Osprey88 From United States of America, joined Jul 2007, 330 posts, RR: 1 Posted (7 years 10 months 3 weeks 6 days 22 hours ago) and read 26261 times:
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?
"Reading departure signs in some big airports reminds me of the places I've been"
Rwessel From United States of America, joined Jan 2007, 2740 posts, RR: 2
Reply 5, posted (7 years 10 months 3 weeks 6 days 22 hours ago) and read 26186 times:
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.
KAUSpilot From United States of America, joined Jan 2002, 1976 posts, RR: 30
Reply 7, posted (7 years 10 months 3 weeks 6 days 22 hours ago) and read 26190 times:
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.
Caribbean484 From Trinidad and Tobago, joined Jan 2007, 2710 posts, RR: 2
Reply 9, posted (7 years 10 months 3 weeks 6 days 22 hours ago) and read 26129 times:
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.
FlyMIA From United States of America, joined Jun 2001, 7583 posts, RR: 7
Reply 11, posted (7 years 10 months 3 weeks 6 days 20 hours ago) and read 25847 times:
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.
"It was just four of us on the flight deck, trying to do our job" (Captain Al Haynes)
RoseFlyer From United States of America, joined Feb 2004, 11166 posts, RR: 52
Reply 12, posted (7 years 10 months 3 weeks 6 days 19 hours ago) and read 25743 times:
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.
If you have never designed an airplane part before, let the real designers do the work!
VC10 From United Kingdom, joined Nov 2001, 1460 posts, RR: 15
Reply 13, posted (7 years 10 months 3 weeks 6 days 17 hours ago) and read 25495 times:
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
Par13del From Bahamas, joined Dec 2005, 8694 posts, RR: 8
Reply 14, posted (7 years 10 months 3 weeks 6 days 16 hours ago) and read 25263 times:
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.
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.
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.
Rscaife1682 From United States of America, joined Feb 2008, 339 posts, RR: 0
Reply 18, posted (7 years 10 months 3 weeks 6 days 15 hours ago) and read 25126 times:
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.
C680 From United States of America, joined Apr 2005, 592 posts, RR: 0
Reply 21, posted (7 years 10 months 3 weeks 6 days 12 hours ago) and read 23813 times:
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)
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.
Donniecs From United States of America, joined Jan 2007, 76 posts, RR: 0
Reply 22, posted (7 years 10 months 3 weeks 6 days 10 hours ago) and read 22616 times:
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.
MD-90 From United States of America, joined Jan 2000, 8530 posts, RR: 11
Reply 23, posted (7 years 10 months 3 weeks 6 days 9 hours ago) and read 21968 times:
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 From United States of America, joined Mar 2008, 27 posts, RR: 0
Reply 24, posted (7 years 10 months 3 weeks 6 days 8 hours ago) and read 20930 times:
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.
Out of the blue of the western skies...
: 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.
: 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 wind
: NASA unfortunately ended their flight testing with the blackbird in October 1999.
: 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 th
: I commented on NASA's SR-71, not the U2
: I was thinking the same thing. Too bad they are still not flying, Really nice bird!
: 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
: 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 wit
: 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 th
: 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 subjecte
: For continuous use (for example, while cruising in an unpressurized aircraft at 40,000ft), it's typically about 40mmHg. For emergency use (IOW to kee