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boeing247

Altimeter issues

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When I press "b" to set the altimeter at cruising altitude (which I have to do frequently because it keeps getting off), the autopilot does not correct my altitude even if I'm 1000 feet off. The only way to get it to climb or descend is to turn VNAV off and then turn it back on again. Now, if I turn off VNAV for the cruise phase and turn on ALT hold instead, everything works perfectly, but aren't you supposed to leave it on VNAV? I'm used to flying the 757 and 767 (Captain Sim), where VNAV worked much like ALT hold at cruising altitude. I'm sure I'm doing something wrong, but what is it?Thanks,boeing247


-Bram Osterhout

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First off stop using B. You do not need to keep changing this while in flight. Depending on what country your in, USA for example the transition altitude is 18,000 feet and in Europe its 6000 feet.At your departure airport set the Altimeter to the current Airport Altimeter. When you pass the transition altitude set it to 2992. That is all you have to do. You do not have to change it again until your on descent and passing through transition altitude.Your never going to be exactly at a specific alitude. Example. You set altitude to FL370. Your gauge will show 37,000 feet but if you look online on the map on say VATSPY for example, you will show up like maybe 37,115 ft. This is normal behavior and all the other pilots are using 2992 as well.


Paul Deemer

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You dont even have to turn the knob to 2992, just push it (the STD button).Also, yes, this is acorrect behaviour for altitude hold modes.


--Peter Fabian 
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When I press "b" to set the altimeter at cruising altitude (which I have to do frequently because it keeps getting off), the autopilot does not correct my altitude even if I'm 1000 feet off. The only way to get it to climb or descend is to turn VNAV off and then turn it back on again. Now, if I turn off VNAV for the cruise phase and turn on ALT hold instead, everything works perfectly, but aren't you supposed to leave it on VNAV? I'm used to flying the 757 and 767 (Captain Sim), where VNAV worked much like ALT hold at cruising altitude. I'm sure I'm doing something wrong, but what is it?Thanks,boeing247
What is your cruising altitude? If you are above Transition Altitude (18,000 feet in North America, and often much lower in Europe), you should set your altimeter to STD which is 29.92 in/hg or 1013 hectopascals, and leave it there. In other words, at typical NG cruising altitudes, you should not constantly reset the altimeter with the B key.In normal VNAV cruise, above transition altitude, if the altitude readout changes because you reset the barometric correction, the aircraft will not automatically climb or descend to recapture the MCP altitude. It WILL do so however, if the change in altitude readout is caused by an actual change in outside air pressure.

Jim Barrett

Licensed Airframe & Powerplant Mechanic, Avionics, Electrical & Air Data Systems Specialist. Qualified on: Falcon 900, CRJ-200, Dornier 328-100, Hawker 850XP and 1000, Lear 35, 45, 55 and 60, Gulfstream IV and 550, Embraer 135, Beech Premiere and 400A, MD-80.

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Okay, I had a feeling that was the case, but I just wanted to be sure. Thanks!


-Bram Osterhout

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Don't forget to hit the Standard Button again on descent after passing through transition altitude and set the altimeter to whatever the current altimeter is at your arrival airport.


Paul Deemer

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There is a disparity of altitude readouts in VATSPY.  Aircraft at cruise above FL180 with altimeters set to 29.92 display significant differences in altitude readouts.

 

Hopefully someone will be able to explain why.

 

Regards,

Tom

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There is a disparity of altitude readouts in VATSPY.  Aircraft at cruise above FL180 with altimeters set to 29.92 display significant differences in altitude readouts.

 

Hopefully someone will be able to explain why.

 

This is due to the nature in which altitude is reported by those programs. Remember that, while flying at 29.92, you're not flying at actual pressure; you're flying at an assumed pressure.

 

Example:

At standard pressure, FL180 is 18,000 feet above sea level.

At 29.82, FL180 is actually 18,100 feet above sea level. (Rolling the wheel down 0.1 inches makes the altimeter indicate 100 feet below the current altitude, so you'd have to climb back up to FL180, which would actually be 18,100. This is why there is the concept of "lowest usable FL.")

 

The programs are probably reporting actual height MSL instead of the altitude being reported off of the altimeter. Of course, with people using potentially different weather sources, the variation may vary.


Kyle Rodgers

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This is due to the nature in which altitude is reported by those programs. Remember that, while flying at 29.92, you're not flying at actual pressure; you're flying at an assumed pressure.

 

Example:

At standard pressure, FL180 is 18,000 feet above sea level.

At 29.82, FL180 is actually 18,100 feet above sea level. (Rolling the wheel down 0.1 inches makes the altimeter indicate 100 feet below the current altitude, so you'd have to climb back up to FL180, which would actually be 18,100. This is why there is the concept of "lowest usable FL.")

 

The programs are probably reporting actual height MSL instead of the altitude being reported off of the altimeter. Of course, with people using potentially different weather sources, the variation may vary.

Kyle,

While crossing the Atlantic this morning @ FL350 Eastbound my altitude readout would vary as much as 700 ft from what was set in VNAV.  All other aircraft in flight over the Atlantic were showing varying degrees of incorrect altitudes.  We were all flying over a flat surface with altimeters set @ 29.92.

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Kyle,

While crossing the Atlantic this morning @ FL350 Eastbound my altitude readout would vary as much as 700 ft from what was set in VNAV.  All other aircraft in flight over the Atlantic were showing varying degrees of incorrect altitudes.

 

Tom,

 

I understand what the issue is. I've seen it. Remember that this is VATSIM. It's not some precision-tuned piece of software attempting to unify world weather between people's sims.

 

Again: people may use different weather programs, which can cause differences in pressure at that point. Who knows if the height MSL metric is actually implemented properly, either. Seems close enough, though.

 

 

 

We were all flying over a flat surface with altimeters set @ 29.92.

 

I think you're misunderstanding pressure here...

 

Just because you're over the ocean, and you've set 29.92 doesn't make pressure irrelevant. It just puts people on a common pressure gradient, in theory, but your altitude is still not what's being reported on the altimeter (see the above example). In the real world, though, we don't have American Airlines flying with ASN, United on FSGRW, Lufthansa on REX, and Delta running no weather add-on.


Kyle Rodgers

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Kyle,

I understand your explanation.....I glad we both agree that VATSPY has a disparity in displayed altitude readouts.

 

Regards,

Tom

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