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jeypee

Wrong altitude in PFD (+200ft) 747 v2

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Hi there,

 

also there is a similar topic around, i started a new one, because its a different problem.

 

Iam running the 747 v2 in p3d 3.35 + AS16+ASCA. I always have the problem that my cruise level is around 200ft too high.

 

For example my cruise level is FL370. The PFD (STD) indicates 37000ft. But the analog altimeter (1013/2992) shows 37200ft.

 

Smartcars reads out iam @FL372. So iam flying at 37200ft and the PFD shows the wrong altitude. The analog altimeter shows correct altitude.

 

http://prnt.sc/d00oxm  (picture on lightshot, cant manage to link the picture via the picture option)

 

Did i miss something, or is it a bug?

 

 

regards, Jens

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Found something on the official support forum.

 

Standby Altimeter / ISFD "incorrect"

 

The Standby altimeter or ISFD is displaying a different Altitude value to the Altimeters on the CAPT and FO PFD's when using the same QNH. This is not a mistake, read below the technical reason for this anomaly.

 
Curtsy from 1 of our Technical Advisory Pilots:
"The left and right Pitot static probes supply data to their respective Air Data computers. The ADC's process this data and the output is in the form of an electrical signal to the respective primary altimeters. Within the ADC's is a stored calibration "look up" table which is a very precise non linear correction curve that can be programmed to account for any errors (which magnify with increasing altitude). This correction curve can be modified as the altimeter gets older. The standby altimeter gets it's data directly from the alternate static source and has no calibrated correction other than what can be achieved within the altimeter itself."

 

 

 

 

But the problem is, the plane still flies the difference too high. so simulator uses the "not calibrated" data from the standby altimeter to determine the plane position i guess.

 

So in future when iam flying online and dont want to confuse ATC, i will set 38800 on MCP for FL390. A solution, but not a very realistc one in my opinion.

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on our aircraft, all three altimeters are out to a certain degree, the effect worsening at altitude.

seems realistic enough to me.

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on our aircraft, all three altimeters are out to a certain degree, the effect worsening at altitude.

seems realistic enough to me.

 

Ok, but shouldnt the autopilot take data/information from the most accurate source? In this case the calibrated data from the ISFD ? In sim this is not the case as i found out.

 

Or is it normal, in rl one plane cruises @ordered FL380 at 37800ft and the next at 38200ft cause of altimeter calibration issues? Seems alot to me. Just have to ask, iam not a rl pilot ;)

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From what you wrote, its not the most accurate display. ISFD gets uncorrected data, by the looks of things.

 

The ADCs supply info to the left and right side instruments, which gets corrected, so they are more accurate.

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so why iam flying 37200, when iam put in 37000 in the window? thats the question, not more ore less. for the ifly 747 and p3d.  is that rl ?

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errrr... this goes beyond the aircraft if you are checking other sources for your altitude, i think you need to ask a developer directly.

 

you are flying at 37000, says so right on the pfd which is where the autopilot gets its reference. so the autopilot is doing what it should. beyond that, i dunno

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so why iam flying 37200, when iam put in 37000 in the window? thats the question, not more ore less. for the ifly 747 and p3d.  is that rl ?

You're not flying 37200. You're flying 37000ft on standard pressure.

 

Smartcars et al will be reading your 'true' altitude, however, which is based on the actual current local barometric pressure (because this is what FS outputs). This is different to real life, where all altitude readouts are based on 1013.

 

In other words, in real life of you were flying below TA at 5000ft on a QNH of 1003, your transponder would be sending 5300ft because it only knows standard pressure. Conversely, when you climb to a flight level and set standard on your altimeter, your transponder should match up.

 

It works the other way around in FS - so when you are flying around with local pressure set, Smartcars will agree with your altimeter, but when you climb to a flight level and set standard pressure on the altimeter Smartcars will continue to see the 'true' altitude as if you still had local pressure set. It's perfectly normal and nothing to worry about.

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if the QNH is 1013, there's no difference to standard. he says he has the standby altimeter set to 1013 also.

 

he says its always 200ft off (the standby and sim altitudes seem to agree with each other), which would imply that even with different prevailing pressures it still is.

 

if the standby altimeter is set to the same as the pfds, then the only difference would be related to instrument error. so the standby altimeter is reading 'correctly' according to the sim, but 'uncorrected' according to the plane, presumably the correction that the PFDs get is based on real world issues which aren't modelled in the sim? Hence correcting the correct reading to an incorrect one. not that 200ft makes a difference in the grand scheme of things. weird though.

 

At least thats my understanding of the ops issue.

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As Simon pointed out, when you are flying a Flight Level of FL370 (using STD altimeter setting), you are NOT flying at 37,000 feet. You , and every other aircraft around you are flying at an altitude that depends solely on the local baro pressure, ie 37,000 feet plus or minus hundreds of feet.

 

You are able to ascertain that the actual altitude is 37,200, but that is irrelevant when flying a FL. The fact that the standby altimeter happens to read the same, is a coincidence, and the fact that it disagrees with the PFD by 200 feet is within normal variation.

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if the standby altimeter is set to the same as the pfds, then the only difference would be related to instrument error. so the standby altimeter is reading 'correctly' according to the sim, but 'uncorrected' according to the plane, presumably the correction that the PFDs get is based on real world issues which aren't modelled in the sim? Hence correcting the correct reading to an incorrect one. not that 200ft makes a difference in the grand scheme of things. weird though.

 

My suspicion is that the fact that the instrument error on the standby is +200ft and Smartcars (which will be reading true altitude direct from the sim) is also reading 37200ft is a coincidence, though -- my guess is that the ambient pressure may be around 1020 (and if this were set on the standby, it would indicate 37400-ish, rather than the actual true altitude of 37200ft).

 

My understanding is that the standby incorporates a simulated instrument error (so rather like the situation with the Smartcars altitude readout vs the way a real transponder works, the situation with the altimeters in the sim is reversed: the PFDs display the altitude out of the sim, which is always correct, and then an error is added to this altitude for display on the standby altimeter: unlike the real world where the standby is unprocessed, in the sim the standby readout is processed in order to generate the instrument error which would otherwise not be present).

 

I may be wrong though -- it depends on whether the standby and Smartcars (or Shift-Z, I think will also display "true" altitude) always agree with each other, or whether they sometimes differ.

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