Flight Level 390

Skipper,

I am interested in reading your explanation about the gps/altimeter discrepancy. So, if you write it, I will read it...

Greetings from Monterrey, México

Wilson
 

What a beautiful picture. You are so lucky to have a job where such sights are commonplace Dave - and I thank you for sharing them with us.

Happy flying.

Dave (EGAD)
 

If you're up in the flight levels, your altimeter is set to 29.92, not the local barometric pressure. Why would you expect the GPS and your cockpit altimeter to read the same thing? I suppose 1000 feet is quite a bit, though, unless there was one hell of a system going.
 

jonathan b.- I don't really expect the GPS and altimeter to match; I am, however, amazed that there are satellites in geo-sync orbit that are saying, "Hey, Captain Dave, do you want to know how high you really are...?" Modern avionics are truly miraculous. I started flying when I was a teenager and I remember the first DME; it was a crude mechanical device with sliding metal scales (10-50-100-200 mile units) and a pointer that looked like Captain Ahab's compass. It was the wonder of all wonders! The aviation writers of the time were fawning over this new invention and the old time flight instructors were horrified. It was the end of an era... What innocent times. I am very lucky that I have lived to see such an aircraft as the A320/319.
 

That almost looks like it has solid form. Having grown up in Kansas, I'm familiar with being on the underside of those. Incredible photo.

Sort of an off topic question, but following Jonathan B's lead, I've always wondered this. How high can an aircraft like yours go? A commercial one, that is. Is it possible to go too high? What would happen if you just kept climbing altitude?
 

Anne,

Specific aircraft engines are only designed to operate up to a certain altitude. As you can see by the Pinnacle CRJ crash of 2004 (two pilots ferrying an empty plane decide to take it for a 'joyride' up to it's max alt.), you still sometimes need to be careful when operating at the aircraft's max cruising altitude

An excerpt from the report: "The cockpit voice recorder (CVR) recording began about 2144:44 with
the captain and the first officer discussing the climb to 41,000 feet. About 2148:44, the first
officer stated, "man we can do it. Forty one it." About 2151:51, the first officer stated, "there's
four one oh my man." About 2152:04, the CVR recorded the first officer laughing as he stated, "this
is great.""....
 

So, if the Altimeter and GPS disagree which is the correct one?
 

The GPS is more accurate, but pilots use the (air pressure based) altimeter for navigation. Air pressure is relevant for flight performance, actual hight above ground has relevance to avoid obstacles (FL 390 is "high enough"), GPS altitude above the reference ellipsoid (sea level) is just a nice statistic.
 

Anne- a contemporary airliner will go as high as the wing design and engine thrust will allow. For example, my aircraft (A320 or 319)max allowed altitude is 39,000 feet. That is an 8000 foot cabin altitude which is considered safe for most passengers. However, if you put on an oxygen mask and you were light enough, you could go higher; maybe 43,000 feet or so... A few business jets are rated to 50,000 feet. To answer your question, if you kept on climbing you would get to the point where stall speed and max Mach merge, affectionately known as coffin corner, i.e., if you tried to slow down you would stall and if you tried to increase speed, you would overspeed.
 

Say hello to Boston for me I will be there next week - I hope its warmer than it was 4 weeks ago!

I don't think on all my transatlantic flights I have ever seen clouds like that at that height. But then what do I know as a passenger I usually either have my head in a book, my laptop or I am asleep!
 

I'd definitely be interested in hearing the explanation for the GPS versus barometric discrepancy. I can think of several reasons, but who am I? :-) And that whole "reference ellipsoid" thing has me intrigued, too.
 

Dave, you are an amazing writer and your pictures are just superb. I love reading your blog and the way you paint out the wonderful scenes of your life :)
 

Hey Dave, Caig at MCO again.. just got to say what a wonderful picture. another for the computer background... So this is interesting as a dispatcher we are always looking to see where the "tropapause" hight is, so we can predict the severity of thunderstorms.. for those reading, when you see a storm with a Flat top.. that means the storm cell has risen to the tropapause and has reached its maximum height before the storm is Puched back to earth, possibly in the form of hail or high winds.. Anyway Dave, In this picture, the trop is not evident here at Fl390.. I would have expected it to be lower? So obviously the jet stream core below you is moving these clouds at 115kts, has it disrupted the trop so that these clouds are still able to continue to rise?

And on the Barometric altitude verses the GPS.. as Dave said, you might want to be spared the dirty details, but I'll take a shot at the simple version.. In order to keep airplanes from running into each other there must be a common altimeter setting used by everyone that is flying above 18,000 feet, this is 29.92. This is actually set and adjusted on the altimeter. So now when a west bound flight is at 37000 feet and an eastbound is at 39,000 the distance is a true 2,000 foot seperation.. With GPS, the system knows the actual measured distance from the earths surface to where you are just like taking a tape measure.. hope that helps..
 

In addition to the altimeter vs. GPS discussion. GPS finds out about position and height by doing trigonometrical calculations which are accurate as long as you can rely on the satellite positions. (If you cant rely on that, there's something really bad happening...)

The altimeter has to be set to the air pressure at zero feet above sea level. This varies depending on the weather.

As Caig already explained, above a height of 18,000 feet all pilots use a setting of 29.92 Hg (inches of Mercury).

The height is then NOT measured exactly, but all planes use the same slightly incorrect measurement so the distance between flight levels is high enough to travel savely.
 

I know those are the tops of TCUs in cirrostratus cloud, but it doesn't actually look that different from embedded Cu in regular stratus, over thirty thousand fee below. I think I have a similar photo in my collection, with a propeller in the picture.

Unless Dave's GPS is in some kind of fancy agl mode, remember that it isn't giving tape-measure distance above the ground, but the distance above mean sea level on a theoretical elipsoid earth. The barometric altimeter, meanwhile is telling him the distance he would be above MSL were the atmospheric pressure outside equivalent to that in a theoretical "standard atmosphere."

If his procedures are like mine, the GPS altitude is merely an amusing number to look at, and is not used for any purpose other than a possible back up to the two independent altimeters. Yay pitot heat.
 

I've been trying to resist adding this rather ordinary comment to the very interesting technical comments above (I enjoy pondering the marvels of the maths and physics) - but with some chocolate sprinkled on top of those clouds they look like they'd make a superb cappuccino!
 

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I've ALWAYS wondered what would happen if I took my PDA-like GPS onboard an aircraft. I've learned to trust its readings not for absolute height, but it may be good for measuring relative changes in altitude over a short time (relative to the bottom of a mountain climb). But it IS interesting, that the GPS channel on airliners is actually the barometric altimeter reading, giving you a perfect number which is the autopilot's programmed altitude (35,000 feet, 39,000 feet). I wouldn't have known!!