Alessio Calandrella

Standby altimeter incorrect reading

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Hello,

i found that the analog stby altimeter is not synched with the PFD altitude values. I was cruising at FL370 and my stby altimeter was reading about 37120ft.

I'll report here just in case you didn't know about it.

Here's a screenshot:

HwtK4rC.jpg

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Two separate systems, can’t imagine they’d be perfectly in sync.  100 foot difference at 37,000 feet seems normal to me. 

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That’s not a bug. The analog standby altimeter is not legally required to be precisely accurate at high altitude. The main air data computers that provide the altitude and airspeed to the primary flight displays are indeed highly accurate under RVSM standards, but they achieve that accuracy using solid state pressure sensors, and digitally-processing the measured pressure, compensating the altitude for temperature and the effects of airspeed on the static ports.

The standby altimeter by contrast is just a mechanical instrument containing an aneroid bellows. It’s not much different than the simple altimeter you’d find in a Cessna 172.

At flight level 370, an analog altimeter is only required to be within +/- 205 feet of the actual altitude.

Analog altimeters become more accurate at lower altitudes.

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1 minute ago, JRBarrett said:

That’s not a bug. The analog standby altimeter is not legally required to be precisely accurate at high altitude. The main air data computers that provide the altitude and airspeed to the primary flight displays are indeed highly accurate under RVSM standards, but they achieve that accuracy using solid state pressure sensors, and digitally-processing the measured pressure, compensating the altitude for temperature and the effects of airspeed on the static ports.

The standby altimeter by contrast is just a mechanical instrument containing an aneroid bellows. It’s not much different than the simple altimeter you’d find in a Cessna 172.

At flight level 370, an analog altimeter is only required to be within +/- 205 feet of the actual altitude.

Analog altimeters become more accurate at lower altitudes.

Ok thanks for claryfing this! I-m used to fly with the NGX and here the stb alt always report the same alt of the PFD. Amazing to see that PMDG has simulated this unprecision of the stby altimeters! 

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1 hour ago, Alessio Calandrella said:

Ok thanks for claryfing this! I-m used to fly with the NGX and here the stb alt always report the same alt of the PFD. Amazing to see that PMDG has simulated this unprecision of the stby altimeters! 

The Maddog does it too.  It’s a nice little touch on these airplanes. 

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1 hour ago, FishermanIvan said:

Two separate systems, can’t imagine they’d be perfectly in sync.  100 foot difference at 37,000 feet seems normal to me. 

LIkewise here to OP's screenshot, at 34000 on the PFD tape and 34100 on the STBY, but as one is digital whilst the other is analogue, I accepted that. Makes no big deal to me.

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Anybody that's been in a cockpit in the days before TSA and RSVM is shocked at how precise the primary flight altimeter is.... back in the day if you were cruising at FL370 and the altimeter read 36940 you considered yourself right on the number.

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Forgive me Dan, but what are TSA and RSVM,aside from the body that currently checks security at US airports for the former?

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2 hours ago, vc10man said:

Forgive me Dan, but what are TSA and RSVM,aside from the body that currently checks security at US airports for the former?

Well... since 911 access to the cockpit for the average guy during a flight has become nil.  There was a time when one could ask to visit and the request would likely be granted on some flights.  That doesn't happen anymore.

To be honest the last flight deck I was on was probably a C-130.

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3 hours ago, vc10man said:

LIkewise here to OP's screenshot, at 34000 on the PFD tape and 34100 on the STBY, but as one is digital whilst the other is analogue, I accepted that. Makes no big deal to me.

Even the altimeter portion of more modern all-in-one digital standby flight displays are often not certified to RVSM altitude standards, but they tend to be a little more accurate at higher altitudes than the purely analog units.

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2 hours ago, vc10man said:

Forgive me Dan, but what are TSA and RSVM,aside from the body that currently checks security at US airports for the former?

RVSM would be Reduced Vertical Separation Minimums (I'm guessing just a slight typo on his part, anyway). Basically, the vertical separation between flight level 290 and 410 got reduced from 2,000 feet to 1,000 feet. TSA would be the Transportation Security Administration.

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Hi Capt Kevin. I thought as much what TSA was, but was not certain in what context Dan was using it in his post. Thanks for the RVSM clue-up.

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