Ask The Pilot

[Harpoon]

Thank you! I can now sit smugly in 38C and listen to the engine whirr starting as the aircon is switched off, and know that... "no....the air con is not dodgey, if off for a reason!!".

Also thanks for being so dedicated to answering these questions. You could just about publish a book with the amount of words you've written answering these questions. Bravo!

One quick silly movie related question, there's the scene in Die Hard 2 where they have a fist fight on the wing of the taxiing 747. Reckon a person, let alone 2 people fighting, could stand on a 747 wing during taxi without being blown/bounced off? Flexing wings look mighty bumpy when taxiing along!

 

[jb747]

One quick silly movie related question, there's the scene in Die Hard 2 where they have a fist fight on the wing of the taxiing 747. Reckon a person, let alone 2 people fighting, could stand on a 747 wing during taxi without being blown/bounced off? Flexing wings look mighty bumpy when taxiing along!

Er...no. But then it was Bruce Willis. 747s taxi at up to 30 knots. It only looks slow because they're so big.

 

[AVC]

Er...no. But then it was Bruce Willis.

That's nothing. Remember the movie Commando?

Arnie climbs down onto the nose gear during the take off roll. Then still manages to hold during rotation, and jumps into a swamp.

No injuries. He's one tough mof_.

We should have a "ridiculous and unbelievable aircraft scenes forum"

 

[MEL_Traveller]

I have a question!

Looking at tthe A380 the wing looks extraordinarily thick at the wing root... and appears much thicker (proportionally) than the 747.

Every time I look at an A380 it just look like this very thick wing root must be very heavy and therefore make the A380 less efficient than it appears.

Is my observation just an optical illusion? (that is, that the thickness of the wing at the fuselage is in the same proportion as the 747 or other jets?)

 

[jb747]

I have a question!Looking at tthe A380 the wing looks extraordinarily thick at the wing root... and appears much thicker (proportionally) than the 747.

Every time I look at an A380 it just look like this very thick wing root must be very heavy and therefore make the A380 less efficient than it appears.

Well, it's hollow, so I don't think that makes it heavy. It is a very interesting wing design (ignoring a couple of manufacturing issues). The curvature is almost the exact opposite of the 787. Don't ask me why...the Europeans and Americans just fed the same data into their computers, and came up with opposite answers.

It isn't as much of a high speed wing as the 747 (and the thickness at the root is just part of that), but it is a very high lift wing, and allows approach and departure speeds that are well below what you'd expect based on the 747.

Is my observation just an optical illusion? (that is, that the thickness of the wing at the fuselage is in the same proportion as the 747 or other jets?)

There isn't a best shape. Designs have to cover the fact that the aircraft will fly at both the fast and low end of the speed regime. Swept wings are nice at high speed, but less so at low. Lots of curvature gives lots of lift, but that doesn't work at high speed. All designs are a trade off against the sort of flying the aircraft does. An aircraft designed for one hour commuter hops (737/320) will have an appreciably different shape to an aircraft meant to fly for 15 hours. If you fly in the .7 mach area, your wing is different to one that flies at .9 mach.

And all of this specialisation means that aircraft can be very inefficient if only slightly off their designed parameters (i.e. too low, fast/slow, or holding).

 

[MEL_Traveller]

thanks jb!!

 

[mjt57]

Fuel is added somewhere between 15% to 25% N3.

How long does it sit at that speed before fuel is admitted?
Our P&W FT4C gas turbines would ramp up to 1,500rpm (N2) and sit there for 30secs before light off was initiated. This was to ensure a proper purge of the combustion chamber was done.

Now, you look at N3, but how can that be if there's no combustion taking place to drive it? Or does the starter spin N3 up rather than N1 or N2?

 

[jb747]

How long does it sit at that speed before fuel is admitted?
Our P&W FT4C gas turbines would ramp up to 1,500rpm (N2) and sit there for 30secs before light off was initiated. This was to ensure a proper purge of the combustion chamber was done.

Now, you look at N3, but how can that be if there's no combustion taking place to drive it? Or does the starter spin N3 up rather than N1 or N2?

I've just made a change to the original to "N2 or N3", to reflect the fact that not all engines are three spool.

The starter spins up the smallest of the spools, be it N2 or N3. N1 isn't spun at all, and may not start rotation until very late in the sequence.

Basically, if we do an automatic start (almost always), the system winds the N3 up until it reaches 25%, and then immediately introduces fuel and ignition. Light off must be within 20-30 seconds (depending upon the engine), but in the RB211 and Trent it is almost alway instantaneous. Curiously, an instant light-off was cause for an aborted start in the old P&W.

The 767 was always manual starts, and in that we'd wait until 'max motoring' (the fastest speed the starter could spin the N2/3 to) before opening the fuel valve. On a hot day, with a tired APU or external air source, we might struggle to get to the 15% minimum. The higher the rpm at fuel introduction, the more likely you were to get a stable start.

 

[Sprucegoose]

I have a question!

Looking at tthe A380 the wing looks extraordinarily thick at the wing root... and appears much thicker (proportionally) than the 747.

Every time I look at an A380 it just look like this very thick wing root must be very heavy and therefore make the A380 less efficient than it appears.

Is my observation just an optical illusion? (that is, that the thickness of the wing at the fuselage is in the same proportion as the 747 or other jets?)

May not be relevant but a few mates of mine who flew in the air force commented at that time their admiration for the 727 wing design. Their civi mates loves flying that plane, it had a wing structure that apparently was well admired.

 

[Sprucegoose]

Not very often. I've seen rubber on the runway a few times...thankfully before we started our roll. I recall a jumbo in Singapore whose jet blast picked up a sign that was beside the taxiway, and blew it neatly onto the tarmac right in front of me...

