Mountain or gorge flying and 400 ft. ceiling question

[Bazza.]

I have an old airplane altimeter installed in my 4x4 and the slightest adjustment on the barometer knob can alter the reading by hundreds of feet.

The adjustment is usually in millibars, 3.5 millibar being about 100 feet or so. You can pick up your local QNH to set the scale from any number of free apps.

Interesting set up, are you usually up a mountain?

 

[Clench M'cheeks]

Installed it for fun.. off roading and camping... mainly it just looks cool and impressed the girls. bought it used and had to sign a waiver that I would never install in an aircraft. Highest I ever drove it was around 11,000ft.. freaking awesome! Mt Evans in Colorado I think is drivable and is around 14,000... someday...

 

[Bazza.]

Installed it for fun.. off roading and camping... mainly it just looks cool and impressed the girls. bought it used and had to sign a waiver that I would never install in an aircraft. Highest I ever drove it was around 11,000ft.. freaking awesome! Mt Evans in Colorado I think is drivable and is around 14,000... someday...

Yeah that’s high. You’d get some good footage from a drone up there on a clear day. Highest point in the uk is only 4,409ft.

 

[Raptorman0909]

Old mechanical altimeters (aneroids) can be pretty accurate, say 100 feet, but they have a problem called stiction that makes the needle jump several hundred feet at a time as the elevation changes. This is due to the very small difference in pressure with a change of a couple hundred feet in elevation. This small change in pressure results in a small force to move the pointer so it hangs then jumps. The trick they tend to use is an eccentric weight spun by a motor that produces a vibration much like the vibrator in a cell phone and that breaks the stiction up so the needle moves smoother.

Fortunately modern altimeters are all electronic so there is very little stiction.

Any altimeter that relies on atmospheric pressure is going to have drift issues as the barometric pressure changes, but for drones with flight times of 25 minutes or less that's seldom much a problem -- usually less than 20 feet. With piloted planes, however, with fight times measured in hours the drift can be a lot, but pilots usually reset the baro setting before landing.


Brian

 

[Bazza.]

My original question was born out of my ignorance, and has been fully answered now, and I thank all involved. As a second question; I plan on flying in an area 60 miles or so north of the Gulf of Mexico, in Alabama. Even though I'm quite a ways from any military base, we routinely see military coptors, jets, and cargo planes flying what we believe to be low altitude training missions; I'm talking really low! The mature pine trees are probably 100 feet tall, and it seems these military craft, especially the coptors, are only 200 feet up or so, and appear very quickly out of nowhere. How best to stay out of their way under these conditions? Is there someone I can call when I'm going to fly? I seriously doubt they're going to give me details about their training missions. Any thoughts on this?

Did you ever get your answer?

Firstly know and check your air space for controlled zones.
Secondly there’s a service called NOTAMS which will inform you of any aviation activity outside of the control zones and if it’s a good idea to fly. In the UK low flying is flagged up under NOTAMS and the GA aircraft will avoid the area. But you will get information on what days and times it will be happening.