# Explained: Why a Spirit Level on a Plane Does Not Show Curvature "Corrections"

Discussion in 'Flat Earth' started by Mick West, May 22, 2017.

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1. ### Neil ObstatMember

I think it would be a great idea if we all could start bringing torpedo levels on our plane trips and prop them up on our laps during the flight. Imagine being a stewardess walking down the aisle seeing all those torpedo levels on passengers' thighs! Wouldn't that just make your day at work!

2. ### Kevin McMillenNew Member

Wouldn't my cup of coffee, Coke or Sprite be good enough to prove the plane isn't dropping 67ft per second as FEers claim?

3. ### Neil ObstatMember

I'm not sure. You never know what a flat-earther has up his/her/unID'd sleeve. They manage to come up with the most bizarre accusations!

But if you want to be really in the "loop" you ought to remember to bring your torpedo level to keep on your thigh, then you'll be sure to get plenty of attention.

4. ### Richard AguirreNew Member

Posting further support to this statement. The altitudes being flown in this experiment are in the Reduced Vertical Separation Minimim (or RVSM) airspace, which requires, according to 14 CFR Part 91, App 6 Section 21 the aircraft pressure altimeters maintain accuracy of +\- 65 feet in still air. Although the autopilot makes continuous tiny changes to maintain essentially 0 feet variation in Pressure Altitude, say for example the Altimeter tolerance was at its limit, and reached 65 feet error before correcting. If we also accept the 5 statute mile drop over 200 miles distance, the average angular change would be 1.43 degrees (-1tan a = 5/200). But a 65 foot change is one 406th of a 5mi drop (5x5280’ / 65’). So the aircraft would make 1.43/406=.0035 degree changes each time it corrected altitude (not realistically how an airplane flies but as a worst case approximation in favor of the experimenter we’ll use it). In that case, even the most sensitive spirit levels (https://www.leveldevelopments.com/sensitivity-explained/), ones with a tube radius of a 100m (most definitely not the one used) would register this .003 angular difference as a .06mm deflection of the ball. Clearly undetectable by the naked eye.

Last edited: Sep 24, 2019
5. ### TrailblazerModeratorStaff Member

I was flying back from holiday last weekend and had my Bluetooth GPS antenna connected to my phone so I could see where we were (it's a lot more accurate than the built-in GPS, especially since Apple ruined the GPS capabilities of iPhones, but I digress...). The GPS altitude - which of course is not the same as the pressure altitude, for a number of reasons - changed very little when we were in "level flight", maybe fluctuating by a metre or two (5ft or so) every few seconds. Certainly nothing as big as 65 feet.

Using GPS altitude has the complication that the aircraft is maintaining a constant pressure altitude, not a constant height above the surface (as Mick mentioned in post #77). So if for example you are flying from an area with low sea-level pressure to one with higher sea-level pressure, you will actually be slowly ascending in terms of GPS altitude, because the pressure at a given height (above sea level) will be higher. GPS altitude also isn't exactly the same as altitude above sea level, but that's not so important for this discussion.

I noticed this when flying from Iceland, where there was a deep low pressure system, towards the UK, which was still under high pressure. When the on-board screens indicated we were flying at 35,000 feet (for example, I can't remember the exact numbers), the GPS indicated we were at about 33,500 feet. After an hour or so, despite the indicated flight level being the same, the GPS showed we had ascended by few hundred feet. So while there were no big corrections in flight, the overall trend was a very gradual ascent above the ground. Of course, flying the other way a plane in level flight would be descending.

6. ### Richard AguirreNew Member

Your observation is correct. There is also the effect that long range flights often use a stepped altitude profile to take advantage of greater engine efficiency and true air speed gained at higher altitude. As the aircraft burns fuel it loses weight, allowing better performance. The pilots can then periodically climb slightly to a more favorable altitude, depending upon winds and other traffic. This can account for a few thousand feet of change over a long flight.

GPS as you may know is also not a perfect measure of altitude, as it still relies upon line-of-sight radio signal reception and requires correction by ground stations which may not always be in range over water. GPS altitude is also referenced to a different geodetic datum than charted MSL altitude so there are variations in comparison. (Nevertheless GPS it is still very, very accurate, accounting for such things as transmission lag and Einstein’s relativistic effects, as well as variations in Earth’s gravitational field to define the actual datum). Here is a great article on GPS altimetry for those who are interested. https://www.esri.com/news/arcuser/0703/geoid1of3.html

7. ### JohnPNew Member

Actually (1) I have no idea what you did there: where does the 8/(6x6) come from?
(2) I think this is all a red herring. The real reason the "change in altitude" is not perceptible is because it's not happening. Your argument would suggest that under the right circumstances, with the right equipment, it could be measured. I'd prefer to just say: level (horizontal) flight is perpendicular to the direction things fall.

8. ### Amber RobotMember

Don’t airplanes fly at a fixed air pressure usually? And surfaces of equal pressure will mostly lie on gravitational equipotentials, so by definition they are always in level flight, since “level” means parallel to gravitational equipotentials.

9. ### LandruModeratorStaff Member

https://en.m.wikipedia.org/wiki/Flight_level?wprov=sfla1

10. ### TrailblazerModeratorStaff Member

In the absence of weather systems they would be, but there's up to a ~100mb range between the highest and lowest sea-level pressures encountered in the atmosphere, which means that a given flight level will vary in actual altitude above the ground by a couple of thousand feet or more depending on where the aircraft is flying in relation to those weather systems.

You can think of the high and low pressure systems as like "hills and valleys" that the aircraft will follow as it maintains a given pressure altitude, just like a car driving over hills and valleys while staying on the ground.

Weather maps often show the "geopotential height", eg on this chart the main colours indicate the height of the 500 millibar pressure level above sea level:

The scale at the bottom shows the geopotential height in decametres (tens of metres).

So for instance if a plane was following a flight level that corresponded to a pressure of 500 millibars, then it would be around 5,900 metres above sea level in north Africa, but only about 5,200 metres above sea level off the coast of Norway.

Generally these geopotential heights roughly correspond to the sea-level weather systems, but not always and not exactly, as the temperature of the air also affects the height. So for instance you can see that the highest and lowest sea-level pressures (shown by the white isobars labelled with black numbers) approximate but don't exactly match the highest and lowest 500mb heights.

Last edited: Sep 25, 2019
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11. ### Richard AguirreNew Member

Technically
I think it’s true there is no appreciable altitude change, because the aircraft adjusts to maintain level flight as oriented to a plane perpendicular to the force of gravity, and the force of gravity is balanced by the lift the plane generates, precluding a continual climb (or drop). But the orientation of the aircraft relative to the original plane it departed from (perdencicular to the gravitational force) has to change or else it would be slowly flipping over backward relative to the new “level plane” as it moves forward over time. It requires stabilization via a pitching moment, which is theoretically measurable, but not by the crude methods employed in the original example.

12. ### JohnPNew Member

It would entail a comparison with the original orientation--so we're probably back to ring laser gyros.

13. ### Clouds GivemethewilliesActive Member

It is only the weight of the density profile of the air above, in turn determined by temperature profile above, that determines the pressure at any level according to the hydrostatic equation, although if you know the temperature profile you can integrate up or down from a known pressure and altitude.