Ask The Pilot

[Boris spatsky]

Just a simple and foolish question - but something I ponder whilst trying to grasp the physics of flight and aircraft:

From a given height, say 30,000 feet, with all engines out, would a jet glide further with full flaps or none? Increased lift vs drag sort of thing.

Best lift on drag or the speed known as Vimd varies with weight and configuration but is the best lift/drag ratio, and thus, best glide speed for greatest glide range. Pilots in a bar will explain this by drawing the drag curve on the back of a coaster, but suffice to say the drag curve explains a lot about the speeds we fly and why, and how a plane behaves at various speeds.

When flap is used, the glide is shortened and is steeper because of the increased drag. So flaps up gives best glide range.

 

[Craigie]

I thought I saw this somewhere in the thread but now I can't find it.

I was wondering past a few gates with MD-80's in DFW the other day and noticed some of them the elevators on the tail were at different angles of attack, ie the left side elevator was pointed down while the other side was flat or something similar. I would've thought the elevators would have been linked for sure? I'm not sure about other aircraft either?

 

[por930]

Hi Craigie,
In a light a/c, we are connected by cables to all flight controls Both sides of the elevator are joined, and when I push/pull the elevator control, the elevator moves as one unit. However, some of the commercial jet boys have varying degrees of automation of flight controls, including splits in the elevators. See this link which specifically talks about the MD80 elevators..
Airline Pilot Chatter: Something Odd About MD80 Flight Controls

 

[RobYg]

On a long overnight flight is the cabin temperature adjusted to perhaps help passengers sleep?

 

[drewbles]

On a long overnight flight is the cabin temperature adjusted to perhaps help passengers sleep?

Absolutely they can. I took a CX flight a few months ago and it was stinking hot in the J cabin. I very politely asked the CSM if they could lower the temperature and she was more than happy to do so. Some pax had to rug up, but as CX have no individual air vents, it certainly made it more comfortable (CX have a habit of keeping their premium cabins very warm for some reason).

 

[Boris spatsky]

I thought I saw this somewhere in the thread but now I can't find it.

I was wondering past a few gates with MD-80's in DFW the other day and noticed some of them the elevators on the tail were at different angles of attack, ie the left side elevator was pointed down while the other side was flat or something similar. I would've thought the elevators would have been linked for sure? I'm not sure about other aircraft either?

The MD80 series has a geared tab and a control that 'flies' the elevator (unlike some other non FBW jet aircraft where the elevator itself moved with hydraulic assistance ). On the ground they can look a bit weird as they can sit at weird angles.

 

[Melburnian1]

In the QF delays/ cancellations thread there was an informative (and sad) post about how due to a lady suffering a heart attack on the Wednesday 4 May QF2 almost immediately after becoming airborne from DXB, the plane had to return to DXB. Total time on the ground was less than five hours, probably about two and a half for the 'second go' at departure.

I asked there but this is really a more general question: how long generally is it before an auxiliary power unit ('APU') might be hooked up to an aircraft at a gate (when the temperature on the ground is say 34 degrees Celsius as it allegedly was at DXB yesterday) to among other things allow the airconditioning to run? In such circumstances there might be some uncertainty as to how long the delay will be, and sometimes passengers may be asked to alight from the aircraft and move to a transit lounge. From what The Flying Brick implied, the latter did not occur with the said case.

And if an APU is not hooked up, do the pilots on board doing their paperwork suffer from the same hot or warm cabin that the passengers do, or do they have a separate mini system to keep them cool?

 

[jb747]

I was wondering past a few gates with MD-80's in DFW the other day and noticed some of them the elevators on the tail were at different angles of attack, ie the left side elevator was pointed down while the other side was flat or something similar. I would've thought the elevators would have been linked for sure? I'm not sure about other aircraft either?

On large aircraft the flight controls are not normally physically linked. In part it would probably be difficult, but mainly it's because they are driven by different hydraulic systems, and having them separate gives the ability to overcome any form of jamming or partial failure. It's often evident on the rudder (when powered down)...the upper section may be pointing a different direct to the lower.

 

[jb747]

On a long overnight flight is the cabin temperature adjusted to perhaps help passengers sleep?

The temperature can be adjusted. There are a number of zones, and then sensors throughout the zone. It's pretty well impossible to get an even temperature though, as ambient heating and cooling differs all around the cabin. The areas near doors will always be cold, no matter how high the temperature is set.

But, if the question is "Are the temperatures adjusted up to put people to sleep?", then the answer is no. It's basically set at 24ºC, and might be adjusted by about 2ºC in response to requests. That's as far as it would be taken though, as the results will be uneven.

