Advise when you have the weather, but don't adjust the altimeter setting?

[buzzin77]

There was a high pressure gradient with a high pressure system moving inland the last time I flew from an inland airport to an uncontrolled, coastal airport, and it presented me with a new situation and a question for you guys.

Upon departure, the controller gave us an altimeter setting of 29.88 with an instruction to get the weather for our destination.

When we got the weather for our destination, still about 20nm away, we changed the altimeter setting to our destination airport, 29.99, obtained from the "one-minute-weather" ASOS and advised the approach controller that we had the weather. We were deviating from our assigned altitude (IFR training flight), about 100' and the approach controller told us we were 200' off and asked us to "verify correcting" and gave us the altimeter setting of 29.88 again.

Question: When should I change to the destination altimeter setting? When cleared for the approach, and not when I obtain the destination weather? This was an interesting case because usually the altimeter setting difference between the two airports is marginal, even with the ASOS reporting the pressure change every minute up to the point when the towered airport finally updates its ATIS.

[sykocus]

The controller's scope probably only gets an altimeter update when there a new observation is pushed out or it may even have to be updated manually by the controller when a new obs comes out. That's all assuming they input the altimeter setting for your destination. From an ATC perspective the correct altimeter for setting is slightly less importantly then making sure all aircraft in an area are using the same setting. That way there is a standard amount of altitude between aircraft when applying vertical separation.

I honestly don't know the correct answer to your question but my gut feeling is to avoid the type of situation you experienced should stick to the the altimeter the controller has given you at least until you are cleared for an approach.

[Casper87]

If you have been instructed to fly at an altitude and the controller has suffixed the level with the altimeter setting then you must not deviate from that. I'm assuming you we're within controlled airspace (CAS) for most of the route; you do not deviate from a clearance if you are flying under IFR within CAS (immediate safety implications aside).

The controller will be separating you from other aircraft, as stated by previous post, thus if you are not on the same pressure setting (i.e the one you have been instructed to use) then you are potentially causing safey to be compromised.

In your case you'd only change pressure setting:

On leaving the frequency
When instructed by the controller
On leaving controlled airspace
Or after negotiating the use of the new pressure setting with the controller

Cheers,

C

[StuSEL]

I would absolutely not suggest changing the altimeter setting after you're cleared for the approach. Regardless of whether or not the altimeter at the destination is higher or lower than the one you received on departure, you're setting yourself up for either a steep descent or a last-minute climb on certain approaches. It is certainly important to have the destination altimeter when you commence the approach, but I would recommend setting it early enough so that you have one less thing to worry about during the critical final approach course intercept or altitude step-down processes in the approach.

If you know the destination altimeter is low or high enough to the point that it will impact your Mode C readout on the controller's scope, give the controllers a heads up before you switch to the new altimeter setting.

I would also suggest filing a NASA report of the incident, because this could be an issue that needs to be clarified in the AIM or the controller's manual, the Order 7110.65. After searching both of those documents, I came up with nothing on the topic. Casper87, what's your reference for those rules?

[7254]

i believe the controller has to manually update his/her scope's altimeter setting and when you input a different setting and you maintain an assigned altitude, it will show up different on the scope, thus potentially causing the controller to break separation.

[beechsundowner]

I honestly don't know the correct answer to your question but my gut feeling is to avoid the type of situation you experienced should stick to the the altimeter the controller has given you at least until you are cleared for an approach.

When I read your response, I thought to myself, well that's how I run my ship, but then I saw on my IFR check list to get ATIS / ASOS 15 miles out which would be consistent with how the OP runs his shop. 

OP was only 20 miles from airport.  "Usually" I'm not cleared for the approach until 5 miles out from the FAF and between that approach clearance given by you and the FAF, this is the time where I am doing the before landing check list and slowing down to 90 knots.  Not so sure I'd want to be mucking around with radios during this phase for ATIS and adjusting the altimeter when I should be flying the plane.  Adjustment to the altimeter at this stage puts me behind the airplane should it change by that much.  I'd have to agree with StuSEL for his additional reasons he puts out on safely executing the approach and also filing that NASA report.

Tough call from a pilots point of view in my opinion.  Whatever the case, I can't imagine that flight being a good flying day with that much gradient in pressure over 20 NM

[Unbeliever]

When the setting isn't that different, I'll change right away.  But normally, "When you're cleared and on a thick black line" is the rule of thumb as to when you change.

--Carlos V.