Altitude inconsistency between Altimeter and HUD?

[Ergo]

There is 40-50 ft difference between HUD altitude displayed and Altimeter altitude displayed.

 

Easy to check : on ground, set you altimeter to 0ft, and check your HUD, it display 40-50 ft.

 

Other thing : when start mission in position "ready to takeoff", altimeter is set to QNH, not same thing on A-10C (QFE setting) or Mirage 2000 (QFE setting)

[bbrz]

The altimeter is only a standby altimeter, which means it has greater tolerances and hence almost never shows the same altitude as the primary altimeter (HUD)

[Precog]

Really? I would have thought they both get the info from static ports. Where does the HUD get is altitude info from?

[Frederf]

Air data computer and in the case of air to ground attack, the INS

[bbrz]

But the ADC uses the input from the static port as well.

[Frederf]

But the ADC uses the input from the static port as well.

Yeah but not only the static port. ADC has a corrected understanding of barometric altitude using temperature, Mach, corrections for alpha/beta, etc.

 

If you put in a bootfull of rudder and create a big vacuum around the static port your standby altimeter is going to go nuts but the ADC-driven HUD baro alt should apply some correction.

 

 

None of this is correct.

One thing is the barometric altitude for navigation and another thing is the altitude for weapons delivery. ADC uses GPS and ins for weapons but HUD uses static port. Otherwise you would not be able to navigate in a crowded environment if your altitude was not according to pressure settings.

The difference between both is only due to scaling.

 

That's what I said: in navigation modes the ADC reports the most accurate barometric altitude it can. It's been run through the computer and messed with numerically to some degree. If you lose the ADC you lose the HUD baro.

 

No it's not fully corrected to true for the reasons you state but it's trying to be an ideal barometric altimeter and remove some sources of error which are not of the type that would risk it running into a 737.

 

 

Explain scaling.

[bbrz]

Again, as I mentioned in my first reply, the tolerances are greater on a standby altimeter, hence less precision, it's the same in almost every aircraft.

 

On ground the HUD BARO alt must show the exact altitude within +30ft and the stndby alt only +60ft.

 

The F/A-18 manual further states at the 10000-18000ft instrument check: Standby altimeter agrees with HUD BARO altitude, smooth operation.

 

So I'm not sure about the less 'resolution' thing.

[Frederf]

I see that tolerance is different but we cannot conclude that is the result of any difference in precision. It could easily be due to a difference in accuracy instead.

 

 

The standby instrument appears to be a completely analog instrument with no inherent granularity to displayed altitude. It's precision is effectively infinite or only subject to how precisely the needle and face can be read by a human.

[bbrz]

I see that tolerance is different but we cannot conclude that is the result of any difference in precision. It could easily be due to a difference in accuracy instead.

Nice example for a typical language problem. When I enter 'accuracy' and 'precision' into the translator the results are for a great part the same and the first hit, the most suitable one, is identical.

[Banzaiib]

I set the altimeter to QNH, and it reads the same as hud... i've never seen that discrepancy before. My first thought is that your hud set to radar altimeter and 50 - 60 ft difference means you're probably on the deck of the carrier...