Pressure Altitude Setting

GMascelli

GMascelli

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Jul 27, 2005
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Ocean City, MD
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Display name:
GaryM
A questions for the Aspen user collective....

What would cause a difference in pressure altitude setting to differ between the altimeter and the Aspen 1000 PFD. I notice in order to be at the same altitude I have to add .07 to the Aspen.
 
Does the Aspen get its data from a standalone blind encoder?
 
Pressure altitude is altitude of the altimeter when set to 29.92.
 
455 Bravo Uniform said:
Does the Aspen get its data from a standalone blind encoder?
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At this time, not sure. I know, that sounds ridiculous. This is a plane I am looking to purchase and read over my notes from a test flight. I’ll know more when we go into annual inspection for the purchase.

I was just wondering if it was something obvious with the Aspen. It’s the first time I flew a plane with that system.
 
If you're reading within 75 feet of field elevation (which is essentially what adding .07 inches does) then the Aspen is generally considered legal per that mickey mouse book of selectively enforceable suggestions we call the AIM.

You'd have to get a static/altimeter/encoder/potato check to see if the aspen meets the technical tolerances described in Appendix E to Part 43 in pursuit of compliance with 91.411.

My opinion? It's an Aspen, so I default to "it's gonna eff me over" Lol. But since you didn't pay for it, it's a sunk cost in the panel. Technically the thing is showing correct enough, so you're good to go. If you're nervous about introducing a 10K box into a static system that requires checks every 24 months for legal IFR flight, then I recommend you stick with steam gauge only IFR birds. Welcome to the fallacy that electronic replacements don't require maintenance $$$. But again to be clear, that aspen is reading legal, so there's no issue here. Certainly none that would make me not proceed with an airplane purchase.
 
Gary, glad your back.

I think I recently had my Aspens altitude adjusted also to match the other altimeter during the 2 year static altimeter and transponders check. So I think your in luck.
 
Take a look at the logbooks for the last altimeter check. You should have a sticker in there both for the panel altimeter and the Aspen. Do they differ there?

Either way, it sounds like they're within tolerance to fly. Hopefully there is an adjustment like others have mentioned so that they can agree - The club's R182 had a bit of disagreement between the panel altimeter and Aspen too, and that always kind of annoyed me.
 
gacoon said:
I think they can change the Aspen +/- during check to zero it in. Talk to Aspen or avionics shop to verify.
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MajorTurbulence said:
Gary, glad your back.
I think I recently had my Aspens altitude adjusted also to match the other altimeter during the 2 year static altimeter and transponders check. So I think your in luck.
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flyingcheesehead said:
Take a look at the logbooks for the last altimeter check. You should have a sticker in there both for the panel altimeter and the Aspen. Do they differ there? Either way, it sounds like they're within tolerance to fly. Hopefully there is an adjustment like others have mentioned so that they can agree - The club's R182 had a bit of disagreement between the panel altimeter and Aspen too, and that always kind of annoyed me.
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Thanks all

I'll have a look at the logs and the most recent alt/trans/static check. I'll also stop in at the avionics shop I use and chat with them about it, since they will be doing my work on this plane too.

I thought there had to be some sort of calibration but my knowledge of Aspen, for now, is limited. More stuff to read and learn.
 
FastEddieB said:
Well, to be precise - and pedantic- pressure altitude is your height above or below the standard datum plane. Setting your altimeter to 29.92 is one way a pilot can determine it.
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To be doubly so, isn't that also your height above said plane without correcting for non-standard temperature?
 
alfadog said:
To be doubly so, isn't that also your height above said plane without correcting for non-standard temperature?
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I guess you could add in that disclaimer, to differentiate it from density altitude, but the PHAK doesn’t see the need:

Pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.92 "Hg (1,013.2 mb) as measured by a barometer. An altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere. If the altimeter is set for 29.92 "Hg SDP, the altitude indicated is the pressure altitude. As atmospheric pressure changes, the SDP may be below, at, or above sea level. Pressure altitude is important as a basis for determining airplane performance, as well as for assigning flight levels to airplanes operating at or above 18,000 feet.
 
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