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Home > Flying the Plane > Does a jet aircraft need to constantly adjust nose down to follow the curvature of the earth?
Does a jet aircraft need to constantly adjust nose down to follow the curvature of the earth?
Flying - Flying the Plane
Wednesday, 11 February 2015 16:51
 
Planet Earth seen from Space
 
Dear Capt Lim;

I have limited flying experience (two introductory flight lessons in a Cessna and several commercial flights, including some Transatlantic trips), so please excuse my ignorance.

I was wondering; since the earth is spherical and the curvature is approximately 6 foot "drop" for every three miles i.e. a 6 foot person will disappear over the horizon 3 miles away, does a jet aircraft need to be constantly adjusted nose down to adjust for the curvature of the earth?

In other words, if a plane was trimmed for straight and level flight, would it "gain altitude" while flying as the earth surface "fell away" due to the curvature?

Thanks for your time and consideration.

Johnson

Hi Johnson,

This is an interesting question. A plane will fly at a constant altitude and will follow the curvature of the earth and would not gain altitude during a level flight.

For instance, if a plane is cleared to maintain 35,000 feet, by regulations, the pilot must maintain that level based on a standard barometric pressure setting (29.92 inHg or 1013 millibars)   Hence it would stay at that altitude (FL350) because the pilot is either controlling the plane manually or has engaged the autopilot to achieve that.

There are two basic instruments that enable this procedure - an altimeter and a vertical speed indicator (VSI). The VSI provides short term changes in pressure and indicates whether the plane is climbing or descending. These changes will give an indication to the pilot so that he would level the plane to maintain 35,000 feet. He will adjust the controls very slightly by use of the elevator and trims. This can be performed automatically by the autopilot as well. As such, the flight controls are constantly moving very subtly to maintain the correct attitude.

You said that, if the plane was trimmed for a straight and level flight, it would ‘gain altitude’ while flying as the earth surface ‘fell away’ due to the curvature of the earth. Well, that would probably happen in a perfectly motionless atmosphere where the plane would fly dead ahead, and over time gain altitude (provided it has sufficient thrust) as the earth curves away from under the airplane.

In reality, a constant altitude must be kept using the standard pressure and that means a fixed distance to the earth center of gravity is maintained, making the path of the plane a curved one.

So, a plane is not flying a straight line - geometrically speaking!


Flying over Planet Earth

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MR
WHAT A BRILLIANT QUESTION AND AN EQUALLY BRILLIANT ANSWER!!!

KEEP IT UP!!!

SONY
SONY THOMAS , 11 Feb, 2015
Level Flight
In fact, there is no real "level flight".
Every flight path is a curve line.
What we should consider a "level flight" is the one when the atmospheric pressure remains constant keeping the plane at the same height all the time.
Carlos Roberto Abreu Moreira , 13 Feb, 2015
wrong assumptions , Low-rated comment [Show]
I'm afraid I'm still a little confused!
Dear Captain,

I'm afraid I'm still a little confused.

The earth is 25,000 miles in circumference curving 8inches per mile squared. If a pilot wanted to maintain altitude at a typical cruising speed of 500 miles per hour you'd have to constantly descend 2777 ft (over half a mile) every minute otherwise you'd be way higher than expected. If as you say this adjustment is performed automatically by the autopilot surely as a passenger you'd be aware of the constant descent? 2777 ft every minute is a considerable drop and would not in my opinion go unnoticed.

Thanks for your time and consideration.

Dharmabro


Dharmabro , 22 May, 2015
I'm afraid I'm still a little confused.
Dear Captain,

I'm afraid I'm still a little confused.

The earth is 25,000 miles in circumference curving 8inches per mile squared. If a pilot wanted to maintain altitude at a typical cruising speed of 500 miles per hour you'd have to constantly descend 2777 ft (over half a mile) every minute otherwise you'd be way higher than expected. If as you say this adjustment is performed automatically by the autopilot surely as a passenger you'd be aware of the constant descent? 2777 ft every minute is a considerable drop and would not in my opinion go unnoticed.

Thanks for your time and consideration.

Dharmabro
Dharmabro , 22 May, 2015
Mr
But wouldn't the passengers feel the constant "dip down" of the plane as the pilot constantly makes adjustments? The speed a plane travels would, I imagine, mean the plane would be gaining hundreds of feet in altitude every few seconds.
cari , 02 Sep, 2015
Level Flight
Or the alternative, the earth is flat.
M1k3 , 20 Sep, 2015
curvature is wrong , Low-rated comment [Show]
follow up
any word from the captain?
jake , 18 Nov, 2015
re: curvature is wrong
no its not. its 8 inches per mile squared. if it was just 8" per mile it would be a long downward plain. the formula for the curve for 1 miles is 1x1x8=8 inches. for 2 miles its 2x2x8=32 inches, for 3 miles it is 3x3x8=72 inches. it gets real big real fast. for 10 miles it would be 10x10x8=800 inches or 66.66 ft.
flying from seattle, wa to new york city is 2857.6 miles by air. that is 1031 miles of curvature the plane would have to encounter in a 6 hour flight. that means the plane would have to descend i have talked to several arline pilots. they said once the plane gets to crusing altitude they make no corrections, as if they were flying on a flat plain. they do not make constant downward corrections in altitude at least not of that severity
ben richardson , 25 Nov, 2015
LEVEL FLIGHT
An aircraft flies at a constant pressure altitude and thus maintains the same relative distance from sea level at all times. If it were to climb as it flew along it would be registering a decreasing pressure which it does not.
Joe , 27 Nov, 2015
Mr.
Okay, Similar question.... How does an airplane's Gyroscopic Artificial Horizon account for the Earth's curvature? As I understand it - a gyroscope keeps absolute position and the aircraft rotates around it, allowing the instrument to show the plane's attitude on the artificial horizon... But, logic would dictate that if I flew around the world, the artificial horizon should show me upside down half the time... What does it do to account for this, and prevent erroneous readings?

