Ask The Pilot

[Quickstatus]

Or a trailer at the local tip

I'll beat my own chest, i can easily reverse a 3 tonne angle horse float into a normal car parking space....with Mrs QS as the "wingwalker".

 

[FreqFlyer46]

Hi all, I’m a newbie here so apologies if I unintentionally break any protocols or AFF etiquette jumping in with my first post. With the advent of the 787 and A350, I notice on Flightradar a lot more commercial passenger flights reaching a final cruising altitude of 43,000 feet these days including the odd Qantas and Jetstar 787 in the final couple of hours of a long haul flight. There is even the odd A380. I am guessing the main factors driving this are the planes now having this capability, weight on the flight and favourable conditions at this flight level on some routes. My question is whether this presents any additional operational or safety considerations that pilots have to take into account for this part of the cruise? Have not found much useful information on the web so far on this topic with lots of ill informed people giving their views. Thanks in advance for any responses.

 

[VPS]

Hi all, I’m a newbie here so apologies if I unintentionally break any protocols or AFF etiquette jumping in with my first post. With the advent of the 787 and A350, I notice on Flightradar a lot more commercial passenger flights reaching a final cruising altitude of 43,000 feet these days including the odd Qantas and Jetstar 787 in the final couple of hours of a long haul flight. There is even the odd A380. I am guessing the main factors driving this are the planes now having this capability, weight on the flight and favourable conditions at this flight level on some routes. My question is whether this presents any additional operational or safety considerations that pilots have to take into account for this part of the cruise? Have not found much useful information on the web so far on this topic with lots of ill informed people giving their views. Thanks in advance for any responses.

and welcome to AFF

 

[jb747]

With the advent of the 787 and A350, I notice on Flightradar a lot more commercial passenger flights reaching a final cruising altitude of 43,000 feet these days including the odd Qantas and Jetstar 787 in the final couple of hours of a long haul flight. There is even the odd A380. I am guessing the main factors driving this are the planes now having this capability, weight on the flight and favourable conditions at this flight level on some routes.

They've been able to get up there, forever. It's just that's it's very rarely worthwhile. The basic rule of thumb that you apply for any climb toward the end of a flight is to ask whether you'll actually get back the fuel used to climb in the first place. So, you'll need that higher altitude to actually become the optimum (not just something you can reach) for at least an hour before you get to top of descent. The 747 could actually go to FL450, but I can only ever recall going there once, and we had zero passengers or cargo. It was just a positioning flight from Sydney to Cairns. And even then, it wasn't actually worthwhile, I just felt like doing it.

My question is whether this presents any additional operational or safety considerations that pilots have to take into account for this part of the cruise? Have not found much useful information on the web so far on this topic with lots of ill informed people giving their views. Thanks in advance for any responses.

As you climb, you end up with a bunch of aerodynamic and performance factors closing in on each other. The aircraft you mention will cruise at about mach .85. That mach number will be more or less the same at all altitudes, as long as you're at the optimum altitude for the weight. Within the FMC the airlines will choose the performance factors that they want to use. For instance the maximum altitude may be defined as the highest the aircraft can reach at climb power, whilst still having the capability of climbing at 400 fpm. On top of that they'll add another requirement for the potential rate of climb, at cruise power, of around 100 fpm. This will be at a specific speed (mach number).

The aircraft itself hits you will a couple of limits. Your mach target might be .85, but the maximum allowed mach might only be .88. Looking in the other direction, slowing down, you'll find that you cannot go much slower without running out of power. The reason for that is that the target speed (that .85) isn't the minimum drag speed, but is slightly faster. This is chosen so that in the cruise, the aircraft is at a 'speed stable' position on the drag curve. If it goes very slightly faster, there'll be more drag, and it will naturally slow down. And if it goes slightly slower, there will also be less drag, and it will naturally speed up again. But, if you slow to a speed less than min drag, you'll now experience increasing drag as you slow, and will need more (and more) power to get back to your target. Ultimately, you won't have enough power to even arrest the speed decay, much less regain it, at which point the only solution is to descend. Add to this the fact that the engines produce less power at altitude.

The minimum drag speed tends to remain about the same as you climb. But, it's an IAS, which means that the minimum drag mach number is getting higher as you climb. Go high enough, and min drag and max speed will actually be the same number.

The simple upshot of all of this is that if you climb high enough, you can't go any faster, you can't go any slower, and you have less power to play with. Airlines build some level of margin into things by effectively chopping the top corner off the allowed climb performance. Nevertheless, very high altitude flight is a place to be treated with caution. The actual margins for those aircraft at FL430 are probably a window of about 20 knots, from slowest to fastest. Oh, and there's not much air outside either.

