IFR Fix: Beating the 24-second clock

February 1, 2013

Here’s the plan. Fly the ILS RWY 5 approach to Lawrence (Mass.) Municipal Airport via the transition from the BEDDS compass locator. Descend to minimums on the ILS, and then fly the published miss. Stay sharp! Things happen fast on the miss. It’s a brief 2.3-mile climbing leg to the Lawrence VOR followed by a continuing climb to 2,000 feet while entering holding.

That’s a great workout: Track an NDB bearing, then the localizer, and then the VOR—all within a compact block of airspace, never higher than 2,000 feet, and most important, never below 370 feet.

Sure, there’s a catch: Somewhere inside WOBMU you will become distracted—your right-seater will see to that—and you’ll only have 24 seconds to undistract yourself, or the game is up.

Unfair? Consider that descending at 500 feet per minute, a pilot reaching typical ILS minimums would have 24 seconds to avoid hitting the ground. This assumes that the aircraft’s altimeter is precisely set, that the altimeter setting is not obsolete; and that the altimeter’s indication doesn’t contain the 75-foot error sometimes described as an acceptable performance.

Assuming accuracy there, the Lawrence ILS minimums will deliver your aircraft to 236 feet above the threshold elevation (THRE) of 134 feet. (Note a 14-foot-difference between the THRE and airport elevation.)

Distractions aside, at many airports another limitation on accurate determination of height above ground arises when the tower is closed or the local altimeter setting is unavailable. Then, IAPs may require you to use a remote airport’s altimeter setting, increase minimum altitudes, let’s say by 40 feet, and increase minimum visibility.

How far away is that other airport, and on what side of the weather-maker does it lie? Good questions to answer before flight.

When a strong frontal system passed through the Northeast on the afternoon of Jan. 31, an aircraft transitioning via BEDDS might have had its altimeter set to 29.23 inches of mercury. Lawrence’s altimeter setting, simultaneously, was 29.21 inches. Only 24 miles away but closer to the low, Portsmouth International Airport registered 29.15 inches. Peak wind gusts measured a thrashy 33 knots.

Distractions, estimated values, erroneous indications, and changing conditions all vindicate the zero-tolerance policy for descending below minimums without a runway in sight. 

Which it isn’t, so let’s get on with that missed approach!