Saturday, August 20, 2011

The Mark I Eyeball

In writing posts for this blog about my weather flying experiences, I've commented favorably on more than one occasion about the utility of weather displays in the airplane. NEXRAD, XM Weather, that sort of thing. I have been, and I remain, convinced that these displays are of remarkable value in dealing with the challenges we encounter when we choose to commit aviation at times and places where the atmosphere is grumpy.

Two flights in the last week have reminded me of how vital it is to stay aware of the limitations of NEXRAD displays and to always remember that they are only one voice in the chorus, one instrument in the band. Depending on circumstances, other inputs may be more important and more informative. All sources of information have to be integrated to arrive at the best decision. You have to meld information from the controller's ASR radar display and input from the view through the windscreen with the picture on the XM Weather display to get the best image of what is happening. Last Monday's flight from Potomac Airfield (KVKX) up to Bridgeport (KBDR) was illustrative.

It wasn't long after departure from KVKX until N631S and I started to see some weather. We were at 7,000 feet, talking to Potomac Approach. There was a fairly energetic cell to the north near Baltimore, and a lesser patch of weather to the south. No problem navigating between them. But it was interesting to correlate the view through the windscreen with the XM Weather display on the Garmin GPSmap 396.

The view at left is out to the right, observing the less energetic but closer patch of weather seen above. It's still fairly benign, although the cumulus core is building. That cell may have gotten a lot more interesting over the next twenty minutes. Bear in mind that this is happening at 7:30 AM Eastern time! Not all convective weather happens in the afternoon. But we flew between the cells with plenty of clearance and pressed on toward New Jersey, soon to be handed off to Dover Approach.

Once past that first bit of weather I zoomed out on the NEXRAD display to see what was cooking up ahead. The situation in the area of the LEEAH intersection looked interesting! Normally, N631S and I stay with Dover as we cross the Delaware Bay and get handed off to Atlantic City Approach as we cross the New Jersey shoreline. I asked the Dover controller to give Atlantic City a "heads-up" that I'd be welcoming a deviation to the left of course approaching LEEAH.

I checked in with Atlantic City and talked to the controller about what he was seeing on his radar. I wanted to turn to the northeast, toward the Coyle VOR (CYN) at a point that would minimize my exposure to the two cells up ahead. A few miles before reaching LEEAH I asked for "direct Coyle", which was approved. That flight path is shown above left. As I skirted the first cell off the right wing, N631S and I experienced some moderate precipitation and some light, to occasional moderate turbulence. Nothing scary! Looking to the left, the Mark I Eyeball reported brighter conditions – I could have deviated to the left if necessary, although that would have required some negotiation with Philly Approach. Flight conditions in the second patch of weather, east of VCN, actually were calmer with only light precipitation and minimal turbulence.

N631S and I broke out into fairly clear skies and continued north toward JFK. A look ahead indicated that only light precipitation was to be expected over Long Island and near Bridgeport. In fact, Bridgeport was still reporting VFR conditions!

The screen shot at left, taken soon after crossing over JFK, shows widespread light precipitation. You would think that N631S and I would be getting wet at that point, but that wasn't the case. The NEXRAD composite radar image can't indicate conditions at any specific altitude.

This screen shot (left) was taken just seconds after the previous one, and shows what N631S and I were actually flying through. We were at 3,000 feet at the time, with no precipitation, no turbulence and reasonable flight visibility. Again, the Mark I Eyeball tells a different – probably more reliable – story.

By the time we got near Bridgeport the ceiling had lowered to 800 feet, so it was necessary to fly the ILS approach to Runway 6. That, and the landing, were uneventful.

Looking back, the flight had relied on information from the NEXRAD display (XM Weather) for the "big picture" and weather avoidance strategy; and on the Approach Radar's excellent real-time information to guide the penetration of moderate adverse weather. And, reference to visual inputs to maintain a real-world picture of what N631S was flying through.

In the next post here, I'll share some details on Friday's flight from Bridgeport back to the DC area – where information from the Mark I Eyeball became the primary input needed for successful completion of the flight on a boisterous evening.


Gary said...

Great post!

I agree, the xm wx is just one piece of the puzzle. Working the system and all the players is what makes for that safe flight. I often ask what control has painted on their screen just to confirm what I see and what the xm is showing me at the time. I still scroll through the map page and bring up the METARs or the flight plan page and check the wx at each location ahead. I’m looking forward to the return trip post. I enjoy looking at the 396 pictures and in fact have started to add them to my blog posts, it adds a bit of realism to what we get to see and now can pass on to our readers.

Frank Van Haste said...

Thanks for commenting, Gary. I agree that approach radar is a great resource and the controllers seem to be very well trained in exploiting its capability. When you get "up close and personal" with the weather you can't afford the latency in NEXRAD. The info from Approach is near-real-time.