Steve In Indiana
Steve , 01 Dec, 2015
Mr
Okay, Similar question.... How does an airplane's Gyroscopic Artificial Horizon account for the Earth's curvature? As I understand it - a gyroscope keeps absolute position and the aircraft rotates around it, allowing the instrument to show the plane's attitude on the artificial horizon... But, logic would dictate that if I flew around the world, the artificial horizon should show me upside down half the time... What does it do to account for this, and prevent erroneous readings?

Steve In Indiana
Steve , 01 Dec, 2015
Curvature calculation
Curvature is approximately 8" per squared mile so in 3 miles it will drop 6 ft.
Joe BLow , 01 Dec, 2015
well , Low-rated comment [Show]
Correct Curvature
The curvature is not 8 inches times the distance since you cannot form a sphere that way. Only the first mile is 8 inches then it grows exponentially. The correct math is 8 inches times the distance squared. So three miles would be 3 x 3 x 8 = 72 inches. At 500 miles the earth will fall away 166,495 feet. If the plane is traveling 500 mph, the earth is falling away at a rate of 2,775 feet per minute. If autopilot is off, the pilot must reduce altitude by 2,775 feet per minute or 46 feet per second to maintain 35,000 feet. Isn't that much greater then descent rates when landing?


Thanks for your time and consideration.
CS , 28 Dec, 2015
...
500 miles an hour divided by 60 mins per hour=8.33 miles a min multiped by 8 inches a mile =66.67 inches a min = 5 and half feet per minutes downward descent needed
Professor , 12 Jan, 2016
Math, how does it work?
Say the plane is traveling @ 600mph. That means they are traveling 10 miles per minute. With a curvature of 2' per 3 miles, the plane would need to descend a little over 6' per minute to maintain altitude. I doubt passengers are going to feel that.
Dunno , 16 Jan, 2016
curvature is right.
Curvature equation under 1,000 miles is: miles squared times 8 divided by 12 for answers in feet.
For a distance of three miles the curve results in a ~6 foot drop. 3X3X8/12=6 feet. Did you follow lakawak? A linear drop in 8 inches per mile is the slope angle not a curve. A curve is inversely proportional to the distance.
Johnson , 19 Jan, 2016
Curvature
Seeing as we're on this topic, I will try and help with the basic arithmetic part. I realize it's been a while since anyone's responded to this thread, but I figured I'd give something for people who keep trying to 'prove' (poorly) that the earth is flat

When it comes to the curvature, if the estimates that you have to account for approximately 8 inches for every mile...

Traveling at 500mph, that's approximately 8.3 miles per minute. Which means that at a curvature of 8 inches per mile equates to a change of 66.7 inches every minute. VERY different than the claim that you'd have to move 2777 feet to account for curvature every minute.
Jesse , 21 Jan, 2016
Level Flight , Low-rated comment [Show]
...
if the plane is adjusting its height to the curvature of the planet why are persistent contrails always perfectly horizontal?
prutt , 12 Feb, 2016
Two words
Newtonian gravity - a cannon fired will not continue in a linear direction due to the constant pull of the Earth's gravity. To break from the Earth's gravity, an aeroplane must achieve higher speed, which is not possible if the aircraft is on autopilot.
Daniel , 21 Feb, 2016
Ship Captain ?????
Does a gyroscope on a normal global commercial flight adjust it's attitude to the curving suface of the Earth (Gravity) while underway or does it remain in the same (horizontal) attitude as takeoff and then have to be readjusted upon landing

This e-mail address is being protected from spambots. You need JavaScript enabled to view it
Anthony B , 28 Feb, 2016
Contrails are not horizontal , Low-rated comment [Show]
Also
Everything is relative.
What do you have as a point of reference when on a plane?
How would you possibly be able to perceive such a gradual descent? And you aren't even descending relative the ground.
It's not as if you have a perfectly straight line drawn in the sky outside the plane window to judge your exact flight path.

The altitude is judged by the planes distance from the ground, so as the earth drops away enough thrust is applied to maintain a set distance from the ground until you want to get closer to the ground to land at which point the thrust is decreased.

Dan , 19 Mar, 2016

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