Ask The Pilot

[AviatorInsight]

Last Wed I noticed a lot of 737/320 and smaller aircraft landing on 16R at SYD. Most of the time you would see them landing on 16L.

At Airports with parallel runway operations how far in advance do ATC inform you of the L or R runway ?

We have an idea of the runway we will be using based on the ATIS. Basically the flow is such that arrivals from the West and South will use 16R. Arrivals from the North and East will get 16L.

We get our STAR (Standard Arrival Route) clearance from the centre controller roughly 150-200nm out from Sydney. A turboprop however won't get the runway with their STAR. They will know of the runway on first contact with approach.

 

[jb747]

Last Wed I noticed a lot of 737/320 and smaller aircraft landing on 16R at SYD. Most of the time you would see them landing on 16L.

At Airports with parallel runway operations how far in advance do ATC inform you of the L or R runway ?

The way the runways are managed varies around the world. In many places there will be a nominated landing runway (on the ATIS), with the other used for takeoff (London). This is then shared around to reduce noise impact. Wake turbulence is a big issue, so, in London, ATC often puts 380s onto the nominated take off runway for landing, as it has less effect on the overall flow.

In LA, they'll normally be using 24R and 25L for landings, with 24L and 25R used for take off. The decision on which side of the airport you'll use is mostly based on where you'll be parking. The 380 tends to be treated a bit differently, because not all taxiways (and runways) are available to it. Mostly we land on 24R/06L, irrespective of parking bay, or the direction we've come in from.

Sydney generally puts aircraft from the north onto 34R/16L, and from the south onto 34L/16R. 16L is too short for most large aircraft landings, so the pilots will mostly require the right. 34R isn't quite as bad, though it will be avoided if at all possible.

In most cases we know the runway before the start of descent, though it sometimes isn't clear until about 20 miles out. It makes very little difference though, as most STARs are the same until nearing finals intercept. If we are unsure of which runway we'll get (and can't confirm it), we simply brief for the alternatives. The FMCs can have multiple routes loaded, and we can either change the active route, or just reselect the arrival on the active pages. It only takes a few seconds.

 

[jb747]

On a related theme, why do aircraft (I guess I mean large commercial ones), when wanting to do (say) a left banking turn, first turn right for a short time (such that the path resembles the question mark at the end of this sentence) ?

They don't. Where did you get that idea?

 

[jb747]

Roster time again.

The current roster has two trips to go:
30/12 MEL-LAX 93
31/12 LAX-MEL 94

07/01 MEL-DXB 09
10/01 DXB-LHR 01
12/01 LHR-DXB 10
15/01 DXB-MEL 10

New roster:

01/02 MEL-LAX 93
02/02 LAX-MEL 94

12/02 MEL-LAX 93
13/02 LAX-MEL 94

24/02 MEL-LAX 93
25/02 LAX-MEL 94

03/03 MEL-LAX 93
04/03 LAX-MEL 94

 

[RooFlyer]

On a related theme, why do aircraft (I guess I mean large commercial ones), when wanting to do (say) a left banking turn, first turn right for a short time (such that the path resembles the question mark at the end of this sentence) ?

They don't. Where did you get that idea?

Sorry if its not standard, but I have experienced it experienced it a number of times, unmistakably, both on Oz domestic and overseas. Start to gently turn one way then maybe 10 to 15 seconds later, a turn the other way onto the heading that was obviously desired. I've been meaning to ask the question before, but your explanation above of banking prompted it this time and I was expecting an answer along the lines of bank angles and g-forces .

 

[Quickstatus]

The "g" is equal to 1/cos(bank angle).

So a banking turn (essentially a normally configured turn) could under the right circumstances put the aircraft into a stall. There would be an altitude where this would occur with a "normal" banked turn and therefore to turn (banked) you would need to go to a lower altitude?. Or would doing this manoeuvre lose some altitude which you would need to recover to get back to assigned FL?

I understand that increasing speed will eliminate stall but there is a limit at high altitudes

 

[jb747]

Sorry if its not standard, but I have experienced it experienced it a number of times, unmistakably, both on Oz domestic and overseas. Start to gently turn one way then maybe 10 to 15 seconds later, a turn the other way onto the heading that was obviously desired. I've been meaning to ask the question before, but your explanation above of banking prompted it this time and I was expecting an answer along the lines of bank angles and g-forces .

Ok, I see what you're getting at. There is no aerodynamic reason for such behaviour. But, there is a much more mundane one. If an object below you is too close to your track, then you won't be able to see it all that well from the coughpit. A bit of bank changes the view out the windows. Once finished sight seeing, you'll need a couple of turns to get back on track.

 

[jb747]

So a banking turn (essentially a normally configured turn) could under the right circumstances put the aircraft into a stall. There would be an altitude where this would occur with a "normal" banked turn and therefore to turn (banked) you would need to go to a lower altitude?. Or would doing this manoeuvre lose some altitude which you would need to recover to get back to assigned FL?

