a sympton of a rushed take-off?>>....an offensive comment you did not need to add. You could have simply asked the question without trying to burrow and discover what you thought may be a defective operation!

I have now removed my answer.

Rainboe,

I was of course grateful for your taking the time to answer the question.

I asked whether the practice was the symptom of a rushed take-off because it can appear that way to passengers. I remember when aircraft always waited on the runway before taking-off.

To clarify: by "rushed" I did not mean to imply inept piloting. Instead, I thought this practice may have been the take-off equivalent to the intensive-braking used to vacate a runway quickly when aircraft are waiting to land.

It is clear from everyone's answers that the practice is standard.

It was certainly not intended to be offensive.

I understand. No aircraft need wait on the runway- pre-takeoff checks can be rapidly completed. The only reason there is a pause before take-off is to wait for a preceding aeroplane to take-off and get clear, or one to land and clear before brakes release. Powering up on brakes is not desired or recommended, though an intermediate power may be required in icing conditions to ensure the engine de-icing can work long enough to ensure there are no problems during high power for take-off. So the ideal is to turn onto the runway and keep it rolling while you spool up the engines, without touching the brakes.

Having watched hundreds of aircraft take off from LPL whilst listening to comms from tower - pilot, It is a fairly regular thing to hear the pilot say "ready on reaching" as he approaches the holding point short of the rwy.
I have also heard the ATCO ask the pilot "are you ready for immediate departure?" this is often when there are aircraft on finals to get the departure off the ground sooner rather than later.

I am aware that LPL is under certain jurisdiction from Manch, there is usually a delay between each IFR departure as dictated by Manch.

Thanks, Rainboe.

Question regards braking on landing.
 

Hi guys I was watching some landings a Liverpool this morning runway wet, a 737 and a airbus think it was a 320 landed while I was there, on touch down with both aircraft I could hear a screeching noise sounded like tyres skidding for quite a few seconds after touchdown,

Would that be what I could hear, if so do the brakes come on automatically.

Nick.

Just the anguished sound of rubber accelerating from standstill to touchdown speed. 'Braking' in reverse if you like.

Perhaps it may be intresting to know that "immediate departure" is in fact an estabilished code phrase used to describe rolling take-off Rainobe and Nicholas have been discussing so eagerly.

That's why I posted the reply. :rolleyes::rolleyes::rolleyes::ugh::ugh:

If one didn't light you could still do the accel. If two failed it was unlikely to work - what dictated whether it worked was not the number of reheats but getting to M1.7 by the 15' accel limit on reheat use - there was no problem with assymetrics. Actually somedays (e.g. cold OATs) the accel could outpace the fuel transfer, the technique then was to switch off one reheat to allow the fuel to catch up...
Again, not quite right. It was a matter of performance - if the TOW was a certain margin below PLTOW (using a graph in the performance manual) then you only needed 3 reheats (called a "GO" takeoff), otherwise you did need all four (called a "STOP" takeoff). This check was made at 100kts not 60kts - the NHP called "100kts" and if all was well the E/O called "power set" and off you went. Interestingly, a single reheat failure after 100kts did not constitute an engine failure and you carried on irrespective of TOW (rare: once lit they tended to stay lit - it was getting them lit that could be problematic....). Two reheats out constituted an engine failure and was treated in the conventional way...
See above - the reheat GO/NO-GO decision was made at 100kts.
Offered purely out of interest - hope it helps...

NW1,
Many thanks!
And apologies to all for providing less than exact info.
CJ

...you're welcome - don't apologise; it's only a chat!

Sure, but as long as people are still interested in the subject and chat about it, it's always nice to get the facts right :8 !

Double engine failure
 

Hi,
What are the possibilities of a quad engined airplane (744, 340s only) having a simultaneous or one after the other engine failure during takeoff and the speed is v1-10~20kts? Would the airplane be able to climb on 2 engines if its fully loaded? if so what would be the biggest rate of climb in feet per min? what is the biggest of climb on these airplanes if only one engine fails?

Another question kind of relating to the above is what if the PF is the FO and an emergency occurs while taking off, would the captain immediately say "i have control" and perform the takeoff, and do the rest of the emergency, while the FO just helps with ATC and checklists?

G'day xxgunnerxx,

Years ago when I first started on the B747-400 we finishd a sim session a little early and had a bit of time to play. The instructor set the a/c at MTOW and the temp 30 degC then during the takeoff run had an engine fail after V1 and the other one on the same side fail at VR. So a double asymmetric engine failure at max weight on a hot day.

