Now that you've taken time to review the various documents available from "I'd rather RAAS," you may be wondering which remote altimeter is best to select as a Remote Altimeter Setting Source (RASS).

First of all, alternate minima are carefully calculated, based upon formula found in Chapter 3 "Takeoff and Landing Minimums" of FAA Order 8260.3 "United States Standard for Terminal Instrument Procedures (TERPS). Minimum requirements state that the remote altimeter setting source not be at a distance greater than 75 nautical miles, or for an elevation differential between the RASS and the landing area that is greater than 6,000 feet.

At first thought, one might think it best to simply utilize the nearest available altimeter setting, so as to achieve the lowest descent minima. However, that is not the intent here. What we wish for is a robust aviation infrastructure, wherein an unplanned AWOS/ASOS outage does not result in instrument approaches becoming NOT AUTHORIZED. So as to achieve this, we need to select an alternate altimeter that will ensure the highest degree of availability. It is my opinion that the remote altimeter setting source selection should be one that is...

1) Redundant. (i.e. based upon an airport that has two altimeter setting instruments, like an ASOS and a Digital Altimeter Setting Instrument (DASI). This equipment is found at most major airports.

2) Full-time. (i.e. from an airport that has 24/7 ATC service).

3) Immediately and always available. The simple formula for this is to ensure that whichever ATC facility may be issuing the approach clearance to the pilot, that they always have instant access to the RASS altimeter setting.