Altimeter Limits

[Captain Kevin 586]

Not sure if this is intentional or not, so I'm asking here before I submit a support ticket. Seems like if I turn the knob to change the barometric pressure setting, the lowest it will go is 28.00 in Hg. If I need to go any lower than that, I have to hit the B key. Is this intentional?

[Budbud 227]

Hi Kevin,

Sorry I don't know the answer but I can't help asking: you want to fly in the eye of a cyclone? 

Edited September 11, 2017 by Budbud
Typo

[skelsey 765]

28" Hg is about 950 hPa. I've not looked in the FCOM, but 950-1100 hPa is a fairly typical range of values for an altimeter subscale so I wouldn't be surprised if this were accurate. 

Now, bonus points if you know how one would get around this problem (QNH outside the range on the subscale) IRL ;)

[JRBarrett 3,070]

Probably 90 percent of altimeters (or air data computers) that I have encountered in my avionics maintenance career have upper and lower limits of 28.00 and 31.00 in/hg.

In the US, Appendix "E", of FAR 43, which details the specific tests that must be done on an altimeter every 24 months, set out some specific test points where the barometric correction must be checked. The lowest test point is 28.10, and the highest is 30.99. 

In r/w flight operations, pressures below 28.00 are extremely uncommon except in strong tropical systems. However, I have always felt that 31.00 in/hg as an upper limit is too low, as pressures higher than that are not unheard of in arctic regions of Canada and Siberia in winter.

One exception is aircraft manufactured by Dassault, which typically have a lower limit that goes all the way down to 23.00 in/hg. This is probably because Dassault, before entering the civil biz jet market in the 1960s, was primarily a manufacturer of military fighter aircraft, whose altimeters are often more rugged and broader range than those found in civilian aircraft.

I'm sure that the NOAA and USAF Hurricane Hunter aircraft have altimeters which will go much lower than 28.00 in/hg.

[kevinh 409]

1 hour ago, skelsey said:

Now, bonus points if you know how one would get around this problem (QNH outside the range on the subscale) IRL ;)

Set 1013 and use QNE.

[vadriver 247]

1 hour ago, kevinh said:

Set 1013 and use QNE.

in what circumstances ..............???

[Captain Kevin 586]

4 hours ago, Budbud said:

Sorry I don't know the answer but I can't help asking: you want to fly in the eye of a cyclone?

I already did this three times so far, the most recent being yesterday, when the pressure dropped to 27.70 in Hg.

[skelsey 765]

2 hours ago, kevinh said:

Set 1013 and use QNE.

We have a winner! 

1 hour ago, vadriver said:

in what circumstances ..............???

Exactly the above: if for some reason the required altimeter setting is outside the range of the sub-scale (either abnormally low or high pressure, or, in smaller aeroplanes where QFE is more commonly used for landing, landing at an airfield at high elevation). In practice very unlikely to be used in a 747 as although pressures of 1050+ have been recorded in Siberia, it is fairly rare and certainly I've never heard of anything >1100 hPa (and at the other end of the scale, a real 747 is very unlikely to be landing in a hurricane!).

QNE is commonly misrepresented as setting 1013/29.92 (i.e. Standard Pressure Setting) on the sub-scale: this is incorrect. QNE is a height reading, not a sub-scale setting: it is "the height indicated by the altimeter on landing when the sub-scale is set to 1013 hPa".

Thus if you know the QNE for a particular airfield, you set 1013 on the sub-scale and you then know your height above the threshold.

[Budbud 227]

49 minutes ago, Captain Kevin said:

I already did this three times so far, the most recent being yesterday, when the pressure dropped to 27.70 in Hg.

Pax or freighter?

The former: hope your pax are still alive! 

[Captain Kevin 586]

3 hours ago, Budbud said:

Pax or freighter?

The former: hope your pax are still alive! 

Passenger variant, but empty, so no passengers to worry about, per se.

[Qavion2 39]

Quote

Probably 90 percent of altimeters (or air data computers) that I have encountered in my avionics maintenance career have upper and lower limits of 28.00 and 31.00 in/hg.

I have a video of a 744 altimeter being scrolled downwards. The tape blanked at -2000', which corresponded roughly to 28.00 (but the green digital display of the baro setting continued to scroll downwards). I stopped turning the knob at 27.68. It may have gone lower.