 

[jb747]

I asked there but this is really a more general question: how long generally is it before an auxiliary power unit ('APU') might be hooked up to an aircraft at a gate (when the temperature on the ground is say 34 degrees Celsius as it allegedly was at DXB yesterday) to among other things allow the airconditioning to run? In such circumstances there might be some uncertainty as to how long the delay will be, and sometimes passengers may be asked to alight from the aircraft and move to a transit lounge. From what The Flying Brick implied, the latter did not occur with the said case.

And if an APU is not hooked up, do the pilots on board doing their paperwork suffer from the same hot or warm cabin that the passengers do, or do they have a separate mini system to keep them cool?

The APU is part of the aircraft systems, and is started during the taxi to the gate. The APU bleed is turned on just before the engines are selected off...so the end result is that the air conditioning packs have an unbroken supply of pressurised air. All of the 380 gates at Dubai have conditioned air available, so, generally, the APU will be turned off after that air and the ground power are hooked up to the aircraft. In the case of a return to the gate, you probably wouldn't bother with the external supplies, and just keep the aircraft running on the APU. It normally does a better job anyway.

There's no separate system for the coughpit..it's normally hotter than the cabin.

 

[747sp]

I am wondering on shorter flights ie. Sydney to melbourne assuming you have an option of any height and no other traffic what are the factors you look at to deside what will be the flight
level. On a similar issue for longer flights besides head or tail winds are there other factors to consider.
I also wanted to thank the pilots for your answers as this forum has given me a much better understanding of what goes on in the coughpit .

 

[Boris spatsky]

I am wondering on shorter flights ie. Sydney to melbourne assuming you have an option of any height and no other traffic what are the factors you look at to deside what will be the flight
level. On a similar issue for longer flights besides head or tail winds are there other factors to consider.
I also wanted to thank the pilots for your answers as this forum has given me a much better understanding of what goes on in the coughpit .

Generally the higher you go, the more fuel efficient the flight. This can vary with weight which may limit your max altitude, or strong headwinds which may lead to you flying lower to get out of the stronger headwind. Turbulence also comes into consideration as we try for a smooth flight where possible.

On flights shorter than about 45 minutes, we generally limit the altitude to lower than maximum.

 

[Kerrodt]

Generally the higher you go, the more fuel efficient the flight. This can vary with weight which may limit your max altitude, or strong headwinds which may lead to you flying lower to get out of the stronger headwind. Turbulence also comes into consideration as we try for a smooth flight where possible.

On flights shorter than about 45 minutes, we generally limit the altitude to lower than maximum.

Am I correct in understanding that the higher the altitude the thinner the air? If so, does this have an impact on the ability of the aircraft to remain "held" in the air? I guess what I am asking is does the higher altitude impact aerodynamics because of thinner air?

Thanks.

 

[jb747]

Am I correct in understanding that the higher the altitude the thinner the air? If so, does this have an impact on the ability of the aircraft to remain "held" in the air? I guess what I am asking is does the higher altitude impact aerodynamics because of thinner air?

An aircraft is a series of juggling acts, with various factors competing for control.

Basically, an aircraft flies because of INDICATED air speed (IAS). That's the actual effect of the air on the wing. What you'd feel if you were able to stick your hand out the window. If the best airspeed is 250 KIAS, then it will (to a degree) be the same at all altitudes...

But, the air is, as you say, a lot thinner at altitude, so to achieve that 250 KIAS I will have to go faster through the air mass as I climb. At FL400, it would be about twice as fast. So, that gives us TAS, or true airspeed. For a constant IAS, the TAS will increase as you climb, and for a constant TAS, the IAS would be reducing.

The aircraft only cares about IAS...that's what makes it fly. TAS is what gets us from point A to B (once we correct if for the wind, which gives us GS, groundspeed).

Overriding all of that is mach number. That's our speed as a percentage of the speed of sound. It gives one set of maximum speeds. It is related to temperature, and basically reduces as you climb. It's TAS.. At sea level it will be around 680 knots, reducing to about 560 at FL400. So, that 560 kts of mach 1 is roughly 280 KIAS, which is not much faster than our airliner's best IAS. Drag rises dramatically around mach one, so it is limiting for a number of reasons. Above about .85, you'll slow to below the best IAS to keep away from it. In a climb, at about FL300 the climb IAS will hit mach .8 (or so) and from that point you climb at a mach number.

An IAS, at a given weight, will give a fixed 1G angle of attack. An airframe has a 'best' angle of attack, so we use IAS to indirectly give us that best AoA. As an aircraft burns fuel, and the weight reduces, it needs a lower angle of attack to produce the required lift. To maintain that best 'alpha', the speed is reduced...which explains why cruise speed is constantly being reduced as a flight progresses.