It was a handful to say the least and if you followed the flight manual it resulted in a gradual descent into the ground, however if you selected the flap up from F20 to F5, regardless of the speed, then there was a huge reduction in drag and the a/c would eventually accelerate and climb away as you cleaned up further. Once you got it clean there was more than enough thrust to complete the approach.

Second paragraph, if the take off is rejected the captain takes over, however if the take off is continued and the FO is handling things well he continues to fly and set up for the approach, though in my company the captain has to do the actual landing.

Regards,
BH.

Gunner- you need to understand what V1 means. In the scenario you suggest, no chance, therefore you should already have abandoned your take-off. Particulary with both on one side out, you would not be able to maintain runway direction at that speed. As Bullethead stated, at V1 you have a severe struggle on your hands, but it can be done. In real life, a significant number, probably a majority, of such failures would result in a crash.

757-200 Spoilers Question.
 

A less aviation inclined acquiantance of mine posed to me a question about something he observed on a flight home from LAS.He noticed the spoilers being deployed while in what appeared to be normal flight.What situations would exist for the spoilers to be deployed in flight?
Landing is the only time I've seen spoilers active, but then I can count on the fingers and thumb of one hand how many times I've flown. <G>
Thanks for your time.............. :)

I'm no expert, but I think if the pilot/ATC made a (big) mistake and they had to make a steep descent or quick deceleration, they can be extended to increase drag, but you don't see it done very often.

I've seen them deployed in flight on a 737 IIRC.

If you are asked to get a move on by ATC before starting a descent, or if you are given a late descent, why not use them?

Ideally probably would not want to as its effectively 'wasted' energy, but if it gets the desired effect for the situation, I don't think anyone has made a 'mistake'.

Disclaimer: I've never flown anything with spoilers on.

On a 757 the spoilers are used to increase drag and reduce lift in flight and on the ground. However they also supplement the ailerons in roll control depending on the amount of control wheel input. Therefore in a turn you will very like see them deploying without any direct input by the pilot other than through the normal control wheel input.

In a descent, or when slowing down in level flight, or when slowing down and descending they will be selected by manual input of the speedbrake lever. This is an almost every flight occurance at some point or other.

Finally the spoilers are fully deployed automatically ( normally ) or manually on landing to reduce lift and transfer weight to the main wheels (and brakes). This occurs as soon as the main gear trucks untilt, or when either thrust reverser is activated, or when manually applied.

It is not likely to be the case they were deployed my mistake, as to do so would be very difficult and extremely unlikely. They are used very often and normally to achieve a desired profile by either increasing a rate of descent, or increasing the drag to slow the aircraft down. The 757 is quite a slippery ship, and often needs assistance in reducing speed particularly in busy traffic environments or where absolute restrictions apply, such as in the USA (and other locations) where a mandatory 250 kts or below is a requirement below 10,000 feet for example.

Thanks for the informative replies. :)

Angle of Engines on MD-80
 

Why are (at least some) airliner engines mounted at an up angle to the horizon--the intake end higher than the nozzle end? I first noticed the design element on the MD-80.

http://www.simviation.com/pageimages/md80a.jpg

Apologies if the question is answered elsewhere on Pprune--I did look, and couldn't find it addressed.

Many thanks.

Lax Pax

Generally aligned with the direction that the air is flowing around the fuselage.

Thanks for the reply, N.

So then wing-mounted engines would not be angled because there's no fuselage and associated air stream to deal with(?).

The airflow ahead of the wing is an upwash, whereas the flow behind the wing is a downwash. If the engines on the wing are angled at all, they should be pointed slightly down. I haven't noticed if this is true or not. Too much of an angle would cause the jet exhaust to impinge the underside of the wing.

Not meaning to hijack the thread but I've recently noticed an MD80(2/3?) with engines removed; there are 3 windows aft of the left main door (assume same on stbd side). Having not noticed them before I presume they are masked when engines mounted. What would be located inside the cabin here? Thinking a galley perhaps?

Thx.

Sometimes there's a galley or such there, but mre likely there are SLF seats behind the engine inlets.

If you want something of an education in fluid flow, sit back there during a night takeoff in light snow, and watch the snowflakes change direction as the ship accelerates down the runway. REALLY interesting!

Flight idle and ground idle
 

What are the reasons for having different idle speeds for jet engines in different flight regimes? I note that there is a ground idle, which seems to be about 25% N1, and a flight idle, which seems to be around 40% N1, and both seem to be automatically set for idle in recent aircraft (I suppose pilots of older aircraft set these speeds manually?).

I've read of two potential reasons for the difference: (1) Higher idle speeds at high altitudes reduce the possibility of a flameout; and (2) flight idle is designed to match the drag of engine nacelles so that an aircraft can glide in a descent with the drag of the nacelles "erased."

