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foliot-pilot
11th Feb 2017, 20:17
Was recently on a jet2 flight out of LBA, in one of the shiny new 737s. The captain said during taxi, that 'manufacturers advice recommends that during takeoff in cold weather operation, they line up, increase power slightly (around 40% from what I could tell) and wait for about 20 seconds, before coming off the brakes going through to takeoff thrust. The runway was big enough and the flight wasn't full and it was also a cold day, so no apparent requirement for TOGA.

Been trying to get my head round why this is but getting nowhere. Any ideas?

safelife
11th Feb 2017, 20:20
Ice shedding (off the fan).

foliot-pilot
11th Feb 2017, 21:09
Not too familiar with that. Could you give me a brief synopsis please?

wiggy
11th Feb 2017, 21:34
As a general point in icing condition many jet types have a requirement to run up the engines to a specified RPM or EPR value at stated intervals when on the ground, e.g. every 20 minutes/every hour...and there's also usually a pre-takeoff run up required, sounds like that is what was being talked about....

As has been said it is done to ensure the engines are free of ice, see my last para.

Out of interest why are you assuming "TOGA" wasn't used? TOGA mode is not just an engine setting, it's actually an automation mode and is tied in with the nav kit, engines, and other things....not using it can cause some interesting complications to your working day. Generally once your pre take off de-icing timed run-up is done you do hit the TOGA switches to let the systems know you are starting your takeoff, as well getting the engines set to takeoff power. Another point is the power setting for de-icing can be quite high and I'm assuming you were in the cabin - I'm not sure you would always be able to ascertain the change from a de-ice run up RPM/EPR to the take off setting in all circumstances, so all in all I suspect TOGA mode was used.

There's probably more than enough info for you here, including comments on ice shedding:

Engine Core and Fan De/Anti-icing - SKYbrary Aviation Safety (http://www.skybrary.aero/index.php/Engine_Core_and_Fan_De/Anti-icing)

foliot-pilot
11th Feb 2017, 22:01
Thanks for the info Wiggy...when I say 'toga' in my mind that means 100% full thrust, although it seems like I'm not quite right there!

wiggy
11th Feb 2017, 22:09
No problem,

In very general terms TOGA (for takeoff) doesn't often give you full power, it will give you the takeoff power that you have entered/selected in the Flight Management computer. That's based mainly on the aircraft weight, weather conditions. If you are light weight on a long runway it is quite possible TOGA won't select anything like 100% power.

FWIW on go-around if pushed once (on a least of couple of types I know of) TOGA gives you the power to climb at 2000 feet a minute, so not full power.....a second push however gives you 100%.

Denti
11th Feb 2017, 22:41
If i remember correctly ice shedding is done at 80% N1 on the 737.

Re the TOGA issue, in the airbus world that would be indeed full take of power, reduced thrust is set in the FLEX/MCT detent. In the boeing world it is as described by wiggy, simply the system to set the calculated take off thrust which can be considerably less than full. The 737 allows "double thrust reduction" by both using derated thrust levels and the assumed temperature method.

172_driver
11th Feb 2017, 22:53
70 % N1 on 737 for ice shedding

lomapaseo
12th Feb 2017, 02:25
Ice shedding (on-the ground) could be addressing ice on blades or ice on stator vanes.

Blade ice is shed via centrifugal load while stator vane ice is shed by areodynamic or windage loads.

The idea is to shed the ice before it buils up to a mass that could cause trouble when shed (damage and/or engine surge)

Ice formed at low power conditions (low temp in the fan area) would take too long to respond to increases in temperature, hence the need for power-ups of N1 to get enough centrifugal load or aerodynamic loading.

Typically The certifying agency would take a dim view having to use these procedures normally on any icing formed in-flight

SeenItAll
14th Feb 2017, 20:41
Another possibility relates to clearing any deicing fluid from the engines. Just returned from a trip to Quebec. Pilot announced that after deicing, he was going to need to run up the engines to ensure all stray fluid was cleared from the engines. He did so, we then taxied to the runway and took off.

DogSpew
14th Feb 2017, 21:57
As a 737-800 Captain I can confirm that what you're describing is Fan Ice removal. The 737-800 FCOM clearly states for Taxi Out:

"When engine anti-ice is required and the OAT is 3C or below, do an engine run up, as needed, to minimize ice build-up. Use the following procedure:

Check that the area behind the airplane is clear.

Run-up to a minimum of 70% N1 for approximately 30 seconds duration at intervals no greater than 30 minutes.

Note: Fan blade ice build up is cumulative. If the fan spinner and fan blades were not deiced prior to taxi out, the time the engines were operating during the taxi in should be included in the 30 minute interval.

If airport surface conditions and the concentration of aircraft do not allow the engine thrust level to be increased to 70% N1, then set a thrust level as high as practical and time at that thrust level.

Note: When operating in conditions of freezing rain, freezing drizzle, freezing fog or heavy snow, run-ups to a minimum of 70% N1 for approximately 1 second duration at intervals no greater than 10 minutes enhance ice shedding."

The FCOM then states that prior to Take Off.

"Do the normal Takeoff Procedure with the following modification:

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

Run-up to a minimum of 70% N1 and confirm stable engine operation before the start of the takeoff roll. A 30-second run-up is highly recommended whenever possible.

172_driver
14th Feb 2017, 22:40
It may even be prudent to do a static run-up after landing to shed ice before shutting the engines down. It can save you the bother of having to put engine covers on or bringing out the hot air for fan blade de-icing. Though, in my experience it's hard to shed the ice completely off the inner part of the blades.