Pity that French Concorde pilot was not as Eagle eyed.......obviously a design fault in adhering that sign. It's always the simple things.

 

[jb747]

Pity that French Concorde pilot was not as Eagle eyed.......

There has always been a question about just what the Concorde was doing so far from the centreline anyway....and even if you do see something, there's nothing much you could do about it.

....obviously a design fault in adhering that sign. It's always the simple things.

Failure of logic more than anything else....don't use sandwich boards near taxiways. It did fly well though....

 

[horseman]

I havent seen this asked yet, Can a commercial passenger jet start and take off without the assistance of a ground crew? eg if you had to land in the middle of nowhere, and stop the engines, could you start and take off again?

 

[Awesom Andy]

I havent seen this asked yet, Can a commercial passenger jet start and take off without the assistance of a ground crew? eg if you had to land in the middle of nowhere, and stop the engines, could you start and take off again?

Yes. Simply press Ctrl+E on the keyboard, and apply full power.

(In other words, no, unless you have knowledge of the start-up sequence, and have fuel in the tanks to actually get somewhere).

 

[jb747]

I havent seen this asked yet, Can a commercial passenger jet start and take off without the assistance of a ground crew? eg if you had to land in the middle of nowhere, and stop the engines, could you start and take off again?

Assuming simply things like fuel, and not needing to be towed or pushed, then sure.

We can start the APU on battery power, and from there power up everything else.

 

[Sprucegoose]

I havent seen this asked yet, Can a commercial passenger jet start and take off without the assistance of a ground crew? eg if you had to land in the middle of nowhere, and stop the engines, could you start and take off again?

Entebbe. Why stop the engines!

 

[harvyk]

Is there a reason why autostart might not be used? (Excluding of course if the plane does not have autostart)
Also if ground power is available, I guess it would be used instead of the APU?

 

[jb747]

Is there a reason why autostart might not be used? (Excluding of course if the plane does not have autostart)

There are obviously some MEL items that will remove its availability. Some forms of start require it do be done manually (for instance a manual override start, in which the starter valve is controlled externally by the engineers). It might also be required after some aborted starts.

On the ground the autostart systems will monitor for almost all of the abort limits, and will quite reliably abort the start if needed. For most aborts, they'll then motor the engine to cool it, and even have a second go. I can only recall a couple of aborted starts (with autostart) on the 747, and I haven't seen one on the 380.

In reality a jet engine start is a fairly simple thing. Basically supply it with enough air to get it turning at a reasonable speed, introduce fuel and ignition. And 99% of the time, you'll have stable engine at idle about 30 seconds later.

Also if ground power is available, I guess it would be used instead of the APU?

Ground power and air will be used as much as possible, to offload the APU, and so allow it to be shutdown, or at least to use less fuel. But, once we get to around the passenger boarding time, we'll take the aircraft back onto internal air and power. So unless we're dealing with a fault, all start-ups will be on aircraft utilities.

 

[Harpoon]

JB, I notice some airlines shut off an engine or two (depending on twin or quad) after landing to save fuel during taxi. I don't think I've ever noticed this with QF? Is it policy not to? Or is it simply up to the pilots on the day?

Is there much difference in control on the taxi way with power on one wing and not the other?

Also, can you and do you need to put brakes on one side of the main undercarriage to aid 90 degree turning on taxiways?

 

[jb747]

JB, I notice some airlines shut off an engine or two (depending on twin or quad) after landing to save fuel during taxi. I don't think I've ever noticed this with QF? Is it policy not to? Or is it simply up to the pilots on the day?

I don't know that I've ever seen this done in a twin (other than when shut down for other reasons). You lose half of everything, and double the jet blast on one side.

Engines have a cool down period...generally about 5 minutes. In most places you'll be at, or close to, your gate in that time frame, so an early shut down won't be available. It causes jet blast problems, and the loss of the ancilliary systems (unless well thought out beforehand) could be embarrassing. Shutting down one engine on a 747 is normally not a big issue, and it was available to us, but rarely used. It's not a procedure on the 380: outboard engines are often used to help us in the turns, and the inboards provide our hydraulics...

In some places there are restrictions on reduced engine taxi. In London you're not allowed to cross an active runway unless you have them all burning, presumably because with less power you'll take longer. Some airlines start engines whilst taxiing out. Issues there come from the fact that engines can have problems during start, some of which you won't be aware of without ground crew input (tail pipe fire for instance), as well as the fact that it takes your mind off the job at hand...taxiing without running into the scenery.

Is there much difference in control on the taxi way with power on one wing and not the other?

On a quad it's noticeable, but not a big deal. On a twin it's much more of an issue (as it is in flight too).

Also, can you and do you need to put brakes on one side of the main undercarriage to aid 90 degree turning on taxiways?

Differential braking....yes we can do that, but don't normally. We do use differential power a lot though. On a very tight turn (a reversal on a runway) we may need both differential braking and power, as well as maximum nose gear steering. The 380 doesn't have body gear steering (unlike the 747), so there is much more tendency for it to stop in turns unless you lead with power.

 

[Harpoon]

I don't know that I've ever seen this done in a twin (other than when shut down for other reasons). You lose half of everything, and double the jet blast on one side.

I landed in SIN the other day on a Singapore 777-300ER and the pilot shut down the right hand engine for taxi. It was a long taxi about 8-10 mins with lots of stopping as well, but it was all right hand turns so imagine it was rather easy to turn right with only the left engine running.

I didn't realise the 747 had body gear steering. Very cool! Will pay more attention next time I see a 747 taxiing to see if i can spot it.

Thanks for the responses.