Ask The Pilot

[jb747]

If powered down, as you term it, what's the sequence of events to get it to the point where you want to start an engine, or get the A/C going? Eg. How do you start the APU? And is it from there that other systems are started?

Normally the pilots don't ever totally shut down the aircraft systems...we leave that to the engineers, and when accepting the aircraft, it's normally powered up, with either the APU running or ground power. But, shutting it down totally is really just a case of taking away all of the electrics...disconnect or turn off the ground power, shut down the APU, and turn off all of the batteries...and it all goes dark and quiet. In some aircraft you may have to pull some circuit breakers to stop emergency batteries activating.

To start from scratch, without ground power, just turn on all of the batteries, do a fire test of the APU, turn the APU master on (which will open the inlet door), and press the start switch. It should wind up within about 30 seconds, and after about a minute both of the APU generators will come on line, and the aircraft will come to life. To get A/C, select the APU bleed on, and the packs will start automatically (as long as the switches are selected on). Some of the computer systems will take a while to completely boot and go through their routines, but a preflight takes about 30-40 minutes anyway, and they'll be up well before you finish.

Oh, like your brother's comment about the hole in QF30. Does he fly too?

No.

Finally, you talk about cruise speeds and low tolerance to speed change. I recall RdC telling a story in his book about a 747 which was flying at a rate which left no room for error, something happened and the crew had to take action to fix it.

I can't find the book to look it up, but would that be related to what you were saying earlier?

Perhaps. I never managed to finish the book, so I don't know the reference. In flight an aircraft has a maximum speed, which at lower levels will be an IAS, and at high levels a mach number. In cruise, an A380's max speed is mach .89. The lowest speed limit is really the stall speed. That's a number that varies with weight, but for example lets consider it to be 200 knots IAS. As you climb, it remains an IAS.

But, as you climb, the mach number of any given IAS increases. At sea level 300 knots is about .4 mach (rough numbers), whilst at 40,000' that same 300 knots is .8 mach. So the mach number of your stall speed increases as you climb. If you had sufficient power (and airliners don't), then it would be possible to climb so high that the mach number of the stall speed would equal the maximum allowed mach. In other words your maximum and minimum speeds would be equal. This is the sort of regime that the U2 flies in.

Airliners don't have enough power to go that high, but another number comes into to play that is also a constant IAS (for a given weight)....minimum drag. It was mentioned a few posts ago, but it is possible at altitude to end up a bit slower than min drag, and to not have enough power to accelerate again. In that case, if you do nothing, the aircraft will continue to decelerate, until it gets very nasty. The only recovery once you run out of power is to trade height for speed, something that's not a good idea in the airways. The upshot of this is that you don't allow the aircraft to slow below min drag at altitude..basically it should be treated as a hard limit.

 

[tgh]

It seems that the safety envelope at cruise in a large airliner is , perhaps , much tighter than many of us would imagine.
Is it too much to ask for a dummies guide to the equation, and the kind of circumstances that might challenge it ?

 

[jb747]

It seems that the safety envelope at cruise in a large airliner is , perhaps , much tighter than many of us would imagine.
Is it too much to ask for a dummies guide to the equation, and the kind of circumstances that might challenge it ?

I thought my last post was a dummies guide....

 

[mjt57]

If we get too slow, the drag will start to increase,

Found that post re: drag.

It must be nightshift but I don't understand this. What is causing the drag to increase in this example?

 

[jb747]

Found that post re: drag.

It must be nightshift but I don't understand this. What is causing the drag to increase in this example?

The overall drag curve is an inverted bell curve.

It's made up of a number of components, but two major ones are form drag, which is what you get when you stick your hand out of the car window, and which increases with the square of speed, and induced drag, which is a byproduct of the production of lift and which is worse at low speeds.

So...going slower may actually give you more drag.

And now a question you can ponder during the night shift.....Flying an aircraft like the F28, which has a split speed brake at the tail (and so does not affect the wing), how could the speedbrake position affect the approach (spool up time is not the answer).

 

[cove]

Just remember that we don't generally score distinctions in our first flight training experience in make believe land JB747.
I had a couple of issues where 2 landings were not so good and I muffed a take off all inside 90 minutes.
I came out thinking flying needs way more concentration than I have available.

