Anyone who has been through instrument training has had to memorize the requirements for alternates, but unfortunately the emphasis is on remembering the numbers rather than on the importance of the concept. I learned this the hard way one time, on a beautiful VFR trip.
We had flown cross-country for nearly four hours to the destination resort airport. The weather was absolutely gorgeous along the entire route, typical for a nice summer morning in the Rocky Mountain West. The FAA requirement was just a 30-minute fuel reserve under these conditions, but I try to never shave my fuel that close. When I started the trip, I had nearly an hour of fuel reserve.
When we were 25 miles from the destination, I tuned into the ATIS and then changed over to the tower frequency. At the time, the tower was trying to communicate with an aircraft that was having some electrical problems that were affecting the aircrafts flaps and landing gear. The tower requested that we circle above the traffic pattern while they worked with the other aircraft.
We held for about 15 minutes while the tower tried to patiently work out the emergency. At that time, it sounded like the emergency pilot would execute one more trip around the pattern and then attempt to land. That attempt failed so they tried again, and again, and again.
As I circled, wondering how much longer wed be delayed, I thought about going to an alternate airport. However, the nearest suitable general aviation airport was about 25 minutes away flying over a tall mountain range. The emergency aircrafts radios began to fade, which only added to the communication problems.
I was chewing into my fuel reserves, and now I wasnt certain if I could really make it to the unplanned alternate. The fuel gauges said we had a quarter-tank of fuel remaining, but the fuel needles hadnt moved much in the last hour and I really didnt trust them anyway.
We were uncertain how much longer it would take before the other aircraft resolved its emergency and the radio was very busy with emergency radio traffic. Soon thereafter, the other aircraft landed, but the gear collapsed on touchdown. Now we were really in a tight spot.
The tower frequency was congested coordinating the air and ground activities, and I wasnt certain that I could make it to an alternate. I notified the tower that I might have a problem. Luckily there was an intersecting runway to which I requested immediate landing permission.
The POH said that we should have landed with 30 minutes of fuel remaining, but when we refueled it was clear that we had been much closer to fuel exhaustion than that.
In Part 91 ops, an alternate airport is required for IFR flight when the destination airport has no instrument approach procedure or if the weather within one hour of the planned time of arrival falls below either a 2,000-foot ceiling or three miles visibility.
However, IFR flights arent the only ones that need an alternate. Every flight should have an alternate of some kind, even local training flights.
My opening example clearly points out that a VFR cross-country flight deserves having an alternate. And you might think having an alternate for a flight in which you intend to remain in the traffic pattern is overkill, but here is a real-world event.
A friend was towing gliders all afternoon, and on his last flight of the day, before he could make that last landing, crosswinds had blown in that were far in excess of what the taildragger could handle. Luckily my friend had enough fuel to divert across a mountain range into another airport, roughly 20 minutes away.
If you fly out of an airport that is served by only one runway, you are forever open to the risk of the runway being closed by another airplanes accident. Youd better have fuel to get to another airport, as having to make an emergency landing on whatever section of runway happens to be available during the accident cleanup isnt the best position in which to find yourself.
What Makes It Good
The factors to consider in selecting a good alternate airport are many. The most obvious is the distance from the destination airport to the alternate. If the closest suitable alternate is 25 minutes away, as it was for me in the opening example, you quickly eliminate the option to divert as soon as you begin eating into your fuel reserves.
If you are in a divert situation, a closer airport means you need less fuel to get there, improving your chances of actually making it there, especially if youre flying in some part of the country where airports are widely scattered. The time and fuel consumed going to your alternate, even for a simple day VFR flight around your home airport, needs to be considered for every flight.
At the very least, an FBO with fuel is a nice feature. Of course, the airport you select should be suitable for your type of aircraft at the time you arrive. An unimproved runway with a rough surface might be a poor alternative for a retract. Too-strong crosswinds or tailwinds should disqualify an alternate from your list.
Other aspects to think about is whether the alternate is equipped with suitable runway lighting systems in case you arrive in darkness or a precision instrument approach if weather is low. If you need to divert because of an equipment failure, a long, wide runway lined with fire and medical emergency equipment might be handy.
When you pick an alternate, make sure the route to it is easy to fly. My friend who was towing gliders had to divert through a substantial mountain range to land at a suitable alternate. Crossing mountains or open water, or flying around congested and complex airspace is more stress than you need.
If the diversion would be under IFR, the route would ideally be a single straight airway with a low MEA. Going to an alternate from a difficult location, such as Jackson, Wyo., where terrain requires a climb to 15,000 feet, can be a difficult and inconvenient proposition that stacks the odds against you. When the situation deteriorates, you want an easy way out.
If weather is the cause of your need to divert, the ideal solution is to fly toward an alternate where the weather is better. Some weather patterns are easier to escape than others. Coastal fog in California has a relatively simple solution of flying inland a bit, or over the coastal mountains. Youll nearly always fly into drier air and resolve your problem.
