Ask The Pilot

[AviatorInsight]

That is a very informative post. Learn something new every day.

AV are your margins as tight in the NG? Do you slow down also?

We don’t really slow down. Normal ECON climb and cruise is around the 280/0.78M which is the turbulence penetration speed. If the autothrottle is still engaged (which it should be) and the thrust is moving too much to maintain speed, then we will disconnect it and set the turbulence N1 which is given by the FMC for the current weight and altitude.

There is usually a bit of a buffer between MMO (maximum Mach) and minimum clean speed unless we’re at max weights.

This photo shows us at FL410 still with some margin left at that altitude.

 

[Quickstatus]

aircraft will defend itself

Thanks, so in summary, the aircraft tries to stay within its predetermined flight envelope whatever the actual capability (and the actual capability might be very close the the flight envelope anyway)

 

[jb747]

Thanks, so in summary, the aircraft tries to stay within its predetermined flight envelope whatever the actual capability (and the actual capability might be very close the the flight envelope anyway)

The A380 certainly did. I expect the FBW Boeings will too, to some degree anyway.

The 380 flight control system was very smart. There were ways to override it, but overall it was well thought out and executed. Of course, they've been able to practice on the 320/330/340 so it wasn't really anything new. It would actively protect you from stalling, which meant that in certain cases it would have been possible to simply pull full aft stick, without any regard to stalling. This was of real use in any ground proximity situation, and it was taught in the sim to simply pull all of the aft stick and to hold it there until the issue was resolved (or you hit the ground!).

As there is no reason to fly this sort of aircraft beyond the stall, even full aft stick would limit you to the exact edge of stalling (from continuously measured angle of attack). It does so way more accurately than any pilot can do it by feel.

At the other end of the speed scale, it would pitch up to limit any overspeed. If you really, really, wanted to do it, you could push against the pull up, and you'd be able to override it. It was not something you could do accidentally.

It's one of the issues with this sort of aircraft though. It is really smart, and protects itself (and you) most of the time. But, with degraded flight laws, these protections also disappear...and you, as the pilot have to put them back, exactly as you would do in any non FBW aircraft. In many ways easy aircraft to fly until something goes wrong, at which point they can become harder to fly than traditional aircraft. The Air France 330 is a great example of degraded flight controls coupled with a pilot who only knew how to fly when it was all working.

 

[Major]

JB, is there much diffence in the height of the coughpit from the ground between the 747 and the 380 ?

Did you notice any appreciable difference the first time you landed the 380 ?

 

[jb747]

JB, is there much diffence in the height of the coughpit from the ground between the 747 and the 380 ?

Did you notice any appreciable difference the first time you landed the 380 ?

The 380 coughpit position was described as being about half way between decks on the 747. So, it was noticeably lower, but it was nowhere near as much transition as it was between the 767 and 747. On those first few landings, there were so many things going on, that you didn't really notice the nuances, you'd just flare on the rad out callouts. By the time it became obvious, it didn't matter all that much.

 

[Quickstatus]

On the 747-200 there was a toilet within the coughpit.

Is the 747 coughpit on the same level as the upper deck or are there stairs

 

[jb747]

Is the 747 coughpit on the same level as the upper deck or are there stairs

It’s on the same level as the upper deck. If you sat on the coughpit of a 737/767 when landing, it looked horribly low over the fence.

One facet of the design that you did have to think about (when taxiing) was your placement vs the position of the nose gear. You were quite a way ahead of it, so sometimes the view from the coughpit would have you out over the grass, even though the wheels were exactly where they should be. People who were vertically promoted on the 747 saw the positioning as normal, but flying with FOs who had come from the 737, you had to be very aware that their version of keeping wide in turns was nowhere near wide enough.

 

[OZDUCK]

The discussion on the Brisbane RON1 engine issues reminded me of something that I witnessed years (and years) ago.

I was airside at Perth Airport in the early 1970s when a CX B707 started its takeoff roll and about a third of the way down the runway there was a series of noises that sounded somewhat like a 'backfire'. It stopped the takeoff and after a short wait returned to the end of the runway and tried again with the same result. It then returned to the terminal, unloaded the pax and was towed off to the maintenance sheds for some work before leaving the next day. I have often wondered if this was the result of some sort of low speed engine stall?

 

[jb747]

The discussion on the Brisbane RON1 engine issues reminded me of something that I witnessed years (and years) ago.

