This is an article about preparing for flight in Instrument Meteorological Conditions (IMC) as opposed to Instrument Flight Rules (IFR). Obviously if we're flying in IMC we're going to be IFR (you want to fly IMC in Class G, except in transition, be my guest). The contents is mainly a discussion about how to go about the decision on whether to fly or not. It's intended for General Aviation pilots flying General Aviation aircraft without radar or de-ice equipment.
We're not going to discuss making sure the aircraft is ready for flight. No discussion of inspections or checking the lights or the pitot heat. We're going to assume we have an aircraft ready in all respects.
We're not going to discuss DPs or STARs or Approach Plates or the lost communication rules or what charts you should have or how you should decide if you, the pilot, are ready to fly. We're going to assume that's all sorted out. No discussion of 1-2-3 rules, we assume you know when you need an alternate and what the rules for picking one mean.
We're not going to discuss Personal Minimums, although they are certainly important. You should have a personal set of minimums and you should apply them as you work through the pre-flight planning process.
There are really 4 crucially important factors we need to check before IMC flight.
We can't fly through thunderstorms so we have to fly around them. If we can't see the thunderstorms then we can't avoid them. So what are our options? Some of our aircraft may be equipped with either weather radar or lightning detection systems or both. These days weather radar may be directly installed in the plane or it may be an uplink or downlink to specialized avionics in the plane.
If you have thunderstorm detection equipment and you know how to use it and are confident you can interpret the display(s) then the pre-flight decision comes down to deciding if there will be a way to thread your way through the unseen storms. Mostly we can pick our way around cells, but if thunderstorms are forming in lines we may not be able to get around them. We can certainly use our detection equipment to fly up to the impassable weather, land and wait it out.
What about the rest of us with nothing or those people who can't quite place their full faith and trust in a box of electronics? Well we have to be able to see thunderstorms, and that means the Mark I eyeball. We either need to stay below the clouds or climb above them in order to be sure that we can see the towering cumulus that marks a thunderstorm. Thunderstorms make VFR pilots of us all. That doesn't mean that the Instrument rating isn't useful if thunderstorms are around, but we can't spend too much time in the clouds if we need to see the weather.
First of all, beware of any indications of embedded thunderstorms. If you'll be flying in an area of embedded thunderstorms you won't be able to see them. This may be you no-go decision maker.
Another strategy may to make shorter flights. Instead of launching on a 4 hour marathon plan a 1 hour flight. If the thunderstorm activity is reasonable predictable you may be able to brief yourself at the airport (FSS or some kind of weather terminal) and make a one hour flight with a reasonable certainty that thunderstorms aren't going to be a problem. Then you land, re-brief on the situation and decide if you want to continue. This is certainly one form of the continue/no-go decision process. We're not making a wholesale "go" decision, we've just decided it's not bad enough to stop yet.
In the end you don't want to tangle with thunderstorms. If you can't see them or be reasonably sure they won't be in your path just wait. Thunderstorms rarely hang around long, in a few hours the squall line will have passed or the heat of the day will take the punch out of clusters of storms in your path.
IFR flight takes a lot of concentration. Even in smooth air you'll find yourself tired at the end of a long day of IFR. Add in a day of moderate turbulence and you may find it's more than you can take. So for turbulence we need to consider the both the issues of airframe damage as well as pilot fatigue. The existence of an auto-pilot, and it's capabilities may influence you decision, but make sure that you have a plan if the auto-pilot quits on you.
Look for information on both forecast and reported turbulence and decide if you can handle a difficult approach in lousy weather with high winds at the end of a flight full of teeth banging turbulence.
Ice is one of those difficult things to predict. While forecasting is getting better the icing Airmet will tend to be widespread and not very precise. The question we have to answer is can we plan the flight to avoid icing and, if we encounter icing, can we find a way out.
One form of icing you can't really deal with is freezing rain. It's quick, rapid accumulation will bring on insurmountable problems too fast to deal with them. When warm air overruns cold air and there is precipitation getting ready to fall it's not going to be good flying weather, IFR or VFR.
Icing occurs in visible moisture, so one way to avoid icing is to stay out of the clouds (and the freezing rain). If the ceilings are high enough we may be able to plan the flight without climbing into the clouds. If the tops are low enough we may be able to climb up through the clouds, but beware the "on top" trap. It may be difficult to determine where the tops are and if you pick up any ice on the climb it's going to slow your climb rate exposing you to icing for longer and that slows your climb rate, well, you can see where this is going. Unless tops are known to be low or you have the ability to get up high (turbo-charging) then climbing on top may not be an option.
Even if we can't stay out of the clouds we may still be able to stay low enough to be below the freezing level, in the clouds and free from icing. The icing Airmet will give an estimate of the freezing levels, the Winds Aloft forecast will provide temperature information and Pilot Reports are also a great source of icing information and temperatures aloft.
If you fly in clouds close to the predicted freezing level you may encounter ice anyway. If you do you need an exit plan you can execute immediately. Conventional wisdom says that upon encountering icing you must do something. Climb, descend or change directions. As we've just discussed climbing may not be an option, if tops aren't low and you can't climb high or quickly then icing may overwhelm you long before you can reach an altitude where it's too cold for ice. Keep in mind that the tops of the clouds may be where the worse icing is found, so you may just be climbing into more trouble. Descending is an option if you have room above the MEA to get down and preferably get out of the clouds. Turning around might be your best option. After all, icing just started, it wasn't icing behind you, make a 180 and figure out an alternative plan.
So in general we need to know if we can conduct the flight below the freezing level and preferably clear of clouds. Of course for people who habitually fly in locations where the surface temperature is so cold that icing is not a possibility, different strategies apply.
Under 14 CFR Part 91 we don't need no stinking takeoff minimums. Many pilots decide that they'd like some kind of takeoff minimums and, since our takeoff minimums are based on our ability to fly an arrival back to the place we started from, takeoff minimum issues are the same as arrival minimums for the basis of this discussion.
Is there sufficient visibility and ceiling for you to be able to land if you need to? If there are the required minimums, how confident are you that they will really exist at the time of your takeoff or arrival? If the forecast is for 200-1/2 and you're comfortable with those numbers you may not be comfortable with 100-1/2 when you arrive. With a forecast of 200-1/2 there is no room for error in the forecast. That may be OK, but be sure you know how much room you have for error and watch the weather reports for changes. Better to make an early decision to go to an alternate than to slog all the way to your destination and find out that you can't get into the airport.
Of special note in takeoff and arrival minimums should be fog. Fog can form quickly and unpredictably and take visibility well below minimums for long periods of time over a widespread area. If you plan to arrive at 9:00AM and the forecast says the fog will be gone by 8:00AM make sure you have a GOOD alternate plan, because the fog may linger in a lot longer than expected.
Before you launch on your IMC flight you need to have plans for:
Small General Aviation aircraft are not all weather aircraft. An instrument equipped airplane and an instrument current pilot have more options than a VFR pilot but the instrument pilot may still have to decided that, sometimes, the weather is just not good enough for flight and a trip needs to be delayed or cancelled.