IFR Fix: The RASS factor

October 28, 2013

It’s been a long cruise in clouds and dark. Now you rally your concentration for the arrival as air traffic control clears you for the approach to the nontowered airport and terminates radar service.

Before switching over to the destination’s common traffic advisory frequency, you dial up the airport’s automated weather broadcast for an update on wind, ceiling, and the local altimeter setting.

That’s strange.  The weather broadcast should be audible by now. And at this hour, there’s probably no one at the FBO. Great time for a glitch to show up; this is the kind of stunt your instrument instructor would have pulled.

The CFII isn’t along for this ride, but the lessons of those artificially glitchy flights have stayed with you. Now you will have to use a secondary source of altimeter information—and that may mean a higher minimum descent altitude, possibly high enough to make the expected easy approach a closer call.

Checking the approach plate, you see that you will be setting your altimeter to an airport 45 nautical miles away; the note in the upper left corner of the plate says to "increase all MDAs 140 feet."

That seems like a lot. How much would the MDA increase if the remote altimeter setting source (RASS) were closer—say about 30 miles distant?

Surprise: You would increase your minimums 460 feet when shooting a localizer approach to the Mercer County, W.Va., airport, using the altimeter setting at Beckley, 29 miles north. But only increase them 60 feet on this approach to Old Town, Maine, with a RASS from nearby Hancock County-Bar Harbor Airport.

That’s because distance isn’t the only factor the FAA uses to formulate what it calls the RASS adjustment. Another variable in the RASS calculation is the difference in elevation between the airport and the RASS.  

In locations where the intervening terrain “adversely influences atmospheric pressure patterns,” the basic RASS-adjustment formula is further modified. (See page 3-10 of the United States Standard for Terminal Instrument Procedures publication, or TERPS.)

The RASS may be from 5 nautical miles to 75 nm from the airport, with a maximum elevation difference between the RASS site and the landing site of 6,000 feet.

Higher minimums are never good news, but certainly preferable to a chart note that says, "When local altimeter not received, procedure not authorized."