Popping and flashes of flame are certainly symptoms of stall/surge. The 707 engine may have been prone to it, if handled roughly, which is why power advance from low speed was always done by the engineers in the days before the digital fuel controls.

Inside the coughpit, you'd be unlikely hear any popping, but the engine wouldn't be producing any power, so you'd get the same swing (yaw)as an engine failure would give you. You may also see the temperature shoot upwards. I'm a little surprised that they just tried it again, unless they had something specific that they suspected (i.e. too rapid lever movement).

 

[OZDUCK]

Popping and flashes of flame are certainly symptoms of stall/surge. The 707 engine may have been prone to it, if handled roughly, which is why power advance from low speed was always done by the engineers in the days before the digital fuel controls.

Inside the coughpit, you'd be unlikely hear any popping, but the engine wouldn't be producing any power, so you'd get the same swing (yaw)as an engine failure would give you. You may also see the temperature shoot upwards. I'm a little surprised that they just tried it again, unless they had something specific that they suspected (i.e. too rapid lever movement).

Thanks for the informative reply.

 

[jb747]

A term that comes up in the media, and which probably causes some level of confusion, is the use of the word "stall".

Most people are familiar with a car engine stalling, and in some ways I expect that they equate that to the use in aviation, but there are a couple of separate uses for the term.

A jet engine stalls when the airflow through it becomes disrupted. That can lead to pops and bangs, and bursts of flame from either end of the engine. It isn't on fire, and it may recover simply by reducing the thrust lever. Like a car though, when an engine is stalled, it isn't producing any power. Unlike the car though, it is actually still running...just very badly.

The wing (or aircraft) stalls when the angle of attack of the air flowing over the wing exceeds approximately 15º. The flow then breaks away, and the amount of lift being produced collapses, whilst the drag simultaneously rises dramatically. To recover from this situation, the angle of attack must be reduced. Normally this means lowering the nose. The aircraft accelerates, AoA reduces, and you can then, gently, start to recover.

 

[Carrots]

Would it be more akin to backfiring JB?

On a side note, likely a question for @AviatorInsight given you’re flying these birds, any reason for a fight path like this into Adelaide? Clearly perfectly doable, but obviously not particularly common for anything bigger than a turboprop.

 

[jb747]

Would it be more akin to backfiring JB?

We sometimes describe it that way, though the mechanism isn't similar. It makes sense to people though (even though I doubt that many understand backfiring either).

On a side note, likely a question for @AviatorInsight given you’re flying these birds, any reason for a fight path like this into Adelaide? Clearly perfectly doable, but obviously not particularly common for anything bigger than a turboprop. View attachment 243368

Beating AV to the punch...there is nothing unusual in this flight path. About as direct as possible. The aircraft's energy is nicely managed, and it rolls out on finals, in the slot.

All approaches in reasonable weather would be like that if it weren't for ATC, and other aircraft.

 

[JessicaTam]

Aviators, is there a set criteria (e.g. altitude) for when to turn off the seat belt sign after take-off?

 

[AviatorInsight]

Would it be more akin to backfiring JB?

On a side note, likely a question for @AviatorInsight given you’re flying these birds, any reason for a fight path like this into Adelaide? Clearly perfectly doable, but obviously not particularly common for anything bigger than a turboprop. View attachment 243368

Firstly, in regards to the compressor stall, yes it’s like a backfire. While I haven’t had one in real life, they’re common in the simulator. Presented as a series of “bangs” and a pulsating N1 gauge with (depending on the severity) a high EGT (exhaust gas temp) reading.

This arrival has definitely been shortened. In this case looks like they got direct to GUGTI, the final approach point on the ILS.

Doing this exact same flight just a few weeks ago we got given a similar track shortening with cancellation of all speed restrictions. The reason? ATC wanted to sequence us in front of aircraft from the East. So it’s definitely doable and not uncommon to join around a 5nm final if weather conditions can permit a visual approach and we don’t have to conduct the full ILS.

Aviators, is there a set criteria (e.g. altitude) for when to turn off the seat belt sign after take-off?

SOPs say 10,000ft. However if the sector length is short (SYD-CFS for example), then anytime above 5000ft and weather conditions permitting we can turn the belts off.

 

[jb747]

Aviators, is there a set criteria (e.g. altitude) for when to turn off the seat belt sign after take-off?

It's going to differ with every airline and regulator in the world. There are some for whom the switch seems to be glued into the "on" position.

QF was pretty flexible with it, not specifying any particular numbers, and leaving it to the flight crews' assessment on the day.