 

[Quickstatus]

maximum allowed mach

how far away is M (critical) when the 747 was at FL450?
When does M( critical) also equal the stall speed) ?

 

[FreqFlyer46]

They've been able to get up there, forever. It's just that's it's very rarely worthwhile. The basic rule of thumb that you apply for any climb toward the end of a flight is to ask whether you'll actually get back the fuel used to climb in the first place. So, you'll need that higher altitude to actually become the optimum (not just something you can reach) for at least an hour before you get to top of descent. The 747 could actually go to FL450, but I can only ever recall going there once, and we had zero passengers or cargo. It was just a positioning flight from Sydney to Cairns. And even then, it wasn't actually worthwhile, I just felt like doing it.

As you climb, you end up with a bunch of aerodynamic and performance factors closing in on each other. The aircraft you mention will cruise at about mach .85. That mach number will be more or less the same at all altitudes, as long as you're at the optimum altitude for the weight. Within the FMC the airlines will choose the performance factors that they want to use. For instance the maximum altitude may be defined as the highest the aircraft can reach at climb power, whilst still having the capability of climbing at 400 fpm. On top of that they'll add another requirement for the potential rate of climb, at cruise power, of around 100 fpm. This will be at a specific speed (mach number).

The aircraft itself hits you will a couple of limits. Your mach target might be .85, but the maximum allowed mach might only be .88. Looking in the other direction, slowing down, you'll find that you cannot go much slower without running out of power. The reason for that is that the target speed (that .85) isn't the minimum drag speed, but is slightly faster. This is chosen so that in the cruise, the aircraft is at a 'speed stable' position on the drag curve. If it goes very slightly faster, there'll be more drag, and it will naturally slow down. And if it goes slightly slower, there will also be less drag, and it will naturally speed up again. But, if you slow to a speed less than min drag, you'll now experience increasing drag as you slow, and will need more (and more) power to get back to your target. Ultimately, you won't have enough power to even arrest the speed decay, much less regain it, at which point the only solution is to descend. Add to this the fact that the engines produce less power at altitude.

The minimum drag speed tends to remain about the same as you climb. But, it's an IAS, which means that the minimum drag mach number is getting higher as you climb. Go high enough, and min drag and max speed will actually be the same number.

The simple upshot of all of this is that if you climb high enough, you can't go any faster, you can't go any slower, and you have less power to play with. Airlines build some level of margin into things by effectively chopping the top corner off the allowed climb performance. Nevertheless, very high altitude flight is a place to be treated with caution. The actual margins for those aircraft at FL430 are probably a window of about 20 knots, from slowest to fastest. Oh, and there's not much air outside either.

Thanks for this very detailed response jb747. With all the factors you mention and the squeezed margins at this kind of altitude, I guess I am surprised it makes sense and is safe to fly at these altitudes as often as airlines seem to these days (as per attached screenshot from Flightradar filtered at FL430). I am probably a passenger who asks too many questions.

 

[jb747]

how far away is M (critical) when the 747 was at FL450?
When does M( critical) also equal the stall speed) ?

There wasn’t a nominated Mcrit that I know of. But, the mach limit for the 747 was .92, and in the days before RVSM (which reduced the max to .90) I tried it out. Max continuous thrust slowly pushed the speed up, and it stabilised at about .915. I’m pretty certain that we already had some level of wave formation over the top of the hump.

Oh, and the reason we were going so fast…it was either that, or descent 12,000’. ATC in other words. You should have heard the bleating from Air France when he realised he’d been beaten for the altitude. Only other time I recall using .90, was going into Melbourne one night, when ATC wanted us to slow down for an A340 that was slightly in front. We couldn’t actually go as slow as he wanted, and the 340 refused to go any faster than .83. So we offered .9, and the 340 was told to slow to .75 or so. Winner.

The 1G stall speed is well below min drag. If you climbed at a speed just above the stall, you’d get nowhere near as high as you would if you climbed at the faster Min drag. Where do the two speeds meet? Above where we could reach. You’d have to be stupidly light to get up there, but that in turn reduces the stall speed. But at FL500 and mach .92 you’d be on the threshold of needing flap (which you CANNOT use at altitude because of the mach effects).

 

[Quickstatus]

I’m pretty certain that we already had some level of wave formation over the top of the hump

Did the stall speed indicator also rise?

 

[jb747]

Did the stall speed indicator also rise?

The lower part of the speed scale wouldn't have even been on display. Lots of red bricks at the top end though. We weren't all that high when we did this. FL370 perhaps, so the speed range would have been reasonably wide. We just happened to be at the top end of it.

 

[JohnM]

Pilots and ATCs: any further comments on this: