Question for the controllers: Altimeter setting

Timbeck2 said:
A lot of controllers I work with are prior radar controllers. When the police helicopter or one of the three medical companies' helicopters checks in (all 500' AGL) they always give them the altimeter. When I asked why, they just gave me the deer in the headlights look and shrug.
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Retired in '88 from ATC and I thought that was in the .65 to issue an altimeter setting on initial call to an ATC facility. That's what I remember anyway. Might been radar facilities only, but I don't recall.
 
Pilawt said:
In the situation I posed initially there was a 0.30" (300 foot) difference between the ATC-issued* setting, for a station 60 nm away that was not my destination, and the ASOS at the airport directly below me.
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What number is going to get you a "call this phone number for pilot deviation" and which isn't?
 
mscard88 said:
Retired in '88 from ATC and I thought that was in the .65 to issue an altimeter setting on initial call to an ATC facility. That's what I remember anyway. Might been radar facilities only, but I don't recall.
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I think he was probably talking about local operators who have requested in writing to not have the altimeter issued. @Timbeck2 , have the local cops and docs done this?
 
No but they don’t complain if a controller gives them an altimeter setting either. Trainees issue the altimeter setting because they heard someone else do it and because they can’t think of anything else to say.
 
I always assumed that giving the altimeter just about always results in you reading it back (and they usually issue it again if you don't), which doubles as confirmation that your radio contact is two-way and working.
 
Capt. Geoffrey Thorpe said:
What number is going to get you a "call this phone number for pilot deviation" and which isn't?
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ATC won't worry about a 200 foot difference. But even a larger difference will result in ATC asking your altitude, by "say altitude". In this situation, the local altimeter setting makes more sense to me and is legal. I have never in 50 years of flying been instructed to set an altimeter setting, just advised of a nearby altimeter setting.
 
I taught this and argued with fellow controllers for years. The pilots ( all pilots) have the 100 mile rule. ATC has a rule to issue the facility (or other more appropriate) altimeter which, by definition, does not NECESSARILY meet the 100 mile rule. The most important goal is that aircraft near each other use the same datum, not wildly different ones to ensure separation.

A controllers airspace may cover 110 miles or more.

That being said. The controller is responsible for separation and needs everyone playing off the same datum, especially when doing approaches to a particular airport. The 300 foot mode c error allowance ( which amounts to 250 feet in reality) means that two IFR aircraft could be 500 apart in reality instead of 1000 before the pilot is required to challenge the mode c error to the pilot. The same error could occur with a 500 foot or 00.50 difference between the two aircraft’s altimeters. The odds of that over a 100 mile stretch are slim in most meteorological conditions.

Tex
 
For traffic separation, it's important so you are flying the correct altitude. If you are concerned about terrain clearance, query the controller/request a higher altitude.
 
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