take off rev hold

[chris lz]

Circumstance: date Jan 1996, TWA 767-200


I am trying to see if anyone can shed light on my memory, which could be all off as it is now very vague, and 25 years ago. It was I believe a somewhat wet or foggy evening at Logan Airport. The pilot announced he would be using a different take off technique than usual. I'm not sure but my memory is he was going to rev the engines to the maximum planned take off thrust before releasing the brakes. I can't remember what the reason was except something to do with either fog or wet runway. Could this make sense?


Thanks

[Vessbot]

It decreases the runway required (or increases the weight you can take off a given runway) since you'll reach takeoff speed quicker with full thrust the instant you start moving, rather than building up as you roll the first few hundred feet.

[chris lz]

Is this a common practice in wet conditions?

[wiggy]

Circumstance: date Jan 1996, TWA 767-200


I am trying to see if anyone can shed light on my memory, which could be all off as it is now very vague, and 25 years ago. It was I believe a somewhat wet or foggy evening at Logan Airport. The pilot announced he would be using a different take off technique than usual. I'm not sure but my memory is he was going to rev the engines to the maximum planned take off thrust before releasing the brakes. I can't remember what the reason was except something to do with either fog or wet runway. Could this make sense?


Thanks

It's going to depend on the aircraft/engine type but in addition to any runway length considerations given the weather you describe engine anti-ice/de-ice might also have been another factor at a guess..

"Icing conditions" can exists at above zero celsius if visible moisture such as fog is present or there is water on the ground - you don't actually need ice/snow on the ground for it to be a consideration.

In icing conditions on many types there is a requirement to have the engines spooled to a high power setting with the aircraft stationary at regular mandated minimum intervals as you taxi-out, and the last run up might have to be done on the runway if it's due, and also on some types in any event in icing conditions there's a requirement to always have a run up before brake release.

Whatever the reason, icing, weight, or both telling the passengers over the PA it's being done because the aircraft is heavy is a good simple cover all.

[bean]

You'll reach v1 quicker so if you need to abort on a wet runway, you increase the available stopping distance as anti skid brakes might come into play. Same asa ABS on a car

[wiggy]

You'll reach v1 quicker so if you need to abort on a wet runway, you increase the available stopping distance as anti skid brakes might come into play. Same asa ABS on a car

True, but I think it's the "on the brakes" run up that the OP is querying, not the use of full power itself.

As I recall it we didn't get any "extra" credit in our takeoff calculations for stationary run up, but on one type I flew there was a caveat that if you were with 10 (?) tonnes of max and had to start the takeoff roll from a stand still (i.e. you couldn't achieve or sequencing didn't allow a rolling take-off from runway entry) then you had to stand the engines up on the brakes.

[cattletruck]

Got the red traffic light once whilst riding around Corfu on a motorbike. What followed next was something I never expected to see. A B737 sitting to the right of the intersection spooled up its engines blowing dust, dirt and garbage downwind for blocks, the screaming engines were deafening. Eventually the brakes came off, the thing roared down the runway and the traffic lights turned green.

[TotalBeginner]

I'm not familiar with the 767, but in the conditions that you describe it sounds like it could have been a static engine run-up. When the OAT is close to zero and there's fog, snow or other precipitation some engine manufacturers stipulate a requirement to perform a static engine run-up at certain intervals / prior to setting takeoff power. This procedure ensures that any ice that has accumulated on the fan blades is shed and also gives the opportunity to check that the engines are stable with acceptable levels of vibration.

Here's an example: https://youtu.be/0T0Lgd9jv-k?t=271

[BBK]

To the OP

It sounds like this may have been the procedure used when Nacelle Anti Ice is used in icing conditions.

On the type I’m familiar with we used the 60/30/30 rule. 60 percent N1 for 30 seconds every 30 minutes. That was on a CF6. it sounds quite dramatic so I’d always make a PA to inform the passengers in advance what was happening.

edited to add: agree with Wiggy and TB above. Hadn’t seen their replies when I posted.

[bean]

ChrisLz, see what you've started🤣

[vilas]

Circumstance: date Jan 1996, TWA 767-200


I am trying to see if anyone can shed light on my memory, which could be all off as it is now very vague, and 25 years ago. It was I believe a somewhat wet or foggy evening at Logan Airport. The pilot announced he would be using a different take off technique than usual. I'm not sure but my memory is he was going to rev the engines to the maximum planned take off thrust before releasing the brakes. I can't remember what the reason was except something to do with either fog or wet runway. Could this make sense?

