Monday, November 2, 2009

Airport Micro-climates

During the first part of last Friday's flight from KBDR down to KVKX there was great VMC on top and a lovely sunset to enjoy.

It was quite warm (about 15 degrees C at 8,000 feet MSL) and there was a strong southerly flow at lower altitudes ahead of the advancing cold front. Most stations below that lower layer were reporting ceilings between 2000 and 3000 MSL.

Sunset happened about 2205Z and by the time I was over Baltimore it was dark. The usual routine is to cross KBWI at 6,000 (headed direct to OTT). ATC will give me a descent to 4,000 a few minutes after crossing over their major airport. So I was not surprised to hear: "Skylane 31 Sierra, descend and maintain 4,000. No weather or traffic information available at Potomac; Washington National is reporting overcast 2,700, wind 170 at 10. Expect the visual approach at VKX."

I expected that, but I wasn't entirely happy with it. Washington National (KDCA) is 7.5 miles NNW of Potomac Airfield. Controllers will usually look at what's happening at DCA and surmise that things can't be too different over at KVKX. I have learned that DCA weather is a poor predictor of conditions at KVKX. So, I responded to the nice Potomac Approach controller: "Skylane 31 Sierra, thanks for that weather, could you please tell me the current wind and ceiling at Andrews?" That got me, "Sure. Stand by."

I already knew the answers, courtesy of XM Weather on the Garmin GPSmap 396. Andrews AFB is 5.5 miles northeast of KVKX and has weather that is much more likely than DCA's to be consistent with conditions at KVKX. KADW was reporting a ceiling of 1,900 feet MSL. The Minimum Vectoring Altitude between OTT and KVKX is 1,700. Not great for trying the visual approach.

The next transmission from the controller was, "Skylane 31 Sierra, Andrews is reporting a 1,900 foot overcast, wind 160 at 8. Ummm, we'll get you down to 1,700 down there and if you don't have good ground contact we'll run you out for the approach."

My answer was, "That sounds good to me, 31 Sierra."

Just before reaching OTT I was cleared to descend to 2,000 feet MSL, and the controller asked for flight conditions. "In and out of the bases," was the response. Then he asked, "I'll get you down to 1,700 in a mile or two - do you still want to try the visual?"

"I'd like the RNAV 6 approach to VKX, please," was the obvious decision. Twenty minutes later, after a routine approach (see below), I was on the ground at home.

Airports all have their own little micro-climates. Even a little local knowledge can be very helpful. The IFR system offers a full toolkit for these kinds of situations and one might as well use it.

Courtesy of FlightAware, here's the track for the 2.9 hour flight:

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