Yet another Quiz question for the student pilots?

Question

§ 91.144 says the following, why is 31"hg ( mercury),

the limit of operations?

Temporary restriction on flight operations during abnormally high barometric pressure conditions.

(a) Special flight restrictions. When any information indicates that barometric pressure on the route of flight currently exceeds or will exceed 31 inches of mercury, no person may operate an aircraft or initiate a flight contrary to the requirements established by the Administrator and published in a Notice to Airmen issued under this section.

Answer 1

Because baro readings of 31.00 or more rarely occur, most altimeters are not calibrated( and some dont even permit you to go higher than 31'hg) to indicate accurate accurate altitudes above 31.00. This is especially important during instrument flying.

Answer 2

Sounds like Charles M has it.  

In Canada, things are different.

TC AIM 12.12

12.12.2 Flight Procedures

When the barometric pressure exceeds 31.00 inches of mercury, the following procedures take effect:

(a) Altimeters of all IFR, CVFR and VFR aircraft are to be set to 31.00 inches of mercury for enroute operations below 18 000 feet ASL. All pilots are to maintain this setting until beyond the area affected by the extreme high pressure or until reaching the final approach segment of an instrument approach for IFR aircraft or the final approach for VFR aircraft. At the beginning of the final approach segment, the current altimeter setting will be set by those aircraft capable of such a setting. Aircraft that are unable to set altimeter settings above 31.00 inches of mercury will retain a 31.00 inches of mercury setting throughout the entire approach. Aircraft on departure or missed approach will set 31.00 inches of mercury prior to reaching any mandatory or fix crossing altitude, or 1 500 feet AGL, whichever is lower.

(b) For aircraft operating IFR that are unable to set the current altimeter setting, the following restrictions apply:

(i) To determine the suitability of departure alternate aerodromes, destination aerodromes and destination alternate aerodromes, increase the ceiling requirements by 100 feet and visibility requirements by 1/4 SM for each 1/10 inch of mercury, or any portion thereof, over 31.00 inches of mercury. These adjusted values are then applied in accordance with the requirements of the applicable operating regulations and operations specifications.

Example:

Destination altimeter setting is 31.28 inches, ILS Decision Height (DH) is 250 feet (200-1/2). When flight planning, add 300-3/4 to the weather requirements, which would now become 500-11/4.

(ii) During the instrument approach, 31.00 inches of mercury will remain set. DH or Minimum Descent Altitude (MDA) will be deemed to have been reached when the published altitude is displayed on the altimeter.

NOTE: Although visibility is normally the limiting factor on an approach, pilots should be aware that when reaching DH, the aircraft will be higher than indicated by the altimeter, which in some cases could be as much as 300 feet higher.

(iii) Authorized CAT II and III ILS operations are not affected by the above restrictions.

(c) Night VFR pilots are advised that under conditions of altimeter settings above 31.00 inches of mercury and aircraft altimeters not capable of setting above 31.00 inches of mercury, the aircraft’s true altitude will be higher than the indicated altitude; this must be taken into consideration. If an instrument approach procedures is to be flown, the night VFR pilot should follow the procedures described in RAC 12.12.2(b)(ii).

(d) For aircraft with the capability of setting the current altimeter setting and operating into aerodromes with the capability of measuring the current altimeter setting, no additional restrictions apply.

(e) For aircraft operating VFR, no additional restrictions apply; however, extra diligence in flight planning and in operating in these conditions is essential.