Thanks
Were two of you Boeing test pilots? I am sure there are procedures in place for 767. If it's not there you just don't do it at least not in today's times. Doing on wet runway is useless as it brakes will slip and depending on which side slips first you can land up in soup.

[airseb]

Both my last type (777) and my actual type (320) have this. It’s there to check for any abnormal engine paramaters (mainly vibrations which would mean icing somewhere on the compressor) when in icing conditions.

[TotalBeginner]

I found a Boeing 767 FCOM online. Here’s the answer...

Takeoff Procedure

Do the normal Takeoff Procedure with the following modification:

When engine anti-ice is required and the OAT is 3°C or below, the takeoff must be preceded by a static engine run-up. Use the following procedure: PF

Run-up to a minimum of 60% N1 for approximately 30 seconds duration and confirm stable engine operation before the start of the takeoff roll.

[stilton]

It decreases the runway required (or increases the weight you can take off a given runway) since you'll reach takeoff speed quicker with full thrust the instant you start moving, rather than building up as you roll the first few hundred feet.


Does it though ?

Stopping on the runway, holding the brakes on while you increase power to take off thrust then releasing brakes means you’re accelerating all that mass from zero speed


A careful turn on to the runway, maximizing the use of all available distance with engines spooled as your line up is completed, then immediately selected take off thrust has the advantage of ten knots or so ‘in the bag’ at the same location as your static technique


I’d rather have the momentum

[Vessbot]

Does it though ?

Stopping on the runway, holding the brakes on while you increase power to take off thrust then releasing brakes means you’re accelerating all that mass from zero speed


A careful turn on to the runway, maximizing the use of all available distance with engines spooled as your line up is completed, then immediately selected take off thrust has the advantage of ten knots or so ‘in the bag’ at the same location as your static technique


I’d rather have the momentum

If you're cornering like that and with the engines spooling, I'd rather not be onboard.

The performance data provider for my airline supplies higher performance solutions with standing takeoff specified

[stilton]

If you're cornering like that and with the engines spooling, I'd rather not be onboard.

The performance data provider for my airline supplies higher performance solutions with standing takeoff specified



Interesting ‘misinterpretation’ of what I said


This has been debated before, at length and on this forum


A 90 degree turn onto a runway requires 5 knots or so to complete or you’ll come to a premature stop, that’s hardly a formula 1 turn, as you align, engines can be spooled and you easily have 10 knots plus in the same spot you’re holding the brakes against power, shaking and juddering, making your passengers nervous

[Vessbot]

Interesting ‘misinterpretation’ of what I said


This has been debated before, at length and on this forum


A 90 degree turn onto a runway requires 5 knots or so to complete or you’ll come to a premature stop, that’s hardly a formula 1 turn, as you align, engines can be spooled and you easily have 10 knots plus in the same spot you’re holding the brakes against power, shaking and juddering, making your passengers nervous

If you're relying on cutesy invented tricks to beat the test pilots' numbers, maybe it's time take a look again with an open mind at the other side of the debate.

If performance requires a standing takeoff, that's simply a burden the passengers are gonna have to bear. Or maybe we can taxi back to the gate to deboard a few of them and reduce weight, and that would ease their nerves?

Spooling the engines before you're aligned is poor technique because it puts you into an unnecessary corner where now you're limited by time to finish aligning, and if you take too long the plane will be rolling too fast before you're aligned, while if you do it too quickly you might not align at all. In mild cases it's annoying and unprofessional to give it like that to the FO, and in extreme cases worse than that (I remember once having to quickly stomp on the rudder as it was pointed off into the grass, and I realized he had given it to me like that and with the throttles mostly forward, without having briefed that. I quickly swung the nose over and continued the takeoff, but the right action would have been to abort and have a talk about it on the taxiway.)

Here's an example of "rolling too fast before you're aligned:" https://www.youtube.com/watch?v=JoGeKdNxH4U

Now you might be able to consistently ride the Goldilocks zone and always manage to get it so you finish aligning before too much speed builds up (and do it accurately), but why put yourself into that situation in the first place?

