Osprey88
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Cruising At 51,000 Feet?

Sat Jun 14, 2008 5:05 am

I often see bizjet manufacturers like Bombardier and Gulfstream advertise their bizjets as having a maximum altitude of 50000ft or above (most often 51,000 ft). My question is, aside from the initial testing of the aircraft where I assume the bring it up to 51000 ft, do private pilots ever fly that high during the normal operation of their aircraft? Is it more efficient at 50000ft? Is their ever any cause to for a pilot to fly his bizjet that high?
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star_world
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 5:10 am

It depends on the winds - in some cases it will be very favourable to fly at that altitude if you have a strong tailwind.

I did a quick search on Flightaware and the highest altitude I found for a bizjet was FL450 on a G5. I have seen flights operating at FL490 a few times in the past though.
 
N766UA
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 5:13 am



Quoting Osprey88 (Thread starter):
Is it more efficient at 50000ft?

Absolutely. The air is much less dense, and thus it takes far less work to punch through it. If they can get up there and the winds are favorable, they'd certainly be at an advantage.
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flynavy
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 5:13 am



Quoting Osprey88 (Thread starter):
Is their ever any cause to for a pilot to fly his bizjet that high?

Sure there is - like avoiding everyone else at lower flight levels.
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brons2
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 5:13 am

Seems like it would be less crowded above FL410 as most commercial jets do not go higher than that. I see that the 777 is certificated to FL430 but I doubt it goes there much.
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rwessel
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 5:25 am

Well it can be more efficient, especially if you can use the extra altitude to dodge, or use, the jet stream. It depends on the aircraft if the thinner air helps efficiency - since you're Mach limited at those altitudes, you may end up flying rather slower (indicated) than most efficient when you’re at those altitudes. You're able to top pretty much any weather. And there's no traffic (other than the odd U-2* and other bizjet) so you can pretty much always get any nice direct routing you want (which is probably the biggest advantage).


*OK, you'll occasionally see a fighter at those altitudes, but aside from the MiG-31 or F-22, it's not that common to operate there. And I think NASA is still flying an SR-71, so you might run into that. And, oh yes, the sailplane altitude record is 50,699ft, so watch out for gliders.  Smile
 
hamad
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 5:26 am



Quoting Brons2 (Reply 4):

in 1999 i was on a british airways 777 from DXB to LHR, and we cruised at 42,000
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KAUSpilot
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 5:26 am

Another advantage of higher altitudes is weather avoidance.

In the summertime thunderstorms with cloud tops into the mid 40,000's aren't uncommon. If you can climb up to 50+ and avoid them, all the better.

The length of the flight, winds, traffic, thunderstorms, aircraft weight, temperature, and tropopause height are all factors that should be considered when choosing a cruising altitude in a jet certified for altitudes in the 40,000+ range..

On flights over 1000 NM or so, in most jets it's usually most effecient to fly at the highest altitude your weight will allow, barring disproportionally detrimental headwnds. The main reason for this is that jet engines burn less fuel for a given mach number at higher altitudes. However, when you get into the 45,000 ft range you start penetrating the tropopause (temps start rising as you get higher) at the mid lattitudes, therefore you gain less effeciency by climbing higher. Most pilots probably don't climb much beyond the tropopause as there is not much effeciency to be gained at that point, even if the plane is certified for it.

Personally the jet I fly isn't certified beyond FL370, well below the tropopause most of the time so I don't have any practical experience wth this, just what I've read.

[Edited 2008-06-13 22:57:13]
 
ikramerica
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 5:27 am



Quoting Brons2 (Reply 4):
I see that the 777 is certificated to FL430 but I doubt it goes there much.

It gets close toward the end of the journey in some cases, but early on, the initial cruising altitude is lower.

I've been on a 777 at FL410 according to the flight tracker channel, but usually they finish up at FL370 to FL390.

The 744 can cruise as high as FL451, but normally won't cruise above FL420, and it can't get there until late in the flight.
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caribbean484
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 5:37 am

Quoting HAMAD (Reply 6):
in 1999 i was on a british airways 777 from DXB to LHR, and we cruised at 42,000

During the summer that year I was travelling on a BWIA 737-700 from SKB-JFK at 41000ft the maximum service ceiling of the 737NG it was a very beautiful day that time, one of the smoothest flights I have ever been on.

