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martinlest2

Two (unrelated) real-world questions: fuel tanks & approach speeds

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1.  Many years ago I am sure I read that although (as we all know!) fuel in centre tanks (when present of course) is used up first, the centre fuel pumps are not activated until after take-off (for fire hazard reasons). I forget the exact reasoning for this, but I have followed this procedure for many years. But now, Googling shows no mention of it. Did I invent this, or is it actually valid???

 

2.  On approach, do pilots select their own speeds or are these always dictated by ATC? I know ATC can and often do give speed instructions, to get the right separation between traffic, but otherwise, are pilots free to choose their aircrafts' approach & descent speeds themselves according to internal & external conditions?

 

Thanks.


Martin Stebbing, EGLF (UK)

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Martin...I can only talk to your second question about approach speeds. As a former air traffic controller rated in approach control, we rarely gave speed instructions unless some aircraft was closing the required separation as you already stated. More often, I would tell an aircraft to execute a 360-degree turn to restore needed separation. And that was only while flying the arrival routing within our controlled airspace and never while on final. Below 10,000, max airspeed is 250. Most commercial jets fly final approach (last 5 miles) in the 140-155 range and is based on aircraft performance criteria, and our separation between jets on final was never close enough to be speed sensitive. We did factor in speed of turboprops and general aviation traffic into the separation we needed, but we didn't tell them what speed to fly. Transition speeds from enroute descent to final approach while under approach control typically was about 220 at handoff and slowed down to 170 while level by 10-miles out on final approach. That might be too much information.

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1.  Many years ago I am sure I read that although (as we all know!) fuel in centre tanks (when present of course) is used up first, the centre fuel pumps are not activated until after take-off (for fire hazard reasons). I forget the exact reasoning for this, but I have followed this procedure for many years. But now, Googling shows no mention of it. Did I invent this, or is it actually valid???

 

It's type-specific and depends on the fuel system configuration.

 

The requirement is that you should not feed both engines from the same tank during takeoff, not for fire reasons but because if the fuel in the centre tank is contaminated for some reason and you're feeding both engines with the contaminated fuel... well, you can work out the problem that might arise! Depending on the aircraft itself it may either be a pilot action to turn off the pumps before takeoff/turn them on again at a safe height, or the switching may be automated (so the pilot simply selects all pumps in tanks containing fuel on prior to start-up and the aircraft systems automatically configure the fuel system to feed "tank to engine" during departure, typically whilst the slats are extended, and then feeding from the centre tank resumes automatically once the aircraft is cleaned up).

 

Separate to this requirement, there are some types where operation of the pumps with less than a certain level of fuel in the tank is not recommended for fire hazard reasons (some early B737 models, for instance, though I think this was 'fixed' in later editions and the pumps can safely be run to empty in more recently-produced aircraft).

 

 

 

2.  On approach, do pilots select their own speeds or are these always dictated by ATC? I know ATC can and often do give speed instructions, to get the right separation between traffic, but otherwise, are pilots free to choose their aircrafts' approach & descent speeds themselves according to internal & external conditions?

 

As David says, it depends -- and it depends on where you are in the world as well. Some places will speed control you quite heavily (particularly these days in Europe I believe there is now a system in place were aircraft will be slowed whilst enroute if there are delays at their destination in order to reduce holding) and at busy airfields at busy times in Europe/the UK you will most likely be speed controlled from quite a long way out. However, there are limits to ATC's ability to impose speed restrictions -- typically laid down in local documentation but usually along the lines of no ATC speed control inside the outer marker or rough equivalent (usually 4 or 5NM from the threshold depending on the speeds required -- as inside this range the pilot will need to slow and configure the aircraft in order to meet the stable approach criteria in time. Typically speeds are no greater than 160kts to 4DME or 180kts to 5DME -- any more than that/any closer in and it would be impossible to configure in time).

 

If there is no speed control applied by ATC then it is up to the pilot to fly whatever speeds he/she deems necessary, taking in to account local rules and regulations, company SOPs, the descent profile required to meet any altitude restrictions, turn radius to maintain whatever ground track/lateral path is required and the requirement to meet stable approach criteria.

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Simon Kelsey

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I sometimes listen in to ATC at Manchester (EGCC). One instruction I hear many times is "one-sixty until 4 DME".

 

Presumably that's to keep optimum separation. Once inside 4 DME I imagine they are left to their own devices and reduce to landing speed.


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Excellent replies by Simon and David!

 

 

 


the centre fuel pumps are not activated until after take-off (for fire hazard reasons).

 

I heard a real 737 pilot saying that, even when there's fuel in the center tanks, they would turn the pumps on only once in cruise so that they don't get a annoying/distracting warning during take-off/climb-out should the pumps "starve" with high deck angles and/or accelerations and the fuel in the back of the tank.


I sometimes listen in to ATC at Manchester (EGCC). One instruction I hear many times is "one-sixty until 4 DME".

 

Presumably that's to keep optimum separation. Once inside 4 DME I imagine they are left to their own devices and reduce to landing speed.

 

Yes, in Stockholm ESSA that's also standard practice. It's even writen in the approach chart. I think keeping the speed up until that "late" into the approach is a bit, "tight", for lack of a better word. Many airlines want their planes to be fully configured and in stabilized by 1000ft, i.e., 3nm out. A Boeing 757 would have to slow down 30 knots in 1 nm while descending on the glideslope, not easy without speedbrakes (and sometimes speedbrakes are not allowed/recommended past a given flap setting)

 

In airports in Germany they have "standard" speeds of 210, 190 and 170. ATC will ask the pilots to keep these speeds during transitions to final. Once established on the ILS and within 5nm the speed is free and the pilots will use their Vapp.

 

On the other side of the spectrum, sometimes ATC will instruct the pilot to "reduce to minimum approach speed", if, for instance, there's an aircraft in final which will have to backtrack on the runway, so that the incoming traffic has enough time to land.


 

 


180kts to 5DME

 

I think you would have a very hard time slowing a 777 down from 180 to 140 past 5nm out, haha. At least I couldn't make it in time and had to go-around. This was an online flight into ESSA on IVAO :)

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Jaime Beneyto

 

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Yes, really nice replies, thanks again!

 

 

I heard a real 737 pilot saying that, even when there's fuel in the center tanks, they would turn the pumps on only once in cruise so that they don't get a annoying/distracting warning during take-off/climb-out should the pumps "starve" with high deck angles and/or accelerations and the fuel in the back of the tank.

.. that rings a bell! Maybe that was what I read, but it was a long time ago, so I couldn't be sure. Whatever, in FS I am in the habit now of switching on the centre fuel pumps only after take-off. According to the above, I should only be doing it when I've reached cruise altitude, but I guess it makes no odds in FS9!


Martin Stebbing, EGLF (UK)

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Whatever, in FS I am in the habit now of switching on the centre fuel pumps only after take-off. According to the above, I should only be doing it when I've reached cruise altitude, but I guess it makes no odds in FS9!

 

But remember that what I said is for the 737 only! And maybe only for the Classics!

 

This is all aircraft specific, as Simon said. Read the documentation on your particular aircraft to find out :)


Jaime Beneyto

 

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Question 2 has been nicely answered by other posters. W.R.T. question 1: I think the reason why the center fuel tank is filled up last is that this minimizes the torque on the wings.

 

Peter


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But remember that what I said is for the 737 only! And maybe only for the Classics!

 

True.

 

None of the aircraft documentation mentions this point (it's a bit abstruse, after all), as far as I can see, which is in part why I posted the question.

 

 

I think the reason why the center fuel tank is filled up last is that this minimizes the torque on the wings

It was really a question of the use of the fuel from the tank rather than the filling, but you may be right.


Martin Stebbing, EGLF (UK)

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I think you would have a very hard time slowing a 777 down from 180 to 140 past 5nm out, haha. At least I couldn't make it in time and had to go-around. This was an online flight into ESSA on IVAO :)

 

Agreed -- I may have misremembered (now I think about it, I think it's more like 180/6DME, 170/5DME, 160/4DME) -- the UK uses 160 to 4 pretty much everywhere I can think of so that's what lodges in the mind!

 

As you say, 160 to 4 with a light jet and a tailwind can be a challenge if you have to be stable by 1000R (some airlines will have an exception to the speed provided everything else is configured and the speed is within 20 knots or so and trending in the right direction, i.e. downwards -- presumably to cater for precisely these situations. 500R is usually the absolute cutoff by which you must have approach speed and engines spooled).


Simon Kelsey

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Hey guys,

 

If you have a certain quantity of fuel in the centre tank, then the centre tank pumps definitely need to be on on the ground before take off.

On the 777 for example, if you have more than about 4800 kgs, then the centre tank pumps need to be on. If you don't turn them on, you will get an EICAS message "fuel in center".

Same thing for the 737NG. If you have more than about 800 kgs, then you will get a "config" alert message on the tanks display.

Fuel in the centre tank needs to go first, above all when you have a large quantity of it.

If you only have a little bit of fuel in the centre tanks, then the pumps stay off until 10000 feet on the NG or whenever the "fuel in center" eicas message appears on the 777, for example.

I "assume" the same principle applies to a lot of other transport aircraft.


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"fuel in center" eicas message

 

OK, yes, I have seen that message in FS9.


Martin Stebbing, EGLF (UK)

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