Alt setting IFR

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800man
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Alt setting IFR

Post by 800man » Sun Aug 14, 2016 7:53 pm

Hey so you're flying controlled IFR, and midway through the flight you pick up the atis for a nearby airport with a different altimeter setting. Do you reset your alt even though atc didn't give it to you? Related, when would you set the alt setting for your destination, if your destination also has an atis you can pick up before your cleared?
References?

Thanks guys
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CitationNation
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Re: Alt setting IFR

Post by CitationNation » Mon Aug 15, 2016 11:19 am

If your cruise level was over FL180 you would set it as you descend through FL180 arriving at destination. During cruise you would have 29.92 set as all other aircraft as well. If under IFR control under FL180 you would set is often as required or when ATC gives you a new altimeter setting.
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Cessna 180
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Re: Alt setting IFR

Post by Cessna 180 » Mon Aug 15, 2016 1:11 pm

Altimeter-setting and Operating Procedures in the Altimeter-setting Region

602.35 When an aircraft is operated in the altimeter-setting region, each flight crew member who occupies a flight crew member position that is equipped with an altimeter shall

(a) immediately before conducting a take-off from an aerodrome, set the altimeter to the altimeter setting of the aerodrome or, if that altimeter setting is not obtainable, to the elevation of the aerodrome;

(b) while in flight, set the altimeter to the altimeter setting of the nearest station along the route of flight or, where the nearest stations along the route of flight are separated by more than 150 nautical miles, to the altimeter setting of a station near the route of flight; and

(c) immediately before commencing a descent for the purpose of landing at an aerodrome, set the altimeter to the altimeter setting of the aerodrome, if that altimeter setting is obtainable.
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800man
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Re: Alt setting IFR

Post by 800man » Mon Aug 15, 2016 3:24 pm

Thanks, and I appreciate the reference.
But lets say you're at 10,000ft, controlled IFR. What if you have a sharp gradient, and the pressure goes from 30.00 to 29.50 over 100 miles that's also along an airway. There's an airport close by somewhere with an atis. You could have people flying around and setting their alts at any random distance away from that station, depending on where they got that atis, and suddenly they could be 500' closer vertically......
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photofly
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Re: Alt setting IFR

Post by photofly » Mon Aug 15, 2016 4:13 pm

800man wrote:What if you have a sharp gradient, and the pressure goes from 30.00 to 29.50 over 100 miles that's also along an airway.
with a pressure gradient of a half inch of mercury over 100 miles you'd find yourself flying in a tropical hurricane.

For comparison that would be 8 isobars (spaced at 4hPa intervals) over 100nm on the GFA.

Which precise altimeter setting to use in that kind of weather is going to be the least of your worries.
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fish4life
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Re: Alt setting IFR

Post by fish4life » Mon Aug 15, 2016 4:29 pm

If it's that sharp of an altimeter change the weather is probably so bad that no one is flying anyway
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lazyeight
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Re: Alt setting IFR

Post by lazyeight » Mon Aug 15, 2016 6:33 pm

If you're controlled you'll get the setting from ATC. Either when handed off or if there is a big change through your flight in their area. Thinking now though, I don't think I've ever had an alt setting change on me while passing through a control zone onward to the next. And even when handed off it's never really more than a couple points off.
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lazyeight
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Re: Alt setting IFR

Post by lazyeight » Mon Aug 15, 2016 6:34 pm

800man wrote:You could have people flying around and setting their alts at any random distance away from that station, depending on where they got that atis, and suddenly they could be 500' closer vertically......
Yeah, but if you're controlled they will be too. Which means they will also get the same setting you have when they have initial contact. Additionally, if you're on freq and you hear them give someone else a setting that is different than yours.. change it to that. It might be only a .01-.02 but stay on top of things while listening.
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Braun
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Re: Alt setting IFR

Post by Braun » Mon Aug 15, 2016 6:50 pm

ATC is required to give any change in ALT setting if it changes by .02. Also we will issue the same altimeter to a/c that may be in conflict.
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PropToFeather
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Re: Alt setting IFR

Post by PropToFeather » Mon Aug 15, 2016 7:26 pm

Also, if you know the controller's busy, and you get a big change, tell the controller before you start flipping switches. Worst case scenario, they'll give you a "yeah yeah, whatever", best case, you avoid colliding with someone because of an altimeter change.
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Tailwind W10
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Re: Alt setting IFR

Post by Tailwind W10 » Tue Aug 16, 2016 7:47 am

Your mode C transponder doesn't care what altitude setting you've put into your altimeter, it always transmits simple pressure altitude. ATC has they're own altimeter setting on their equipment so they're seeing all aircraft under their control with the same setting. It's quite possible for your altimeter to be showing a different altitude than ATC is seeing for you, but it should be close within tolerances, that's what your biannual transponder check is about.

here's an excerpt from an Avweb article

"The altitude-reporting capability of your transponder transmits your aircraft's PRESSURE ALTITUDE (rounded off to the nearest 100 feet) whenever it receives a Mode C interrogation and is switched to ALT mode. You might recall from your private pilot groundschool that pressure altitude is what the altimeter reads if you set it to 29.92 In. Hg. Because the transponder reports pressure altitude, the altimeter setting that you dial into your altimeter's Kollsman window has absolutely no effect on your Mode C altitude reports. It is this fact that makes "blind encoders" (which are mounted behind the panel and have no setting knob at all) practical."

Gerry
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Maynard
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Re: Alt setting IFR

Post by Maynard » Thu Aug 18, 2016 4:36 am

I've had centre tell me to stay on my current altimeter even though I was getting over the half way to the other station, because there were other planes on the same altimeter.
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