[Edited 2008-06-13 22:40:26]
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SOBHI51
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 7:01 am

The Concorde used to fly at over 50,000 feet.
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flymia
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 7:08 am

The only Private jet I have been on was a Citation Sovereign. I was flying from OPF-HPN we cruised at 47,000ft for a three hour flight. The pilot explained to me that we do fly slower up there but its much more efficient and looking out the window you could see all the airliners below us as we had almost no traffic at all near out flight level. The flight back was at FL450.
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roseflyer
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 7:54 am



Quoting N766UA (Reply 3):
Absolutely. The air is much less dense, and thus it takes far less work to punch through it. If they can get up there and the winds are favorable, they'd certainly be at an advantage.

That's not entirely true. There isi a most efficient altitude for each stage of a flight depending on weight of the airplane. Just because a plane can climb higher, does not mean that it's more efficient. Your engines lose performance as you climb and can be operating less efficiently to make up for the lower density and drag as you imply.
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vc10
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 10:12 am

At 50,000 ft and above the winds generally become quite light about 20 kts that is all. One advantage therefore is if your aircraft is capable of doing it, is to climb to these altitudes to avoid a large headwind at the more normal cruise altitudes

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par13del
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 11:38 am



Quoting RoseFlyer (Reply 12):
Your engines lose performance as you climb and can be operating less efficiently to make up for the lower density and drag as you imply.

One of the more important aspects of altitude flying, the thin air has an effect on engine performance. Engines are designed to operate efficiently at certain altitudes, high altitude flying is a combination of a/c design wings and engines. A U2 wing and engine is optimized for 50,000+ altitudes, its a jet a/c which will also operate at 20,000, but not optimal. One problem commercial a/c other than the Concorde had was the location of engines on the frame to aid in airflow into the engine when the air density starts to fall, engines hung on the wing don't get much help from the frame in aiding air flow.
 
iRISH251
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 11:45 am

There is a radiation issue also and if crews spend extended periods at very high altitude they can rack up an excessive amount of such exposure. More detail here:

http://www.iop.org/EJ/abstract/0952-4746/21/1/003
 
skymiler
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 11:48 am



Quoting SOBHI51 (Reply 10):
The Concorde used to fly at over 50,000 feet

The normal flight profile took it over this altitude regularly, sometimes up to 56,000. I flew on it once and can still remember the deepening colour of the sky, and could discern the curvature of the earth. At these altitudes there was a concern for solar radiation (and perhaps other unkown phenomena) that could affect those on board.
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David L
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 12:10 pm



Quoting SOBHI51 (Reply 10):
The Concorde used to fly at over 50,000 feet.



Quoting Skymiler (Reply 16):
The normal flight profile took it over this altitude regularly, sometimes up to 56,000.

I think 58,000 ft was common on the JFK route and 60,000 ft closer to the equator, e.g. Barbados. I managed four trips between LHR anf JFK, two in each direction, and we reached at 58,000, Mach 2.00, each time. Of course, it used a cruise-climb so it would drift very slowly up to those maximum levels rather than step-climbing and staying there.

Quoting Skymiler (Reply 16):
At these altitudes there was a concern for solar radiation (and perhaps other unkown phenomena) that could affect those on board.

Yes, but it turned out that, compared to a similar subsonic flight, there was approximately double the dose for approximately half the time - call it a draw.  Smile
 
Ryanair737
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 12:17 pm

I have been at FL430 on the 744 between JNB-CPT in 2002 before they flew direct to CPT. We were very lightly loaded though.
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rscaife1682
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 12:18 pm

Most of the tails we deal with fly between 450-490 I have never filed a tail at 510 even on ling flights it is normally a step climb from the high 300 to the high 400. The altitude helps due to less turbulance, traffic and weather.


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AEROFAN
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 1:41 pm

When I flew Concorde LHR/JFK in 1997 we cruised at 53,000
 
c680
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 3:33 pm

I think everyone hit the key points:

- Less weather / bumps
- *MUCH* lower fuel burn for a very small speed penalty
- Less air traffic (more direct routing)

...but...

Just like the airliners, most planes that are certified to FL510 can not climb to those heights at gross weight. They have to step climb as they burn.

The climb performance up there isn't very good either.

I fly a Citation Sovereign, which is certified up to FL470. It is a great climbing airplane (one of my crewmates who also flies a Gulfstream III once said that the Sovereign climes like a "raped ape") at mid weights, we have to set initial climb to over 6000 fpm to avoid overspeeding. It seems to "give up" at about FL350 (goes from about 2000 fpm down to about 500 fpm by FL410) so the last few thousand feet take FOREVER. Unless there is a compelling reason (weather, winds, or fuel) we like to fly at about FL410.
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donniecs
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 5:10 pm

We've climbed to FL490 in once in a G550 (wx avoidance, could have gone around but we choose to go up) and have hit FL470 on occasion in the GV's and G550s. It's not uncommon to see Gulfstreams in the FL430-FL470 but not usually any higher. I doubt that a Gulfstream in regular operation has ever seen FL510, its just not in their normal flight profile (FL510 is for advertising, just like the G650 max speed is faster than a Citation V).

You have to be ultra light to get that high with well less than 10,000 lbs of fuel on board and if your ultra light your just not going far enough to make it practical to climb that high. If you are are on a long flight the first few hours you'll be lucky to make FL450 (with a full load of fuel) and by the time your light enough (if you get to that point) your ready to descend and it would be pointless. I've never seen a GV or G550 perf out with a max altitude of more than FL490 but normally FL450 through FL470 is seen.

If you can get that high practically there are numerous benefits.
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MD-90
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 6:08 pm



Quoting C680 (Reply 21):
I fly a Citation Sovereign, which is certified up to FL470. It is a great climbing airplane (one of my crewmates who also flies a Gulfstream III once said that the Sovereign climes like a "raped ape") at mid weights, we have to set initial climb to over 6000 fpm to avoid overspeeding.

From the pictures it looks like it's got a lot of wing area for its size.
 
FlyingCrown
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 7:43 pm

The lamented Boeing Sonic Cruiser was intended to fly in the mid 40's for efficiency. Cessna's 600 MPH Citation X, many Learjets and a number of Gulfstreams are certified to 50k. Up there, winds are generally light, but aircraft performance can suffer: turbines are air breathing engines, so power decreases with altitude. Most turbofans deliver less than 25% of their rated power at cruise altitude, but air drag drops off so much that they only need a fraction of the power, so they get their best fuel economy up high. But as power drops off, aircraft fly slower, and with stall speeds increasing with altitude, the spread between stall and maximum mach can be very small. That was the case with early Learjets. The closer to stall, the higher your wing angle to your flight path, and that increases drag, so sometimes a lower altitude is more fuel efficient.

Very high altitude is a strange environment: Critical mach number doesn't decrease; in fact it can increase because air temperatures stabilize at the tropopause and actually increase with altitude after that. The speed of sound is based on temperature, not pressure, so it can increase as well.

The US FAA requires stringent structural fail-safes because the environment at FL510 is very hostile. You literally cannot put on an oxygen mask fast enough to keep from passing out, and even if you could, the oxygen partial pressure at that altitude is barely enough to keep you alive; O2 partial pressure measures a blood cell's ability to capture O2 from the air; they get lazier with lower air density. Aircraft have to be built like spacecraft to be safe at those altitudes, that's why one company is using a modified Learjet as the vehicle for a space-tourism company.
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flylku
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 7:53 pm



Quoting Brons2 (Reply 4):
I see that the 777 is certificated to FL430 but I doubt it goes there much.

Our final on a BA 777-200ER from LHR to KKR (Calcutta India) was 410. I think my dad said he was on a 747SP once when the final was 430 or 440.
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pjflysfast
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 9:15 pm

A Global 5000 crew I know was doing a flight from Tahiti to Ft. Myers Florida and by the time they got over Mexico they were light enough and the winds were in their favor and they were able to cruise the rest of the way at 51000ft! They have done it a few times since but they most always file for 47000 and 48000 because it is a whole lot smoother up there and they do not have to worry about weather and other traffic for the most part.
 
fritzi
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 9:55 pm



Quoting Rwessel (Reply 5):
And I think NASA is still flying an SR-71, so you might run into that

NASA unfortunately ended their flight testing with the blackbird in October 1999.
 
Bellerophon
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RE: Cruising At 51,000 Feet?

Sat Jun 14, 2008 11:40 pm

FlyingCrown

...You literally cannot put on an oxygen mask fast enough to keep from passing out...

I think you'll find that it was possible, as both the CAA and FAA accepted. When I flew at those altitudes we were tested on exactly that ability every year.


...and even if you could, the oxygen partial pressure at that altitude is barely enough to keep you alive...

The O2 partial pressure, at ambient pressure at 60,000 ft, is certainly inadequate to support life, which is why a special pressure-breathing mask system was necessary, unlike the demand-breathing mask system found on most other jet transport aircraft, such as the one I now fly.

Being force-fed 02 from a pressure mask meant that the 02 partial pressure in your lungs was adequate to support a pilot whilst an emergency descent was carried out.

The limiting factor in determining the amount of pressure delivered by these pressure breathing masks, in civilian use, was the fact that civil pilots do not normally wear jerkins (which would constrain the chest against over-expansion) and thus the pressure delivered to the mask had to be carefully calculated and controlled, lest someone’s lungs and/or chest be over-inflated.

For me, the hardest part of the annual proficiency test was not donning the mask in the required time, rather it was meeting the requirement to be able to talk whilst using such a mask, a lot harder than it might sound!

As soon as you opened your throat to speak, the pressure mask would pump 02 the wrong way across your vocal cords. The only way to speak was to take a deep breath first and then force the air out of your lungs, overpowering the inflowing 02, to utter a few words at a time.

Quite tiring, and a skill that had to be learned!


Best Regards

Bellerophon
 
fritzi
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RE: Cruising At 51,000 Feet?

Sun Jun 15, 2008 2:17 am



Quoting PJFlysFast (Reply 28):
Yea but U-2's!

I commented on NASA's SR-71, not the U2
 
YWG747
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RE: Cruising At 51,000 Feet?

Sun Jun 15, 2008 4:55 am



Quoting Fritzi (Reply 27):

Quoting Rwessel (Reply 5):
And I think NASA is still flying an SR-71, so you might run into that

NASA unfortunately ended their flight testing with the blackbird in October 1999.

I was thinking the same thing. Too bad they are still not flying, Really nice bird!
 
njxc500
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RE: Cruising At 51,000 Feet?

Mon Jun 16, 2008 11:50 pm

I know of an operator of a Bombardier global and they only go to fl510 about once a year. fl430 and 450 are very common though. I'm sure the aircraft is capable, but in the case of an executive flight crew, they carry extra fuel pretty much everywhere they go, and they never push the range of the aircraft to its limit.

I did fly on a 744 Qantas that stepped to 45,000 from LA to Sydney. Must have been a light load that night. It doesn't seem like that would be very common.

Following the original question, do operators of these high flying birds change their mach number to maintain a given indicated airspeed?

For instance say a jet is at fl450 and needs 470, might they raise their mach number so they would maintain the same indicated airspeed, to stay ahead of the curve so to speak?

Nick
 
DashTrash
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RE: Cruising At 51,000 Feet?

Tue Jun 17, 2008 1:58 am



Quoting Njxc500 (Reply 31):
Following the original question, do operators of these high flying birds change their mach number to maintain a given indicated airspeed?

For instance say a jet is at fl450 and needs 470, might they raise their mach number so they would maintain the same indicated airspeed, to stay ahead of the curve so to speak?

Indicated airspeed pretty much gets thrown out the window up high. It's all about mach number, unless you're talking about lower speeds. The idea with lower speeds is to avoid them...
 
rwessel
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RE: Cruising At 51,000 Feet?

Tue Jun 17, 2008 6:10 am



Quoting DashTrash (Reply 32):
Indicated airspeed pretty much gets thrown out the window up high. It's all about mach number, unless you're talking about lower speeds. The idea with lower speeds is to avoid them...

That's not true - many aircraft run into the situation where the indicated airspeed at their mach limit decreases to near the stall because of the thinning atmosphere. Often called the coffin corner - you're simultaneously flying too slow (IAS near stall) and too fast (Mach number near the upper limit).

Get too far into the corner and recovery from any kind of upset can be quite challenging - you can't pull the nose up to slow down or you'll stall, and you can't push the nose down to increase speed without exceeding the Mach limits, and losing control that way.
 
kris
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RE: Cruising At 51,000 Feet?

Tue Jun 17, 2008 8:23 am

Bellerophon

Do you know the pressures you were subjected to through those masks? In my youth (as a mad scientist training with the RAF) I was subjected to between 50-70mmHg with counterpressure garments for experiments on my blood pressure... apart from my face looking like a ripe tomato (I have photos!) the most interesting sensation had to be the air blowing into my eyes through my tear ducts...!

[Edited 2008-06-17 01:24:22]
 
rwessel
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RE: Cruising At 51,000 Feet?

Wed Jun 18, 2008 5:37 pm



Quoting Kris (Reply 34):
Do you know the pressures you were subjected to through those masks?

For continuous use (for example, while cruising in an unpressurized aircraft at 40,000ft), it's typically about 40mmHg. For emergency use (IOW to keep you alive while you descend to a lower altitude after a pressurization loss), systems deliver up to 70mmHg.

Breathing with a pressure demand mask definitely takes a bit of getting used to.

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