The switch itself had three positions. ON, AUTO, OFF. The exact way in which the AUTO setting works can be varied by airline configuration changes. QF had it set this way:
FASTEN SEAT BELTS signs (AUTO selected):

• landing gear not up and locked or
• flap lever not up, or
• cabin altitude above 10,000ft, or
• passenger oxygen on

The upshot of that is that unless it was specifically selected to ON, the seat belt sign would go out when the flaps were selected to UP (which would be around 3,000' on most departures) and it would come on at the first flap selection on arrival, which would be around 15 miles, and 4,000'.

 

[harvyk]

We don’t really slow down. Normal ECON climb and cruise is around the 280/0.78M which is the turbulence penetration speed. If the autothrottle is still engaged (which it should be) and the thrust is moving too much to maintain speed, then we will disconnect it and set the turbulence N1 which is given by the FMC for the current weight and altitude.

There is usually a bit of a buffer between MMO (maximum Mach) and minimum clean speed unless we’re at max weights.

This photo shows us at FL410 still with some margin left at that altitude.View attachment 243139

Hi AV,

Just looking at this picture, how much have I got right?
It looks like you've entered in an offset to the route, but have then set a heading to fly on to go around some weather?

If I have interpreted the above correctly, is there a reason why you'd set a heading rather than simply let LNAV take you to the offset path, and is there a reason why you're flying to the east of the weather rather than the west?

 

[AviatorInsight]

Hi AV,

Just looking at this picture, how much have I got right?
It looks like you've entered in an offset to the route, but have then set a heading to fly on to go around some weather?

If I have interpreted the above correctly, is there a reason why you'd set a heading rather than simply let LNAV take you to the offset path, and is there a reason why you're flying to the east of the weather rather than the west?

Great questions!

You are correct. There is an offset to the right of route and currently flying in Heading select. Here is why we chose that.

1. We chose Right of route for the simple fact that it was ever so slightly upwind of the storm. Most of the time it’s smoother when going upwind, usually because the anvil has travelled downwind.

2. The offset had not yet been executed (shown in white), about 2 mins after that photo was taken we asked for further right of route to get around that storm sitting in the way of the new track.

3. When flying an offset that bit of airspace becomes ‘yours’. We can manoeuvre as we like within that distance. By flying LNAV from this heading, it’ll intercept the new track at a 45° angle and then obviously keep it there (adjusted for wind). If there’s anymore weather we’ll have to come off that track (using HDG SEL) and reintercept until we’re clear. By keeping it in HDG SEL it is easier to manoeuvre around individual cells, but you still need to monitor it so you don’t fly outside your cleared offset.

So for example, if we have asked for 50nm right of route then we will enter 50nm into the FMC to show us a picture (it helps with situational awareness) as to how far we can go before a new clearance is required to go out further if we need to.

 

[Flying Fox]

Sorry if this has been asked before but in another thread there is a discussion about passengers being bumped from row 4 to row 28 for weight & trim balance or other similar technical reasons.

1. As a pilot of a B737 or larger, how much real difference does moving 3 people around really make when taking off and landing?

2. How many people would you have to move round to make any real difference on an A380 or B747?

3. As we are told we are free to move around when at cruising altitude, why does this make no difference when flying?

4. Wouldn't freight (with its higher density) make a larger difference?

TIA

 

[jb747]

Sorry if this has been asked before but in another thread there is a discussion about passengers being bumped from row 4 to row 28 for weight & trim balance or other similar technical reasons.

1. As a pilot of a B737 or larger, how much real difference does moving 3 people around really make when taking off and landing?

In practical terms, none. But, legally, huge. It can be the difference between legal weight and balance, and operation outside of the limits.

2. How many people would you have to move round to make any real difference on an A380 or B747?

We used to move fuel into and out of the tail for weight and balance reasons, and every 1,000 kgs, was equal to 1% of movement. And we had to be within 1% of the calculated numbers at take off. So, at the extremes of the cabin, call it 10 people on a 380.

3. As we are told we are free to move around when at cruising altitude, why does this make no difference when flying?

It would if you all moved to the same end at once!

The limits in the cruise are greater than they are for take off. CoG margins are established there to give appropriate rotation, and control responses, at given stab settings, and flap configurations. All different in the cruise.

4. Wouldn't freight (with its higher density) make a larger difference?

It sure does. Pallets could weigh many tonnes, so in effect, they might be the equivalent to block of 50 or so passengers.