But.....the engines have an RPM at which they function best too (actually close to full power, call it 90%). So, reducing the speed means the engines may not be at that best power setting any more, so after a while you'll be better off climbing. The 'best' altitude would be the one at which the IAS giving that best AoA simultaneously requires the engines to be set at their best RPM. The upshot of this is that you end up with a series of climbs and decelerations during a flight.

Hopefully a little clearer than mud.

 

[Quickstatus]

So :
IAS is used to keep the plane in the air
TAS is used to navigate
GS is used to navigate and predict destination arrival??

Thanks clear as mud!

Re "series of climbs and decels" do flight crew have to ask for permission each time they want to change FL in order to optimise performance especially on those transpacific and JNB/SCL flights - I'm assuming there radio black spots?

 

[jb747]

So :
IAS is used to keep the plane in the air
TAS is used to navigate
GS is used to navigate and predict destination arrival??

TAS comes into other things...but for the purpose of this discussion, that will do.

Re "series of climbs and decels" do flight crew have to ask for permission each time they want to change FL in order to optimise performance especially on those transpacific and JNB/SCL flights - I'm assuming there radio black spots?

All data link, so it's unlikely that you'll be out of comms for any lengthy period.

Any change of altitude MUST be approved by ATC. You can sometimes get a 'block' clearance, where ATC will clear you to operate between two levels. Changes of speed may also need to be reported.

 

[juddles]

An aircraft is a series of juggling acts....

jb, awesome post - that first sentence really states the reality - it is bloomin complex! The detail of the rest of the post really helps a non-pilot like myself actually understand a bit better what you guys do. Thank you.

A question that was probably answered somewhere in the past, but something that I wondered about today - how soon before takeoff do pilots need to board an aircraft? Is it much less on smaller aircraft (say 737 vs A380), or is it pretty much the same? Has it been reduced in modern times by improvements in systems and so forth?

 

[Skyring]

Thanks, jb747, for the explanation of speeds and altitudes. I guess you aren't exactly sitting around, feet up in the coughpit, while the autopilot churns on. It looks a bit more complex than simply setting cruise control!

Recently I flew Qatar from Athens to Doha. We crossed the Mediterranean coastline just south of the Israel border, and we were at an enormous altitude. I'm guessing that this was to be as safe as possible from any potential SAM threat?

 

[jb747]

Thanks, jb747, for the explanation of speeds and altitudes. I guess you aren't exactly sitting around, feet up in the coughpit, while the autopilot churns on. It looks a bit more complex than simply setting cruise control!

I can simultaneously understand this stuff....and drink coffee, with my feet up.

Recently I flew Qatar from Athens to Doha. We crossed the Mediterranean coastline just south of the Israel border, and we were at an enormous altitude. I'm guessing that this was to be as safe as possible from any potential SAM threat?

You are safe from MANPADs once above about FL200. Airliners do not have anywhere near the performance to get out of the envelope of real SAMs.

It's a relatively short flight, and whilst you don't say what the aircraft is, I'd expect the most efficient altitude to be in the very high 30s...so no higher than usual.

 

[jb747]

A question that was probably answered somewhere in the past, but something that I wondered about today - how soon before takeoff do pilots need to board an aircraft? Is it much less on smaller aircraft (say 737 vs A380), or is it pretty much the same? Has it been reduced in modern times by improvements in systems and so forth?

I've never flown any airliner smaller than a 767, so I don't specifically know about them, but, their systems are very much the same as the larger aircraft, so I'd expect they need similar time. About the only thing that would get shorter with a smaller aircraft is the time taken to do the exterior walk around. On the other hand, the guys get a lot more practice, and that makes things faster.

We normally get to the aircraft with about 45 minutes to go. The means we can do everything we need, and be waiting for others to finish (fuel, loading, etc). You're behind the 8 ball if you're the hold up. Getting ready isn't just a case of putting the switches in an eye pleasing manner, but much of it is getting your head around what you're doing, and the procedures you'll be following.

The minimum time...if you align the IRSs it will take about 9 minutes for them to be ready...so that gives a limit to the lowest time possible. I've seen 767s turned around in not much more than that, but some of the planning work had been taken care of during the previous cruise. Best cold start in a 380 was just on 25 minutes, but that was up against a curfew, with no passengers.

In many ways the technology has not made it quicker. Both of you need to go through every entry on the FMCs. Computerised take off data is more capable, but no faster, than working it from a chart or tabulated data.