People need to stop doing everything at a breakneck pace trying to scrap for every 2 seconds they can save. It lowers your blood pressure and fosters a cockpit environment conducive to thoroughness. (I'm talking more generally now than just this particular bit of flying)

[pineteam]

] Does it though ?

Stopping on the runway, holding the brakes on while you increase power to take off thrust then releasing brakes means you’re accelerating all that mass from zero speed


A careful turn on to the runway, maximizing the use of all available distance with engines spooled as your line up is completed, then is and immediately selected take off thrust has the advantage of ten knots or so ‘in the bag’ at the same location as your static technique


I’d rather have the momentum

I asked the question to Airbus Customer Services and they consider that the rolling take off technique is equivalent in terms of performance to the static take off. Also the computation logic in Flysmart considers a take off with initial thrust at minimum idle rating computed at 0 kt and brakes released form the beginning. They have no particular recommendation to perform static or a rolling take off; It's up to the flight crew decision. I personally most of the times do rolling take off. It decreases the risk of FOD ingestion. It's actually recommended by the engines manufacturer to do rolling take off on A320 equipped with PW1100 for this reason.


Here's an example of "rolling too fast before you're aligned:"

This is crazy. I don't see the point: Trying to save few meters? It's not bush flying. And adding so much stress on the nose gear on top of that. I never add power until the aircraft is fully aligned. Performance calculation assumes the worst case scenario brakes released at 0 kt and idle power on Airbus at least so you well covered.

[Cough]

Have a really big think before you open up the taps to full chat prior to releasing the brakes. A few airliners over the years have managed to lift the runway surface when doing this which has then blasted into their tail surfaces. Really expensive, really embarrassing and in some parts of the world I imagine career limiting.

[oceancrosser]

I´m with Stilton et al here. Static take-offs are so yesterday. They were really a thing on the old radial piston engines...

[tdracer]

]

I asked the question to Airbus Customer Services and they consider that the rolling take off technique is equivalent in terms of performance to the static take off.

BINGO! Boeing has said pretty much the same thing - there is no meaningful performance advantage to static takeoff, full power statically is really hard on the engine (actually banned on some engine types), and there is not an insignificant risk of an engine surge if there is any side or tailwind present (inlet separation).

[chris lz]

ChrisLz, see what you've started🤣

As long as I'm not in the cross hairs of any skirmishes.

Thank you ALL for your replies.

[megan]

Static take-offs are so yesterdayUnless you're flying Concorde, SOP, sob, so yesterday.

[Check Airman]

BINGO! Boeing has said pretty much the same thing - there is no meaningful performance advantage to static takeoff, full power statically is really hard on the engine (actually banned on some engine types), and there is not an insignificant risk of an engine surge if there is any side or tailwind present (inlet separation).

Why’s full power harder on the engine at a standstill?

[tdracer]

Why’s full power harder on the engine at a standstill?
Inlets don't work particularly well without some forward airspeed. As a result, when sitting still, it doesn't take much of a crosswind (or worse, tailwind) to cause the inlet to start to separate when at high power - and a separated inlet can easily lead to an engine stall or surge (which are really hard on the engine). You're also more apt to suck up FOD when sitting statically than with some forward speed.

Back pre-FADEC, it was necessary to run the engines to takeoff power to check for proper operation of the bleeds and vanes - often requiring 'trim' adjustments to the hydromechanical control. At one time, I was involved in the engine run functional test requirements and would sometimes sit in for the engine runs. They'd always try to position the aircraft into the wind before the run, but that wasn't always practical - the engine run guys had developed a finely tuned ear to listen for inlet separation during the runs when the winds were less than favorable. Originally I was skeptical that you could actually hear the inlet separate with all the noise and vibration of an engine at takeoff - but then I was sitting in on an engine run and I started to hear this unusually growling noise - I was just about to say something like 'what's that' when the engine run guy snapped the throttle to idle - I'd just heard inlet separation...

[Check Airman]

Your posts from the flight test world are an endless source of fascination. Thank you. When you talk about "separation", am I right in understanding that the airflow separates somehow? If so, I'm having trouble visualising how airflow could separate in front of a what' essentially a giant vacuum.