Are these the real reasons? Are there others? Is the notion of flight idle and ground idle common to all jet aircraft, or only certain aircraft? Are there other idle speeds?

Another reason is a safety buffer, since it takes a LONG time for a CF6 (for example) to accelerate from 25% N1 to 40%.

There is also a consideration in some airplanes of reliably providing enough bleed air for all the systems airborne.

Anthony,

Some aircraft use different idle settings, others don't. Generally that's an engine function, rather than an aircraft function; airframes tend to use a variety of engines either as a customer choice or just as the airplane evolves...and what's an option on one engine installation may not be the same on another...even though it's the same type airplane. The B747, for example...three different basic engine makes to hang on it, with differnt models engine from different manufacturers.

I flew the C-130. The early models I flew didn't have a ground idle. These were turbopropeller airplanes; the T-56A-9 engine had no ground idle, but the T-56A-14 engine did. It was quieter, cooler, and burned less fuel on the ground. As previously mentioned, in flight, a higher idle speed means the engine spools up faster, and is less likely to flame-out...as well as producing more bleed air for pressurization, air conditioning, anti-ice, etc.

At low engine speeds, small openings have to be used in the engine, sometimes called acceleration bleeds, which dump air out of the compressor to keep it from stalling. As air enters the engine, it's compacted or compressed, and pushed through a series of stages which continue to do so, before it's slowed, pressure increased, and then fuel added to burn. In those compression stages, an orderly flow of air is necessary; increase the pressure too much without enough airflow, or allow the blades to turn slowly enough, and the airflow in there stalls; it can reverse direction in the engine, the flame in the engine can go out, and somewhere in there, engine damage can also occur. The acceleration bleeds are small valves which open to let some of that pressure out when the engine is operating at low speeds.

Wasting this compressed air in flight is pointless...especially when the bleed air is used for so many functions in the airframe. Preventing the engine from going so slowly as to open these bleeds is an important operational consideration, then, and is designed into the operating design of the engine. Some engines revert to ground idle on their own after touchdown, others must be put there manually.

Keeping a constant taxi speed
 

Is there some secret to getting a large aircraft to roll at a constant speed during taxi? I'm having a terrible time getting my aircraft to taxi at a reasonably constant speed, and I'm wondering if airline pilots have some special technique for doing this. Or is it just practice?

For example, it seems that a lightly loaded 747-400 will lazily start rolling at 35% N1, but then it will speed up and up and in no time you're rolling along at 30 knots. And yet if you set power to 32% N1, it will gradually slow down and stop. The "sweet spot" seems to be about 0.00008 knots in width. Sometimes I hit it entirely by accident, but otherwise constant tweaking of the throttles is required. I must be doing something wrong.

Ditto for the 737-800, except that you see results a bit quicker when you adjust the throttles. A 767-300ER seems to want to roll even at idle, but it does seem to speed up and slow down more rapidly, which makes it a little easier to set the throttle.

So, how do you taxi at a steady speed in real life? Do you try to find the right spot for the throttles or just constantly adjust them. And is it cheating to set the throttles so that you gradually accelerate and then tap the brakes a bit to slow down when necessary? I don't know how much this heats the brakes. On a small plane I'd use differential braking to help in turns but I don't know if this is allowed or a good idea on large aircraft (and in any case differential braking doesn't seem to make a lot of difference in turns, at least in simulation).

Keeping a constant taxi speed
 

Is there some secret to getting a large aircraft to roll at a constant speed during taxi? I'm having a terrible time getting my aircraft to taxi at a reasonably constant speed, and I'm wondering if airline pilots have some special technique for doing this. Or is it just practice?

For example, it seems that a lightly loaded 747-400 will lazily start rolling at 35% N1, but then it will speed up and up and in no time you're rolling along at 30 knots. And yet if you set power to 32% N1, it will gradually slow down and stop. The "sweet spot" seems to be about 0.00008 knots in width. Sometimes I hit it entirely by accident, but otherwise constant tweaking of the throttles is required. I must be doing something wrong.

Ditto for the 737-800, except that you see results a bit quicker when you adjust the throttles. A 767-300ER seems to want to roll even at idle, but it does seem to speed up and slow down more rapidly, which makes it a little easier to set the throttle.

So, how do you taxi at a steady speed in real life? Do you try to find the right spot for the throttles or just constantly adjust them. And is it cheating to set the throttles so that you gradually accelerate and then tap the brakes a bit to slow down when necessary? I don't know how much this heats the brakes. On a small plane I'd use differential braking to help in turns but I don't know if this is allowed or a good idea on large aircraft (and in any case differential braking doesn't seem to make a lot of difference in turns, at least in simulation).