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The ADDS Flight Path Tool - How to Use for IFR Go/No-Go Decisions

Early 2015, I found myself flying an Angelflight from KOAK to KSMO on an IFR flight plan in N2875Z, flying into deteriorating IMC conditions as I approached socal. KSMO was reporting VFR with 4200 ft ceilings and 4 miles visibility, but to get there, I had to fly through some solid IMC – mainly some stable clouds and drizzle conditions. It wouldn’t have to be a big deal, if it weren’t for a flight plan that called for a 10,000 ft MEA, where the temperature on our nearly 40 year old OAT indicator was at best .5 degrees above freezing, and numerous jet transports descending into the area were reporting moderate rhyme ice accumulations – something that would not be pleasant in our Archer, with limited power to stay aloft and nowhere to descend to. Though I never got any icing, I decided I needed a way to be able to predict and avoid this situation while planning an IFR flight.

Terrain, Clouds and Darkness

One of my CFIs taught me some simple wisdom about risk management for an instrument rated pilot in a single engine airplane: of the three main risk factors of terrain, clouds and darkness, accept only one at a time.

Intuitively, I think we all know this is probably a good idea: avoid night IMC, don’t fly through clouds over obscured high mountains and don’t fly over unpredictable mountains in the dark.

Obviously, there are some nuances, but the day of my Angelflight, I was clearly violating one of these rules and though never in real danger, I was clearly feeling the risk I was taking and wanted to find ways to minimize the probability of doing that again.

Aiming for the Top(s)

I decided that the situation would have been a lot less hairy had I been ‘on top’, looking down at an undercast and some mountain peeks sticking out. There would have been no risk of icing, the flying would have been a lot easier, and though an engine failure would still have caused an issue, but the probability in cruise of that happening (assuming plenty of fuel) are minuscule, and if it happened, I’d still be much rather on top than in a cloud…

So I started looking for usable ‘tops’ information. We have all been taught that tops information can only be obtained through the area forecast (somewhat) and through pilot reports, but that forecasting doesn’t typically provide that type of data. So this is when I discovered the ‘Flight Path Tool‘, available at ADDS.

The Flight Path Tool provide a wealth of information, including relative humidity for a cross section of your flight path, which is a very good predictor of the probability to encounter clouds. The Flight Path Tool runs on Java (so not on iPhones and iPads) and you have to download a Java run time environment, available free at Oracle.

The Flight Path Tool has supported me in several No Go decisions to Southern California on days that were technically VFR, but that I decided were too risky even for IFR. In those cases, the flight path cross-sections looked something like in figure 1. Other than what we may be used to, green in this diagram means ‘No Go’: the darker the green, the higher the relative humidity. The horizontal line is the 10,000ft line, and the reason this led to a No-Go decision is that in my airplane, there would be no way to climb above the cloud layer to get across the mountains. It’s a No-Go, even though all stations along the route are likely reporting VFR or MVFR conditions.

Figure 1. Flight path cross section KLHM to KSBA leading to No-Go decision

I made at three No-Go decisions that were based on the Flight Path Tool providing this type of a picture.

Than came an Angelflight opportunity last week from KSAC to KRNM (near San Diego). The weather forecast was not great – scattered layers between 3,000 and 3,500 and broken to overcasts layers around 4,500 to 5,000. Widely scattered rain showers and Isolated thunderstorms were in the forecast as well, mainly around my departure airports starting after my departure and ending before my return. The flight path tool showed something like what you see in Figure 2.

Figure 2: Flight Path from KSAC to KRNM supporting a Go decision

(BTW – I manually manipulated these to make it kind-of looks the way it looked then).

So based on Figure 2 and the rest of the forecast, I decided:

§ I could climb above the cloud layer – predicting that I would break out to blue skies at about 8000 ft

§ In case of emergency, I could easily descend and break out with plenty of time to pick a landing spot.

§ The approach into Ramona should be easy.

§ On top, I could spot build-ups and circumnavigate the widely scattered showers and isolated storms.

It worked out pretty much like that. Because of prevailing winds, we flew the first 90 minutes or so below the clouds, cutting through some scattered cumulus, but mostly in the clear with a slight tail wind. Then we climbed through a layer to 11,000, breaking out at 7,800 (but now with a 20kts head wind. We cruised above the clouds and could see all kinds of holes where they would have reported ‘broken’ clouds or better on the ground. The approach into Ramona was easy, and we circled and landed with no incidents.

Figure 3. The clouds look a lot friendlier from the top

The way home was the reverse, but there was a bit more build-up.

Garmin Pilot gave me confidence that the path back would keep me clear of the worst rain:

Figure 4 – Garmin Pilot with GDL39 connected

For the final stretch, ATC provided vectors keeping me away from the heaviest precipitation still in the area, and in the end, I broke out at about 3000 ft in light rain at 10 miles visibility.

How to Use the Flight Path Tool

1. Install Java on your PC

2. Launch the Flight Path Tool

3. Work through a number of warnings to actually download, run and load the file (note: Java programs have unlimited access to your PC once they run, so you have to be careful what you load and run).

4. Choose ‘Relative Humidity’ by checking that box at the bottom

Figure 5 – Flight Path Tool Start Screen, check Relative Humidity

Figure 6. Relative Humidity Selected

5. Enter a flight plan

Figure 7 – Enter Flight Plan

6. Move your altitude line to your cruising altitude. Remember: Green is No-Go…

Figure 8: Flight Path from KSAC to KRNM supporting a Go decision under IFR

7. Enjoy

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