How high am I?

…or maybe better termed confusion over pressure settings, altitude, and weather.

Most pilots know the memory aid “high to low, hot to cold, look out below”. This reminds us that without local altimeter settings or understanding of temperatures, we may not be where we think we are altitude-wise. Many fewer pilots seem to understand the following situation:

KSBD 192350Z 25013G18KT 7SM SKC 34/11 A2988 
KL35 200035Z AUTO 06007KT 10SM CLR 22/M01 A3028

If flying from KSBD to L35, many pilots might think there’s a significant pressure system somewhere nearby. I can guarantee you there isn’t. You see, these airports are only 21 nm apart; you’d be hard pressed to get that much pressure gradient in a hurricane. The real issue is that they’re roughly 1 sm different in altitude.

But you say, what has altitude got to do with it? Both the altimeter settings are corrected to sea level pressure. What that answer covers up is all the errors you daily live with, but may not have realized. It usually doesn’t even matter, but here’s a case where it really may.

As we take off from SBD with the local altimeter setting, we expect the altimeter to read very close to the airport elevation (1159′ MSL). If you have a WAAS GPS, it should read a very similar altitude. As we climb, those number often start to diverge. But why?

The altimeter measures pressure differences between the static port and the number you put into the Kolsman window. It converts that pressure difference into an altitude by assuming pressure will drop 1″ of Hg per 1000 feet of altitude. While that’s a good average and works well at low altitudes, it often isn’t very accurate the higher or the further from standard temperature we are.

If you landed at L35 without adjusting your altimeter to the local setting, you’d find the altimeter reading around 6450′ MSL, instead of the field elevation of 6752′ MSL. This means a 300′ error over less than 6000′ of altitude; that’s 5%! Using the wrong altimeter setting in this direction gave us greater terrain clearance, since we were 300′ higher than our altimeter said.

Now, what if we had set the altimeter properly in Big Bear, but flew back to San Bernardino without resetting our altimeter? We’d find the ground 300′ before we might expect according to our altimeter. Hopefully as a VFR pilot you aren’t that dependent on your altimeter, but in IFR, we can be. [fly the ILS into Ontario and you might be in trouble].

This also has implications for what altimeter settings you use as you fly along. You need a local altimeter setting, but if you made the choice to listen to Big Bear and entered that into your altimeter, you’d be 300 off  from the rest of LA Center and SoCal approach traffic using an Ontario altimeter setting in the area.

Lessons to take away

  • Be careful of altitudes and altimeter settings when going between high and low altitude airports in one flight.
  • Be wary of using altimeter settings from high altitude airports in cruise, when also near low altitude airports.
  • Use flight following and use their altimeter setting

One thought on “How high am I?

  1. Pingback: METAR SLP – Sea Level Pressure | Wings by Werntz

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