I understand that increasing speed will eliminate stall but there is a limit at high altitudes

Any time that you pull some 'g', be it in a turn, pulling up, or even pulling down (if you happen to be inverted), you increase the stall speed. The stall speed will increase as the square root of the g loading. So, looking at an A4, which had a clean stall around 110 knots, it would stall at 6 g at around 270 KIAS. In manoeuvring that sort of aircraft, the term "pulling to the judder" was used...it meant pulling until the aircraft was mildly shaking in the early stages of the stall. You'd hold it there by feel, sitting right on the edge of the stall, as the speed varied through the manoeuvres.

If you're flying fast enough, you'll hit the g limit before the stall IAS. So, if I was faster than 295 KIAS (or so, this all varies with weight), then I'd hit the 7.2 g limit before the judder.

Applying this to an airliner....At altitude, your maximum speed is limited by mach number. The IAS for a given mach number reduces as you climb. At sea level, maximum mach is well above the maximum IAS allowed (by about 200 knots). At somewhere around FL300, max IAS will be greater than max mach, and from that point upwards, mach is used. The effect of this is that as you climb, your speed is being limited by mach to a reducing IAS.

But, the wings work on airflow, and IAS is a measure of this. The stall IAS remains more or less the same as you climb. So, your minimum speed, as measured by IAS, remains the same. (True airspeed, TAS, is increasing, but is not relevant to this.) If we climb high enough, the mach limit will continue to reduce until it meets the stall IAS. That's the real coffin corner, but it's also way higher than any airliner can fly. If you happen to have a U2 in your hangar, you'll be familiar with it.

The IAS for the stall is the most obvious limit, but a speed somewhat above that is more relevant. Vmin drag is generally only a few knots slower than the actual cruise speed. If you slow below that speed, then the aircraft may not have sufficient power to accelerate again, so it will continue to slow. The only solution is to trade height. The power margin may be such that as little as 5 knots below Vmin drag would be the tipping point. And, when you pull that 'g', not only does the stall speed increase, but so too Vmin drag.

Airlines and manufacturers counter this by placing some level of margin into the aircraft performance data. In the case of maximum altitude, the FMCs calculate it with a defined stall margin, specified in G. The Boeing standard is 1.2, but some airlines (QF) increase this to 1.3. That means that nominally, you could roll to a 47º angle of bank turn before running out of margin. You wouldn't have sufficient power to offset the drag, but that's a somewhat different issue. The is no need for 47º, so it's a nice margin. By using the 1.2 margin, an airline will be able to climb higher/earlier than an airline using 1.3. But, they also run the risk of issues if they reach any turning point that is more than the usual slight kink. I've seen this once in the Middle East, where an aircraft was unable to maintain altitude at a turning point (I'm told they have nice champagne).

This has all been about increasing the 'g', but what happens to the stall speed if we reduce it below 1...or even go to zero?

 

[Quickstatus]

Well at Zero G no aircraft will Stall, even if the angle of attack is past critical. Actually It wont even need wings.

But no aircraft operates in space. The closest to zero G would be at the top of a ballistic arc and then only for a moment. If the IAS was zero at zero G no problems but G at the top of a ballistic arc does not stay zero for long...

Is it true to say the reason that aircrafts tend to go into a death spiral during a stall is because during a stall the airfoil does produce some lift, though not enough?

 

[Major]

As a matter of interest what hours will your log books show for the year ? I notice JB still has 2 sectors to complete and no doubt Boris still has flying ahead of him as well.

 

[jb747]

Well at Zero G no aircraft will Stall, even if the angle of attack is past critical. Actually It wont even need wings.

The angle of attack couldn't be past critical..otherwise it would be stalled. It's the AoA that produces no lift.

But no aircraft operates in space. The closest to zero G would be at the top of a ballistic arc and then only for a moment. If the IAS was zero at zero G no problems but G at the top of a ballistic arc does not stay zero for long...

A bit longer than you might imagine. Back in the old days, if you wanted to get out of a fight, zero G and max power would give you max acceleration. Once you reached about 40º nose low, you'd stop the push and just run.

Is it true to say the reason that aircrafts tend to go into a death spiral during a stall is because during a stall the airfoil does produce some lift, though not enough?

In my military days, we used to play with the stall, in particular holding the CT4 in very deeply whilst using rudder to keep it wings level. When you got it wrong it would flick though. The Macchi would spin if you mishandled it...but that didn't stop us. It was quite a nice spin. A4 was pretty classic delta, in that it remained quite controllable, but the sink rate was off the clock.

Aircraft are not exactly symmetrical, so they often stall slightly earlier on one side. Same effect if you haven't got it exactly in balance, or ailerons exactly neutral. The civilian world tends to keep away from stalls and departures, the military much less so. Many times I've departed an aircraft, and had to sit on my hands for a while whilst it sorted itself out.

