The go/no-go decision is, of course, a matter of pilot judgment. While weather conditions are important ingredients in the decision, they're not the only considerations. The other variables are the pilot, the aircraft and its equipment, and the geography along the proposed flight path.
First, the weather. We all know it's changeable; we all know that forecasts can be incorrect; and we all know that we should be prepared for unanticipated adverse weather by constantly having an alternate airport in mind. But each weather situation is different. The go/no-go decision can be clear-cut, in which case the decision is easily made, or quite indefinite, in which case the decision can be stressful.
Let's take a clear-cut situation: A convective sigmet for a widespread area of moderate icing is forecast along your route of flight. Your route takes you near a center of low pressure, so there's plenty of lifting, which means tops are apt to be high. There's plenty of moisture in the air, as shown by close temperature-dew point spreads at stations along the route, so cloud bases are low. There are mountains along the route, which always make for the worst icing conditions. All of this bad news is backed up by a slew of pireps confirming the accuracy of the reports and forecasts. Here, the decision is easy: Don't go.
But what about situations where things aren't so definite? Let's say an icing scenario involves cloud tops reported at a relatively low altitude, and there are no pireps confirming actual icing encounters. The cloud bases are right at the minimum en route altitude (MEA), which would keep you clear of any high terrain. Here, icing would seem to be less of a factor-but a possibility nonetheless. You might be able to climb to ice-free conditions on top or scoot beneath the cloud deck. It's a setting where you'd definitely want a safe alternate plan of action ready in case things turn sour and your pirep is the first of the day.
For another example, let's say that a center of high pressure has moved over your proposed route. Ordinarily, high pressure means good weather, but what if overnight temperatures drop to the dew point and dense fog forms in valleys, and your destination airport is next to a river, surrounded by high ridges? The forecast calls for the fog to burn off, but at daybreak your destination is zero-zero in fog. It's wintertime, so it's been a long, cold night, and the sun won't burn so high in the sky as the day progresses. Will the fog really burn off by the time you plan to arrive? That may be difficult to predict, even for an experienced meteorologist, so this situation is a tougher call.
We could list an infinite number of these kinds of tough-call weather scenarios, but in the end, the go/no-go decision should depend on the variables mentioned earlier. Let's go back to the icing situations. OK, in the first case the weather is confirmed bad. Now what about you, the pilot? (We'll presume that you already meet the I'M SAFE launch criteria: free of illness, medication, stress, alcohol, fatigue, or emotional upset). Are you instrument-rated, current on instruments, and experienced and comfortable flying in those conditions? If so, fine. In the second case, conditions seem more favorable. If the pilot's instrument-current and has planned an escape route, the flight may not be unduly complicated-if all other available sources of information indicate a go.
What about the aircraft? Is it approved for flight in known icing? Does it have the kind of performance that will let you climb above the clouds? Does it have the necessary system redundancies to let you continue flying safely in the event of a component failure? Let's say your aircraft doesn't have these requisites. In this case, end of discussion. It's a no-go for the first icing scenario. But in the second situation, the airplane may-may-be suitable for the flight.
As for geography, we know it's mountainous in the first case, so that earns another no-go vote. The pilot's good, but the weather, aircraft, and geography argue for waiting. In the second icing case, flight at the MEA might provide an adequate margin against flying into any ice-laden clouds. But if the airplane does enter clouds, and does ice up, the quality of the terrain below will have everything to do with the go/no-go decision. If it's flat and the minimum obstruction clearance altitude permits a descent, this could be a way out of the situation-especially if above-freezing temperatures exist below. If there is high terrain below, play it safe, and just say no.
In our valley fog situation, let's say that you're not an instrument-rated pilot, don't have a whole lot of cross-country time, and are unfamiliar with the destination airport. The airplane is well-equipped for instrument flying but you aren't, and the weather may or may not be VFR at the time of arrival. The fact that the airport is in a valley and next to a river argues for a lingering fog, so it could be a dicey situation. Shadows cast by the ridges could keep temperatures in the valley low enough to stay near the dew point. Should you plan on a morning arrival? You can call the local FBO at your destination and ask what the usual fog experience is, but each situation is different. Besides, the person at the other end of the phone may be totally unqualified to provide forecasts.
Another option could be to locate a nearby fog-free airport, land there, and wait out the fog at the destination. But if it's your first trip to the area, locating and landing at an alternate airport can add to the stress level. What to do? It's up to you, and the decision can be agony.
Will the fog burn off in time? Will you recognize the airport surroundings if it's VFR but there are patches of low clouds nearby? Should you just set up a waypoint and circle the area VFR, above any fog layer, until it lifts? What if it doesn't lift? Will you have enough fuel to go to an alternate with good VFR?
As with anything connected to weather, the questions could go on and on, and there will be few definite answers. I recall one pilot who cornered me after receiving a briefing that forecast the possibility of instrument weather at his proposed destination. He has an instrument rating but wasn't current at the time. Frustrated by a briefer's equivocations and the all-too-standard "VFR not recommended" admonition, he demanded, "Just tell me. Can I get through there VFR? I want the truth!" He wanted a definite answer to a very indefinite situation.
Another pilot, a student about to go on a cross-country, called me after performing a meticulous job of preflight planning. She was concerned about the weather briefing, which mentioned turbulence and strong surface winds. "I know it's going to be good VFR and all," she said, "but I just don't feel good about it." There must be a reason why you feel this way, I recall telling her, and that reason must be a good one if you've taken the time to call and express your thoughts. Ultimately, she called off the flight, as did the frustrated, non-current instrument pilot.
They both made the right decisions. Their doubts showed that their judgmental skills were alive and kicking. If everyone followed their example, we'd have fewer weather-related accidents. It's the ones who don't question, who don't feel self-doubt, who are most apt to get in over their heads.
So before answering the question "to go or not to go?" think long and hard about the weather at hand, as well as the skills and tools that you have to deal with it. More often than not, you'll see that if you have to ask the question, you already know the answer.