 

[tgh]

The only recovery once you run out of power is to trade height for speed, something that's not a good idea in the airways. The upshot of this is that you don't allow the aircraft to slow below min drag at altitude..basically it should be treated as a hard limit.

I might be perceived as leading you down the AF 447 street perhaps…seeking understanding of just how much wriggle room you normally have and the factors that erode it.

 

[markis10]

And now a question you can ponder during the night shift.....Flying an aircraft like the F28, which has a split speed brake at the tail (and so does not affect the wing), how could the speedbrake position affect the approach (spool up time is not the answer).

Allows slower approach speed as the speedbrake reduces minimum drag speeds.

 

[jb747]

Allows slower approach speed as the speedbrake reduces minimum drag speeds.

Exactly...

It was a question on a 2FTS aerodynamics exam.

 

[jb747]

The only recovery once you run out of power is to trade height for speed, something that's not a good idea in the airways. The upshot of this is that you don't allow the aircraft to slow below min drag at altitude..basically it should be treated as a hard limit.

I might be perceived as leading you down the AF 447 street perhaps…seeking understanding of just how much wriggle room you normally have and the factors that erode it.

At the maximum altitude the margins are small. But, not for a 447 scenario..that would only start to apply if you ran out of margin, and still tried to stay up there. Even then the crew would have to ignore aerodynamics, and hold more and more backstick...with no actual attempt at a stall recovery.

In the 747, maximum altitude in the FMC was defined as the altitude at which the aircraft could produce a given 'g' in a turn, with the standard being 1.2g, but 1.3g being a company option. That equates to the standard being 33º angle of bank, with the option requiring more margin (a capability of 47º). That's one reason why you might see otherwise identical aircraft, but from different airlines, climbing at different positions. Both are actually capable of the climb, but one airline requires a different margin to another. Additionally the aircraft had to be capable of a climb of 100 fpm....which defines the power margin.

Remember though, that that is the max. We rarely go there...it isn't efficient, and there's way more margin only 2,000' feet below.

 

[Hvr]

Hi JB, following on from a discussion about slide rules in the totally off topic thread I read the wiki page on slide rules and saw this paragraph.

One slide rule remaining in daily use around the world is the E6B. This is a circular slide rule first created in the 1930s for aircraft pilots to help with dead reckoning. With the aid of scales printed on the frame it also helps with such miscellaneous tasks as converting time, distance, speed, and temperature values, compass errors, and calculating fuel use.

The so-called "prayer wheel" is still available in flight shops, and remains widely used. While GPS has reduced the use of dead reckoning for aerial navigation, and handheld calculators have taken over many of its functions, the E6B remains widely used as a primary or backup device and the majority of flight schools demand that their students have some degree of proficiency in its use.

I'll assume you learnt to use a slide rule in your original flight training, but do you still use it in day to day flying? And are they still in any Australian coughpits?

Thanks.

 

[Fruitcake]

And are they still in any Australian coughpits?

I purchased a Jeppesen CR-3 this morning and will be using it in an Australian coughpit. Does that count?

 

[jb747]

Hi JB, following on from a discussion about slide rules in the totally off topic thread I read the wiki page on slide rules and saw this paragraph.

I'll assume you learnt to use a slide rule in your original flight training, but do you still use it in day to day flying? And are they still in any Australian coughpits?

In my original life as an Observer/Navigator, I knew how to make one dance. It was a very neat little tool, not just working as a circular slide rule, but also allowing calculations of drift, groundspeed and wind.

I haven't carried one in many years, although I think there may be one hidden away at home somewhere. Occasionally you still see one around though...the smaller ones fit nicely into a shirt pocket.

 

[legroom]

Hi JB,

Perhaps each route has different waypoints where you are required to contact the local ATC but in general how many hours would pass between such contact ?

You initiate the contact or the ATC asks first ? Does that matter ?

Have you ever 'forgotten' to make a contact and would do so some time afterwards ?

Or would the flight computer automatically flash a reminder message when a certain waypoint is reached ?

Thanks again

 

[jb747]

Perhaps each route has different waypoints where you are required to contact the local ATC but in general how many hours would pass between such contact ?

