Sunday, July 22, 2012

Sufficiently Interesting

My friend Craig from Oregon sort of wondered, in a recent comment, where N631S and I have been. He said: "FlightAware says N631S has been flying. Any details to share?"

I responded to the effect that "the flying has been pretty uneventful (i.e., not great blogfodder)" and in fact it's been six or seven weeks of pretty routine back and forth between Potomac Airfield (KVKX) and Bridgeport's Sikorsky Memorial (KBDR). However, this past Friday's southbound trip was interesting enough to talk about.

It was clear from the beginning that, at the least, we were going to get wet. The early chart from NCEP valid for 18Z showed a double-barreled surface low over southern Pennsylvania and Maryland with a stationary front trailing off to the east-southeast and widespread showers.

The Terminal Area Forecasts issued at 12Z looked pretty reasonable:

KBDR 201120Z 2012/2112 09013KT 6SM -SHRA SCT020 OVC050 
     FM201500 08014KT 4SM -RA BR OVC020 
     FM202300 07012KT 5SM -DZ SCT020 OVC025 
     FM210400 05010KT P6SM OVC035
KDCA 201125Z 2012/2112 33006KT P6SM BKN015 OVC060 
     FM201600 VRB05KT P6SM VCSH FEW030 BKN040 
     FM202000 31008KT P6SM VCTS BKN040CB 
     FM210300 03006KT 4SM BR VCSH SCT015 BKN030 
     FM210500 01013KT 4SM BR SCT009 BKN015
The forecast for a late-afternoon departure from KBDR was fine – four miles visibility in light rain and mist, moderate northeast wind and a 2,000 foot overcast. Nothing to be concerned about.

The arrival forecast for the Washington area (using KDCA) had something that would bear watching. From 4:00 PM local time, it anticipated thunder-showers in the vicinity and a broken ceiling of cumulonimbus clouds at 4,000 feet. Supporting that, the mid-morning Collaborative Convective Forecast (CCFP) map valid for 21Z showed widespread sparse convective activity, predicted with low confidence (the gray stuff) but with areas of higher confidence (the blue stuff) located where it might prove interesting to me.

I talked about that "VCTS" tag in the TAF back in May. It seems to follow from a forecast for scattered or sparse convective activity and is flyable with good on-board weather depiction and basic visual conditions. I'd have both, so the VCTS wasn't a "no go" item, but it did demand respect.

So, I was good to go but I continued to keep a watchful eye on the evolving forecast. And I was pleased to see the 18Z TAF's. Neither the forecast nor the view out of my office window had changed much regarding my departure, but things were looking up for arrival.

KDCA 201733Z 2018/2118 32007KT 6SM -RA BKN010 BKN035 OVC100 
     FM202000 36010KT P6SM BKN025 OVC150 
     FM210200 02012KT 5SM -SHRA VCTS BKN015 OVC040CB 
     FM210600 01008KT 4SM BR BKN007 
     FM211600 02007KT P6SM SCT007 BKN015
Basically, the forecast thunder-showers had been pushed out to later in the evening, well after my ETA. That was a nice piece of news. So, shortly thereafter, off to the airport!

N631S and I took off in light rain, but it was clear that the weather was breaking up quickly. After departing Runway 6 at KBDR and turning to the north, I was able to snap the picture at left showing what amounts to visual conditions over the Housatonic River. There were clouds and bursts of precipitation during the initial climb to 6,000 feet but it was the ragged edge of the weather. Just a few seconds later I captured the NEXRAD screen below.

This image makes it clear that there would not be much precipitation to deal with, at least for the first half of the trip. Of course, NEXRAD doesn't show clouds. Upon reaching the Hudson River, ATC asked for a climb to 8,000 feet. That put N631S into the clouds and there we stayed for most of the flight.

Having gotten the departure under control, it was time to look at how the weather was developing at the destination. I panned the NEXRAD display down to the south and got this:

Here too, the precipitation seemed to be departing to the east, and there was no sign of the convective activity that had been in the earlier forecast. Things were definitely looking good. The next step was to check recent METAR's for airports near KVKX to get an idea of current conditions. Early in the flight, here's what Washington National, Andrews, and Davison Army Airfield were reporting:
METAR KDCA 201952Z 35005KT 10SM -RA BKN017 BKN023 OVC040
           27/22 A2992 RMK AO2 SLP130 P0000 T02720217=

METAR KADW 201955Z AUTO 06007KT 7SM -RA BKN007 OVC015 24/23
           A2992 RMK AO2 RAB1941DZE1941 CIG 006V008 SLP131
           P0001 T02410232 $=

METAR KDAA 201955Z AUTO 29004KT 10SM -RA CLR 27/21 A2992
           RMK AO2 RAB1950 SLP132 P0000 T02690214=
The most interesting part of that was, of course, the marginal weather at Andrews AFB. Only about four miles from KVKX, Andrews is a better predictor of conditions at my home field than Washington National. Andrews had good visibility in light rain, but a broken ceiling at 700 feet. That's pretty low! It was time to break out the approach plate for the RNAV Rwy 6 instrument procedure at KVKX and hope that conditions would be improved on arrival.

The Minimum Descent Altitude (MDA) for the approach to Runway 6 at KVKX is 680 feet MSL – 562 feet above the touch-down zone elevation (TDZE). A 700 foot ceiling cuts things fairly thin. One piece of good news was that the METAR's indicated winds favoring Runway 6. I would not need to worry about circling to Runway 24. The MDA for a circling approach is 60 feet higher, which would cut the margin further.

Just to make things a bit more entertaining, I hadn't flown the RNAV approach into KVKX since March. I'm current, but all my recent approaches have been at Bridgeport.

The flight continued smoothly and uneventfully past Allentown and Lancaster, and down to Baltimore. Passing Baltimore I rechecked the conditions at Andrews. Not much had changed:

METAR KADW 202155Z AUTO 04006KT 10SM OVC007 23/22 A2992 RMK
           AO2 RAB2102E212136DZE2102B2136E2138 SLP131 P0004
The rain had ended and visibility was a bit improved. The ceiling, still at 700 feet AGL, was now a solid overcast rather than broken. The recorded ATIS at Potomac Airfield (KVKX) was reporting winds five knots at 080. It doesn't report ceiling.

ATC asked my intentions and I said I'd like the RNAV 6 approach. "You can expect that," was the reply. I planned to shoot the approach, and if I had to miss, I'd go around and try one more time. A second miss and I'd divert to Manassas. I have a rule about never shooting an approach a third time.

Potomac Approach vectored N631S and me to the southwest and brought us down to 2,000 feet. Then we were cleared direct to WOBUB, the Initial Approach Fix (IAF), to begin the instrument approach. This picture, approaching WOBUB, looks off to the northeast toward KVKX, about 10 miles away.

The approach was routine and we broke out of the clouds at 800 feet, two to three miles from the threshold. It was nice to land and put N631S away, but if the cloud bases had been 200 feet lower the tale would have gotten longer and more interesting. I thought it was sufficiently interesting as it was.


Gary said...

Good post!

Good instrument pilots plan and plan some more. I have the same rule for shooting approaches, no sense raising the strsss level and forcing the situation.

Good deal on having the "plan B" and walking the readers through the process. I like to learn something from everyone I fly with, at least a good review on the process. In your case Frank, I learn from reading. Thanks!

Frank Van Haste said...

Gary, I'm glad you found it interesting.

There's an old military adage that says, "No battle plan ever survived contact with the enemy." I think that analogously, we might say, "No IFR flight plan ever survived encountering the weather." We have to distrust forecasts and stay adaptable.

Best regards,