Messages

1

General Discussion / Re: Mode-S Altitude

« on: November 20, 2015, 08:03:52 am »

I think I've probably got a duff altimeter even though the altimeter reads correctly on the ground.

I'll set QNH on the altimeter; note the difference between QNH alt and average GPS height, climb to 3000' or so in the same locality and measure the difference between QNH alt and average GPS height again.  I know the limitations of using GPS height but it is not too bad provided the GPS antenna has a good view of the sky. (The GEOID undulation is 155' at EGHH and I'm at Newton Peveril)

All Shadow airspeed indicators over-read by about 15% and this is covered by the pilots' manual, mine is not so different in that respect.

P.S. If anyone is interested in checking the GEOID undulation in their locality then consult the AIS web pages for Aerodrome Index - Specific, open the textual data for their chosen airfield and search for "Geoid Undulation" (without quotes), which should be in the "RUNWAY PHYSICAL CHARACTERISTICS" section.

2

General Discussion / Re: Mode-S Altitude

« on: November 15, 2015, 10:25:59 am »

Ade,

<snip>
I could envisage  a situation where the reverse could occur and the cockpit be at a lower pressure if a Venturi effect is created by airflow passing over the rear of an ill fitting canopy etc.
<snip

Moffrestorer, you are correct and I just came to the same conclusion this morning. I think my cabin pressure is less than ambient (static) pressure because my IAS is overreading   i.e. difference between dynamic and cockpit pressure is greater than dynamic and ambient (static) pressure.

I still think my altimeter is under reading but this must be due to an inaccurate altimeter rather than effects of cockpit  pressure.   I've got some testing to do with GPS and a separate barometric source.

Ade

3

General Discussion / Re: Mode-S Altitude

« on: November 11, 2015, 05:36:53 pm »

Do you fly a pressurised aircraft?

If not, I doubt whether the pressure difference between static line and cockpit pressure at the speeds most GA travel at will be very significant in terms of relative height for PAW purposes.

Moffrestorer,

There can be a significant difference between the cabin altitude and the true altitude in some GA aircraft.  The CFM shadow usually has no static system and shadow pilot manual warns the IAS over reads by about 15%.  I own a Streak Shadow but did not realize straight away that the altimeter is also affected and it under reads by about 300' at 2500'.  I found out when I did some airborne trials with another a PA28, thinking I had 500' separation but the cold reality was that it was less than half that.

A bit of experimenting with a couple of PA28s showed there is little difference between cabin alt and static alt on a PA28.