Depends upon just where you are. Sometimes you change frequencies, and call a new agency every 15 minutes, whilst at others there might be 40 minutes between calls, and many hours between agency changes.

You initiate the contact or the ATC asks first ? Does that matter ?

At position reports we generally call ATC, though sometimes they call you.

Have you ever 'forgotten' to make a contact and would do so some time afterwards ?

Sure, and mostly there's a not to subtle reminder from the ground.

Or would the flight computer automatically flash a reminder message when a certain waypoint is reached ?

Waypoints just pass. The old aircraft used to give a two minute warning on waypoints, but that was because you might have to actually load the next one. FMCs contain the entire route.

Much of the time we use data links. Once we have established the link, ATC keep track of the aircraft automatically, and messages can be sent in either direction.

 

[mjt57]

Allows slower approach speed as the speedbrake reduces minimum drag speeds.

Is this because the speed brakes aren't interrupting flow over the wings?

 

[jb747]

Is this because the speed brakes aren't interrupting flow over the wings?

No. It has nothing to do with the wings.

The inverted bell curve of the drag profile is the combination of the form and induced drag. Form drag is on the right of our graph, and increases with speed. If you increase the base level of form drag, the effect upon the overall drag is to increase it, but also to move the minimum drag to a lower speed....there's more of it, but it happens slower. As it happens, the best speed to fly an approach is a little bit faster than min drag..so that ends up slower too.

 

[Trakman]

I have a question relating to an issue that has been raised in the MH370 tragedy, but I am posting it here because I am only interested in it as a theoretical question not related to any practical issues with any particular flight.

Is it technically possible for a hijacker (or wayward pilot) to disable a commercial plane's comms and then fly it "below the radar" for any period of time? We hear about it in adventure stories but can it actually be done in the real world? How low would they need to fly and can that be achieved for any length of time by a large commercial airliner?

 

[ReLoad]

JB as always thanks for you patience with us mere mortals. Please excuse the barrage of questions!

I'm sure you have seen the 767 bounce landing video doing the rounds

http://m.youtube.com/watch?v=5x85YYLuCY4

I have a few questions about it.

Firstly given the wind conditions do you have permission as the pilot to refuse to go for an alternate airport without even trying to land?
At what point can you no longer do a "go around" can you even after the bounce throttle up and take off again?

I've been in 4 go a rounds in my flying time, 1 in syd where we had lost separation to the plane in front and had to go around, the QF pilot was funny, he said the plane in front was on a go slow. The second and third were on the same flight, trying twice to land in WLG before giving up and going to CHC and the last was in MEL where we were very close to the ground and due to wind change he did a loop and landed on the other runway. So I'm 1 in every 120 flights. how often do you experience a go around?

Do the larger planes such as the A380 have better adverse weather performance and less go arounds?

In relation to the 767 in the video, after a landing like that (I'm surprised the tyres didn't pop!) would you schedule engineers to check out the plane? Is there a procedure you would follow about this sort of landing post event?

Do you sim for these sorts of weather conditions?
Do earlier flights let you know about what they experienced when they landed, or does ATC let you know?

Thanks very much!

 

[jb747]

Is it technically possible for a hijacker (or wayward pilot) to disable a commercial plane's comms and then fly it "below the radar" for any period of time? We hear about it in adventure stories but can it actually be done in the real world? How low would they need to fly and can that be achieved for any length of time by a large commercial airliner?

Firstly, in relation to the Malaysian 777, 99% of everything I've seen so far has been total rubbish, sprinkled with a liberal dose of fabrication.

But, to answer your question. You don't really need to disable to comms...you just don't have to answer. You would need to go into the FMC and turn off the data link, and you'd also need to turn the transponder off. Additionally, you'd have to go into the ACARs and turn it off too. Primary radar would still be able to see you, though I don't know how often the controllers actually look at it. It would still, most likely, be recorded.

Below the radar. If you are within about 20 miles of the site, there is no such thing. The radar horizon varies with your height, and that of the radar site. In nautical miles, it is roughly 1.23 times the square root of your height (in feet). Consideration of it in regard to an airliner falls in the silly category.