I’ve another one.
Am I correct in believing that the altimeter is calibrated to the altitude of the bottom of the wheels? That is the altitude shown should match the runway threshold elevation when stopped at the threshold?
Obviously for the aircraft most of us for the difference between the bottom of the wheels and panel height is trivial but for some aircraft it could be 1 MB.
If that is true then that would lead to the odd situation where I could legally be flying 1 ft below controlled airspace but, with the undercarriage retracted, 100% of the aircraft is actually inside controlled airspace!
So my question is which part of the aircraft is the altimeter calibrated to?
Obviously for the aircraft most of us for the difference between the bottom of the wheels and panel height is trivial but for some aircraft it could be 1 MB
It’s the reason why on heavy stuff you need positive VSI > 0 to retract the gear, the cockpit can rotate higher than ground and climb 30ft while wheels and static ports remains on tarmac…
Obviously, some still say “rotate, positive rate of climb, gear up” in Arrow, although there is no Vr speed and no VSI > 0fpm in these aircraft
Yes when you set QNH yourself to read AD ELV, that assumes wheels are on threshold by design even if the altimeter is feed from static
When you get QNH from ATC, I have no idea if it needs some tower/wheel offset? but it won’t vary that much?
The aircraft where this matters tend to have radar altimeters and they use the latter when DH<200ft with all adjustments from wheels and pilot eye heights
dublinpilot wrote:
Am I correct in believing that the altimeter is calibrated to the altitude of the bottom of the wheels? That is the altitude shown should match the runway threshold elevation when stopped at the threshold? ….. So my question is which part of the aircraft is the altimeter calibrated to?
The altimeter is not calibrated to any position on the aircraft. It simply shows the altitude corresponding to the pressure at its static port.
E.g. AMC1 to NCO.OP.101(a), PRE-FLIGHT ALTIMETER CHECK: “A serviceable altimeter indicates the elevation of the point selected, plus the height of the altimeter above this point, within a tolerance of ± 60 ft.” (My emphasis.)
Thank you!
If we’re going to be drilling this deep in to the essence of the thread, then try this on for size;
As you are casually flying along, you notice a fly buzzing around in your cabin. Does the mass of the fly add to your auw?
As you are casually flying along, you notice a fly buzzing around in your cabin. Does the mass of the fly add to your auw?
Of course it does add to your AUW 
I have not seen a fly that make 150kts, so it has to be “carried” by aircraft during takeoff and acceleration…if I fly Balloons at 1kts drifting with the winds, I may buy that the fly managed to join my flight without adding to AUW
Balloon or airplane or hell or airship….if the fly is flying in a fully enclosed cabin, in a static condition it does not matter whether it is airborne or seat-borne : the aircraft will be carrying its weight.
However what about a dynamic condition ? (Ieif it jumps up from the seat withou flapping its wings ) ?
The centre of gravity of the aircraft will change as it jumps.
whether it is airborne or seat-borne : the aircraft will be carrying its weight.
Yes, but why?
Because flying objects fly relative to their immediate frame of reference, which is the air.
That parcel of air is in turn carried by the aircraft internally.