Cold fronts tend to have rather steep frontal gradients and the conditions usually rapidly improve behind a cold front. However, getting through a cold front can be rather stressful sometimes, particularly if it is associated with strong contrasts in the temperatures and moisture. In that event, severe weather is likely, and again, youd rather have an easy way out if you need an alternate.
Warm fronts can be rather troublesome. As the air wraps a low pressure from the south, it warms up and tends to pick up that humid air. As it moves counter-clockwise around the low-pressure center, the warm humid air tends to rise up over the cooler air. The slope of the front is very shallow and the frontal conditions can exist for hundreds of miles. It can be very difficult to escape these conditions quickly.
The weather at the alternate is obviously important, but you also have to consider the terrain and the available instrument approaches. For Part 91 operations, the quick answer is that weather conditions must be better than 600 feet and two miles visibility at the time of arrival at the alternate if it is equipped with a precision approach. For an alternate with a non-precision approach, the forecast has to be better than 800 feet and two miles visibility.
But theres more to it than that. The 600-2 and 800-2 limits apply only if the airports instrument approach procedures do not specify alternate minimums.
To make that determination, look at the approach plate for the alternate carefully. If there is an NA at the bottom of the chart, it means the airport is not authorized for use as an alternate. Generally speaking, an airport wont be authorized for use as an IFR alternate if it does not have weather reporting capabilities.
It would be risky to divert to an airport without knowing for certain that the weather is above minimums. Can you imagine diverting to your alternate and shooting the approach down to minimums only to find yourself still in the soup? That would really be a bad situation.
If you fail to notice the NA section on the approach plate and apply the 600-2 or 800-2 rule, you may be in for a rude awakening. For an extreme example, say you are trying to fly into Eagle, Colo., for some winter skiing and there is a chance of snow showers at the time of arrival. You might want to choose Aspen as the alternate because its nearby and has a non-precision approach (VOR DME-C). If you miss the NA symbol on the Aspen plate and apply the 800-2 rule, you wont even be close. Aspens circle-to-land approach requires a minimum ceiling of 2,385 feet and 2 miles visibility.
There are some other pitfalls in simply applying the 600-2 minimum without carefully looking at other factors. Lets suppose your proposed alternate has an ILS, but flying into that runway will result in having a strong tailwind for your arrival. There is nothing in Part 91 that requires you to assess the suitability of that approach.
In practical terms, you have to determine whether you can actually shoot the approach and land under the weather conditions forecast. Most large aircraft are prohibited from landing or taking off with more than a 10-knot tailwind, a practice light plane pilots would do well to adopt at all but the longest runways.
If the ILS approach into your alternate is accompanied by a 10-knot tailwind, it could make the landing very risky – possibly outside the operating limits of your aircraft. In this case it might be wiser to consider circling-to-land, which requires higher minimums.
Check your approach plates against the forecast wind to ensure you arent confronted with a downwind landing because the weather minimums wont allow a circle-to-land.
Qualities of Alternates
The alternate airport should be easy to find, regardless of whether youre flying VFR or IFR. It would be disconcerting to circle over an area trying to find an airport, certain that you were within a few miles but still unable to spot it – particularly with dwindling fuel reserves.
While GPS makes it much easier to find airports, even in the haze, an airport that has distinguishing features makes it a better bet in case the GPS goes down. An airport thats near a prominent geographic feature such as the intersection of two highways or a distinct bend in a river is easy to find. If you dont have GPS, an airport with a VOR or NDB on the field is much easier to find than one without.
When planning an IFR flight, remember that some instrument approaches require radar. Murphys Law will have the radar out of service at the time you are diverting there.
If the winds favor an instrument approach from the distant side of the field, you will likely fly farther while maneuvering for the approach – and that consumes fuel. If the airport has an approach control facility, you can reasonably expect to get radar vectors to the final approach course, but not always.
Read the Notams regarding both your destination and alternate airports. Its easy to miss the sections listing inoperative approach aids. It would do you little good to plan on an alternate whose navaids were out of service if you were making the trip in instrument conditions. If the alternate is served by just one navaid, youd really be out of luck if that navaid was out.
Ideally, you would be flying an airplane equipped with an approach-approved GPS as well as a VOR and ADF receiver into an airport with a full complement of instrument approaches.
Whether flying VFR or IFR, the alternate should fulfill whatever number of what if scenarios you can dredge out of your imagination. Consider the kinds of systems failures your airplane may be subject to, and think of how suitable the alternate would be. Make sure the runway is long enough for a no-flap landing.
If your electrical system fails en route and a deviation has you airborne after dark, you dont want to be counting on an airport with pilot-controlled lighting. Gear malfunction? Plan on an alternate that has fire equipment available.
Part 91 leaves many of these decisions up to the judgment of the pilot. Only by considering the whole range of potential pitfalls can you stack the odds in your favor. Select a suitable alternate airport for every flight, giving full consideration to the plausible contingencies you may encounter on that flight.
Also With This Article
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-by Pat Veillette
Pat Veillette is an aviation safety researcher who develops training programs for a large carrier.