 

[jb747]

As a matter of interest what hours will your log books show for the year ? I notice JB still has 2 sectors to complete and no doubt Boris still has flying ahead of him as well.

This has been a pretty quiet year. I took a six week chunk of leave, and then that was closely followed by an 8 week blank line roster, which would have to have been the quietest one I've ever done. Generally you reach the average flying on a blank line, and often exceed it, but that one gave nothing.

Total will be right on 600 hours. 2015 was 845. No blank line planned for 2017, and very likely little leave, so I expect I'll be bouncing off 900 (max) hours.

At the end of the year, my total hours will be 20846, of which 14851 are heavy jet command.

 

[Kerrodt]

The civilian world tends to keep away from stalls and departures, the military much less so. Many times I've departed an aircraft, and had to sit on my hands for a while whilst it sorted itself out.

JB, what does it mean when you refer to "departures", and "had to sit on my hands for a while whilst it sorted itself out"?

Thanks.

 

[jb747]

JB, what does it mean when you refer to "departures", and "had to sit on my hands for a while whilst it sorted itself out"?

Departure = departed controlled flight. Which means it's now doing whatever it wants, and not whatever the control input you've got in would normally give. Sometimes the behaviour is quite predictable, and can be stopped, but at others, you put the controls back into neutral, throttle to idle, and wait until it settles into something stable. Stable could well be a spin (either erect or inverted), but that's something the correct control inputs will ultimately cancel. If it never stabilises, then, at best, you'll be walking home.

Lots of interesting video on youtube, but ensure you don't read the comments from the flight sim kiddies. Do a search for "youtube aircraft departure from controlled flight". The F22 is a doozy.

 

[Quickstatus]

When an airfoil is at a positive AoA producing lift does it have a tendency to return to a zero AoA?
How do aircraft systems maintain a certain AoA?

 

[jb747]

When an airfoil is at a positive AoA producing lift does it have a tendency to return to a zero AoA?
How do aircraft systems maintain a certain AoA?

An aerofoil doesn't exist in isolation, but is part of a system. In the case of the wings, the angle of attack is controlled by the tail surfaces. So, when the tail is trimmed it gives a pretty constant angle of attack, and results in very predictable and stable behaviour. Slow down, and that constant AoA won't be enough to maintain level, nose will drop, aircraft will accelerate, and ultimately pitch up, lose speed, pitch down. It tends to be self correcting.

The vertical tail is also an aerofoil, but one that is normally presented to the airflow at zero angle of attack. Unlike the wings, it's symmetrical, so at zero AoA, no lift is produced. But, if you get some sideslip, it will start to produce some, and that will lead to a yawing input...which in turn reduces the sideslip. Stability in that case is zero sideslip, and zero lift is the normal situation.

Fly by wire aircraft measure the angle of attack (and sideslip), and will use that data to allow the normal laws to hold constant alpha, and or to protect from the stall.

 

[TheRealTMA]

Was thinking yesterday when we decended into AKL and I had to relieve pressure in my ears, I know that one can't physically open the door of a pressurised aircraft in cruise or at altitude, but I wonder what is the minimum altitude that it is actually possible. I.e. at what altitude does the pressure inside the aircraft exceed the outside air pressure to prevent this. Or I guess at what altitude does the aircraft pressurise?

 

[Boris spatsky]

Was thinking yesterday when we decended into AKL and I had to relieve pressure in my ears, I know that one can't physically open the door of a pressurised aircraft in cruise or at altitude, but I wonder what is the minimum altitude that it is actually possible. I.e. at what altitude does the pressure inside the aircraft exceed the outside air pressure to prevent this. Or I guess at what altitude does the aircraft pressurise?

The aircraft begins to pressurise on the takeoff roll - the cabin descends below airfield elevation for a short period then ascends slowly once airborne.

 

[Boris spatsky]

As a matter of interest what hours will your log books show for the year ? I notice JB still has 2 sectors to complete and no doubt Boris still has flying ahead of him as well.

No idea; i'll have a look. I did a 737 course and then training this year so my hours will be quite low compared to a normal year i would think. Still a day or two to go...

 

[jb747]

Was thinking yesterday when we decended into AKL and I had to relieve pressure in my ears, I know that one can't physically open the door of a pressurised aircraft in cruise or at altitude, but I wonder what is the minimum altitude that it is actually possible. I.e. at what altitude does the pressure inside the aircraft exceed the outside air pressure to prevent this. Or I guess at what altitude does the aircraft pressurise?

They start to pressurise during the take off roll, and the vent any remaining pressure at touchdown. The aircraft initially pressurises itself to a level lower than the airport. So the answer is zero feet. Even on the ground, there's a few hundred pounds of pressure on the door.