I am still curious as to how 'aerodynamic' the fan blades are and how much they are just big paddles shuffling air backwards. The amount of ice 'acceptable' during walk around will always be a subjective assessment.

Turbine D
14th Feb 2017, 23:30
172 driver,

They are very aerodynamic, even more so today with the ability to manufacture shapes not before possible. Not only that, there is less of them in a rotor doing more work than when there were 1/4 or more in a rotor.

As fat as ice observed on fan blades doing a walk around, it is somewhat subjective as I recall seeing photos on a PPRuNe thread that depicted ice on the back of the fan blade that is hard to visually observe...

FE Hoppy
15th Feb 2017, 09:22
As a general point in icing condition many jet types have a requirement to run up the engines to a specified RPM or EPR value at stated intervals when on the ground, e.g. every 20 minutes/every hour...and there's also usually a pre-takeoff run up required, sounds like that is what was being talked about....

As has been said it is done to ensure the engines are free of ice, see my last para.

Out of interest why are you assuming "TOGA" wasn't used? TOGA mode is not just an engine setting, it's actually an automation mode and is tied in with the nav kit, engines, and other things....not using it can cause some interesting complications to your working day. Generally once your pre take off de-icing timed run-up is done you do hit the TOGA switches to let the systems know you are starting your takeoff, as well getting the engines set to takeoff power. Another point is the power setting for de-icing can be quite high and I'm assuming you were in the cabin - I'm not sure you would always be able to ascertain the change from a de-ice run up RPM/EPR to the take off setting in all circumstances, so all in all I suspect TOGA mode was used.

There's probably more than enough info for you here, including comments on ice shedding:

Engine Core and Fan De/Anti-icing - SKYbrary Aviation Safety (http://www.skybrary.aero/index.php/Engine_Core_and_Fan_De/Anti-icing)
I really dislike this airbus "TOGA" description. I'm not sure what part of the EASA ATP introduces it but I have tonnes of initial Type rating students who require that airbusism squeezing out of them as they come through their Ejet/CSeries/Avro Type ratings.

172_driver
15th Feb 2017, 23:29
Turbine D,

Thank you, the old CFM56-7B look less aerodynamic than more modern engines but I take your word their aerodynamic attributes are nonetheless critical.

A tactile check will reveal ice on the blades. The 'little of it is alright'e' jargon is just another pilot's guesswork.

wiggy
16th Feb 2017, 00:17
I really dislike this airbus "TOGA" description.

FE and Denti, must admit that it hadn't occured to me there was a difference between a Boeing TOGA and an Airbus/EASA TOGA.....

Everyday is a school day....

riff_raff
18th Feb 2017, 08:02
172 driver, They are very aerodynamic, even more so today with the ability to manufacture shapes not before possible.

Indeed. Most thrust from modern turbofan engines is produced by the fan. In order for the fan to perform efficiently, the blade OML surfaces must be free from any debris that could disturb the boundary airflow over its surface.

lomapaseo
18th Feb 2017, 13:41
I am still curious as to how 'aerodynamic' the fan blades are and how much they are just big paddles shuffling air backwards. The amount of ice 'acceptable' during walk around will always be a subjective assessment.

Of course as others have said the fan blades are aerodynamic and critical to propulsive thrust.

However the thread was mostly about ice on the blades and its detection and removal.

The aerodynamics is mostly about high power operation and the ice build up problem is about low power operation. At low power approach, taxi in and taxi out, those blades are not working like they would at high power and hence that is why the ice sticks.

For sure there is always the subjectivity of what your eye sees at walk-arounds, but this is addressed by your training and procedures. So in the end as long as you follow your training the ice issue is well managed

Swedish Steve
18th Feb 2017, 18:08
As fat as ice observed on fan blades doing a walk around, it is somewhat subjective as I recall seeing photos on a PPRuNe thread that depicted ice on the back of the fan blade that is hard to visually observe...

I am surprised with the word subjective. I am recently retired, but I taught the deicing operators to always check the fan blades on every aircraft they were called to, and deice anything they found with a hot air blower. There was no subjective. If the pilots called for deicing, the deicing crew came out and deiced the fan blades if there was anything to deice. We had a contract with deicing that they would inspect al aircraft if asked, and not charge us if they were OK.

RAT 5
18th Feb 2017, 19:01
foliot-pilot

May I ask; were you a pilot on the flight or were you a pax?

172_driver
18th Feb 2017, 22:29
What I mean with the slightly gray area is:

Ice is detected on the back of the fan blades during walkaround, engine covers are put on for X amount of minutes (perhaps 10 min of your scheduled 30 min turnaround). You taxi out for remote de-icing in the same conditions that possibly gave you the ice on the way in. No blade de-icing is possible on a remote de-icing with engines running. Prior brake release a run-up is performed, no abnormal sounds or vibrations. Is the fan performing as advertised? My experience of having done run-ups during taxi in (to shed ice, to avoid having the hot air out) is that it doesn't always shed everything. Especially the innermost part of the blade has residual ice.

lomapaseo
19th Feb 2017, 01:52
Is the fan performing as advertised? My experience of having done run-ups during taxi in (to shed ice, to avoid having the hot air out) is that it doesn't always shed everything. Especially the innermost part of the blade has residual ice.

Nothing is perfect ... but the only remaining issue .... is the engine able to achieve rated thrust and operability over the range of flight conditions as presumed in it's certification ?

Experience has shown that the answer is yes.

172_driver
19th Feb 2017, 08:30
Experience has shown that the answer is yes.

That's basically what I was getting at.