B737-800/900ER CABIN ALTIMETER

ATA 21 (AIR CONDITIONING)

Cabin Altimeter

I determined I’ve looked at the indication from an opposite, less operational (real life) point of view and it puzzled me.

The concluding explanation toward unpressurized altitude operations is that if you decent to 10.000 feet (not FL 100), the lower barometric pressure is working against the decision to remove the mask.

Reason for that is that the aircraft is at 10.000 feet with an actual (983 hPa) reference and the cabin at a 10.000 feet difference from standard (1013 hPa) on that same reference line as in the image. While flying unpressurized at 10.000 feet, this results in a Cabin Altimeter indicating a higher altitude. (± 10.800 ft) Also you’d have the amber CABIN ALTITUDE Alert indication which is adjusted the same as the indicator.

Correction on the image, it should read actual cabin altitude iso indication like in the values of the explanation.

Cabin Altitude indication.

The QRH states that in a depressurized condition, oxygen masks may be removed when the Cabin Altitude is at or below 10.000 ft but is it really 10.000 ft?

The needed knowledge is indicator calibration, the current QNH and the possibility of problems that can arise when depressurization results in Hypoxia. In the NAVY every 3 years we had to attend “Height Indoctrination Courses” (HIC) to determine our personal Hypoxia symptoms so we knew them. When dropping Parachutists up to 13.000 ft, we’d still function relatively normal without the use of O2 when performing little exertion. Regulations talk about 12.500 feet MSL for more than 30 minutes as a rule to provide extra Oxygen.

Anyway, the QRH tells us that during a depressurization, increasing the Cabin Altitude to 10.000 feet and above requires supplementary Oxygen. The indicator is calibrated at 1013 hPa and will show the Cabin Altitude related to MSL in a standard atmosphere. Because of this, the Cabin Altimeter indicates basically “Cabin Flight Levels” during flight as you cannot set QNH on the indicator.

When a cabin depressurization occurs above 10.000 ft, it depends on the QNH what the actual Cabin Altitude will be vs the indicated Cabin Altitude. When you select the PFD Altimeter subscale to the current QNH, you’d see what the actual (inside & outside) PA is, provided there is a 0 PSI Differential Pressure indication. The QRH procedure will guide you to a relatively safe Altitude but be aware it will differ from what you see on the Cabin Altimeter when the actual QNH is not standard.

The next Altimeter indications apply with a total Cabin Pressure loss and at FL 100:

With a QNH of 1013 hPa;

PFD Alt. with STD selected = 10.000 ft (related to 1013)
PFD Alt. with QNH selected = 10.000 ft (actual PA)
Cabin Alt. = 10.000 ft (related to 1013)

With a QNH of 983 hPa;

PFD Alt. with STD selected =10.000 ft (related to 1013)
PFD Alt. with QNH selected = 9.200 ft (actual PA)
Cabin Alt. = 10.000 ft (related to 1013)

With a QNH of 1043 hPa;

PFD Alt. with STD selected =10.000 ft (related to 1013)
PFD Alt. with QNH selected = 10.800 ft (actual PA)
Cabin Alt. = 10.000 ft (related to 1013)

The conversion is ± 27 ft for each hPa in a standard atmosphere that most of the time is rounded up to 30 ft so the values here are approximate using 27 ft and checked in the FFS.

Finally the concern would arise when QNH is extremely high which is exceptional, so hardly an issue but not impossible. A little research show a record low in hurricanes of 870 hPa (-4290 ft) and a record high in Siberia of 1085 hPa (+2160 ft) where normal pressures easily is ± 1050 hPa.

Using the high record still gives us at FL 100, a Cabin (Pressure) Altitude of 12.160 ft, remember the para-drop story from above to find out it is of course an issue but not exceptionally dangerous for a short period of time.

By the way, when cranking the subscale to its limits it'll stop at 745 hPa (- 8040 ft) and 1084 hPa (+ 2130 ft).