You are planning your IFR flight but your destination airport is forecasting 1,000-foot ceilings with three statute miles due to a thunderstorm in the vicinity. According to FAR 91.169,
- For aircraft other than helicopters. For at least 1 hour before and for 1 hour after the estimated time of arrival, the ceiling will be at least 2,000 feet above the airport elevation and the visibility will be at least 3 statute miles”.
You conclude that your best decision would be to file an alternate airport. Which one do you choose? Here are a few tips to think about in picking the right alternate airports for your IFR flight plan.
What type of storm is it and what direction is it moving?
Believe it or not, knowing the type of storms in the vicinity could be very important to your alternate airport. A few different storm types include single-cell, multi-cell, supercell, frontal thunderstorms, and squall lines. Particular traits such as the duration of the storm, how fast it moves, the environmental instability that will support vertical development, and the length of the storm are all factors to consider. The last thing you want to do is pick an alternate airport that is in the direction of the moving storm. The regulations suggest that you remain 20 NM away from the storm as hail, lighting, and other factors could put you in greater danger.
Does this alternate airport have precision, non-precision, or no instrument approaches?
Depending upon the type of approaches available at the alternate airport, you need to ensure you have the minimum weather at the estimated time of arrival. For an airport with a precision approach procedure, you need a minimum ceiling of 600 feet and two statute miles of visibility to file it. For a non-precision approach procedure, you need a ceiling of 800 feet and two statute miles of visibility. Lastly, for an alternate airport without an instrument approach available, the ceilings and visibility minima must allow you to descend from the MEA, approach, and land under basic VFR conditions. In some cases, the 200 feet of lower guidance from a precision approach can be the determining factor of if you can land or not. Choose an airport that will give you the best chance at landing. Be careful to observe nonstandard alternate minimums and if that airport can be used as an alternate altogether.
Fuel on board (FOB) and other factors that will affect your alternate choice.
It is important to pick an alternate airport that allows you to fly to the first airport of intended landing, fly from that airport to the alternate airport, and still fly after for 45 minutes at a normal cruising speed. Make sure to calculate FOB because the airports that will permit you the diversion may be specific to the aircraft’s performance.
Additionally, look at your alternate airport’s runway surface type, lengths, slope, and services provided. Calculate runway takeoff and landing distances for that airport and assure that your calculations are feasible for your payload and aircrafts specific performance. Do remember that your airfield conditions (wet runway) will adversely affect your landing and takeoff distances. The slope of the runway can also provide benefits and disadvantages to your performance. Just remember that for the most favorable performance, you want to takeoff downhill and land uphill. Lastly, make sure to choose an airport that provides you with necessary services such as fuel, hangar space, maintenance, etc.
There are a lot of things to consider when choosing an alternate airport. One thing to remember is by taking time to consider several different factors in picking a good alternate during your preflight planning, you can establish early threat and error management before you are airborne. Stay ahead of every flight!