[Vessbot]

Your posts from the flight test world are an endless source of fascination. Thank you. When you talk about "separation", am I right in understanding that the airflow separates somehow? If so, I'm having trouble visualising how airflow could separate in front of a what' essentially a giant vacuum.

Like a wing stalling, but the top wing surface is the inside of the duct.

When going fast, the airflow is coming from straight ahead, actually expanding outward kind of as if the engine pod is just a blunt front end of a cigar and the air has to go around it. This is the design condition, and the cowl is designed for that with a slightly inward-turned lip to match the air expanding into it.

The slower you go, the more it sucks air from all around rather than straight ahead, so air comes at the lip inward from the outside. The most extreme of this, a standstill, has air coming in from all directions and is why you see engines on test stands (and some helicopters) have these big tuba horns put on the front to allow the air to flow in smoothly. A standard cowl can't do that, so the air has to make a nasty sharp turn around the lip, and it doesn't take much upset to stall it.

http://edallcoin.fatcow.com/ecollege/upload-files/Subsonicandsupersonicinletforjetengine_9A05/clip_image0024.jpg

[Check Airman]

Thanks. I learned something new. This is more relevant than a lot of the ATP written questions.

[pineteam]

Thanks. I learned something new. This is more relevant than a lot of the ATP written questions.

Engine Thrust Management – Thrust Setting at Takeoff | Safety First (airbus.com) (https://safetyfirst.airbus.com/engine-thrust-management-thrust-setting-at-takeoff/)

Hi Check Airman. Check this out. Pretty much what Tdracer explained with one example of an incident and some graphs.

[Vessbot]

That being said, I'm just an interested nerd, who's also waiting for Tdacer to expand on, and fix any inaccuracies. For one, I think it might be that the design condition has the air coming from straight ahead (the slug of air that enters the inlet has the same diameter as it does far ahead before any disturbance) while the expanding flow is at a speed faster than design. And the primary reason the inlet expands is to slow down the air and increase its pressure. And I'm wondering how this interacts with the "matching" of the airflow field at the lip I mentioned earlier.

[Check Airman]

Engine Thrust Management – Thrust Setting at Takeoff | Safety First (airbus.com) (https://safetyfirst.airbus.com/engine-thrust-management-thrust-setting-at-takeoff/)

Hi Check Airman. Check this out. Pretty much what Tdracer explained with one example of an incident and some graphs.

Thanks. Will give it a go.

[tdracer]

That being said, I'm just an interested nerd, who's also waiting for Tdacer to expand on, and fix any inaccuracies. For one, I think it might be that the design condition has the air coming from straight ahead (the slug of air that enters the inlet has the same diameter as it does far ahead before any disturbance) while the expanding flow is at a speed faster than design. And the primary reason the inlet expands is to slow down the air and increase its pressure. And I'm wondering how this interacts with the "matching" of the airflow field at the lip I mentioned earlier.
Actually you explained it rather well in the previous post.
Inlets are designed to work very well and be very efficient at cruise - not takeoff. When sitting statically, as you explained the air is coming from all around - not just straight ahead - and the air to the side or rear has to turn a pretty sharp corner going around that inlet lip, that's where the inlet can separate (it can also happen if you over-rotate on takeoff - the bottom of the inlet lip can separate and cause an engine surge at a really bad time). Fans really dislike lots of distortion at the fan face - and in severe cases that distortion can even make it to the core compressor which likes it even less...
Now, this can all be made better by putting a bigger radius on the inlet lip (i.e. making the inlet thicker/fatter) - but - that makes the inlet less efficient and increases cruise drag.
One solution that has been used is called 'blow-in doors' - spring loaded doors in the inlet that open inward due to the suction at low airspeeds so the air doesn't have to turn that sharp corner around the inlet. These are used on some military aircraft (I believe the Harrier had them) and were on the early 747s. The 747 was originally designed to cruise at Mach 0.87 and higher - so it had a very thin inlet lip - but that was prone to separation at low speed so they added blow-in doors. But that created a new problem - blow-in doors are noisy. Not only do they remove some of the acoustic treatment in the inlet, they create vortices that enter the fan and that creates a lot of noise. After the 747 entered service, it was determined that it seldom cruised that fast, so the later 747s got a redesigned inlet with a thicker inlet lip that didn't need blow-in doors.

[Check Airman]

Science lesson and history lesson at the same time. Thanks:D