When I start flying, the barometer is at 29.92. When I get to 5000 feet, I hit the “B” or the barometer knob on the aircraft panel and it radically changes by a thousand feet or so. The barometer setting becomes 27.99. My problem is I keep missing the final altitude at the 'APPROACH" of RNAV. It seems I’m always coming in high. I see items on the forum about the transition altitude of 18,000 feet … I’m nowhere near that. So which altitude do I use, the one I have on the ground or the “corrected” one when I hit the “B” at 5000 feet in the air? Flying this low, when the ATC gives me the barometric pressure, should I put that into the barometer? The RNav is not trustworthy with this problem (which I’ve probably created for myself).
Above the transition altitude, you set the baro to standard pressure (29.92in or 1013hPa).
Below the transition altitude, always set baro to ATC or ATIS, especially when on the ground 
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This. Let me attempt to elaborate…
When you load in, most planes (even the jets) load in with 29.92 as the default altimeter. As different airports in different places have different weather and atmospheric conditions, you need to change the planes altimeter to match those local conditions. Its one of the first things you need to do when you start the plane.
Depending where you are regionally, the transition altitude changes; The 18000 feet number you saw is the US Transition altitude where you go from local pressure at your departure/arrival airport to the standard pressure of 29.92. This is to allow uniformity across the airspace for ATC to maintain spacing and other related concerns. As a result, ATC will typically have you repeat the given baro pressure to ensure you and ATC will have similar speed and height readouts when being cleared down from Class A Airspace (AKA above 18,000 feet) and into the approach
In your case, it seems you had the process backwards:
- Press B on the ground. This sets local altimeter
- At your transition altitude (assuming your locality is set at 5000’) press the STD button or change to 29.92/1013
- Cruise
- On approach, set your altimeter to the arrival pressure found in the ATIS and METER at your transition altitude (again, assuming it’s 5000’ for you). Configure the plane as required for your desired approach from there.
Personally, i set my GS intercept height as my final descent height when configuring my approach. This way when i pass my transition (in my case 18,000 feet) my plane will be at the correct height and will agree with my charts by the time i get to the intercept fix and ATC won’t yell at me for being where i shouldn’t…
And yeah, sometimes it does sure seem pretty low to the ground even when configured correctly. Sometimes at DEN you seem dangerously close to the ground on certain approaches but having conferred the charts in these situations i’ve found myself exactly where i need to be 99.9% of the time. The weather just sucks…
Absolutely WONDERFUL and thoughtful responses. These are the clearest explanations I’ve gotten. Hopefully, others who are quietly facing this same problem will also benefit from your clear solutions. Alas, I had it backward (as usual for me). And now it is all working. Thank you all very much.
UPDATE: There was a second problem (which your answers guided me to). My “B” on the keyboard was bound to the RADIO “Set Altitude” not the INSTRUMENTS “Set Altitude”. This was why my BP was jumping all over the place seeming at random. Just flew again out of my home airport, up to 5000 feet, headed back on RNav, and landed with 300’ visibility. It is working beautifully. Thanks again!
Doug
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