Instrument Insights Part 7 of 12

Precise but Flexible Flying

July 1, 1998

The practicals of alternates and minimums

As VFR pilots we are dealt engine failures, communication failures, and other "emergencies" to test our ability to make safe piloting decisions. As instrument pilots we're taught to fly by the books and stick to the plan. But, as we all know, even the best-laid plans get changed by unforeseen circumstances. Often the unforeseen circumstances involve weather.

Federal aviation regulations (FARs) require pilots to prepare for some of these unplanned weather changes by carrying extra fuel and filing an alternate destination on an IFR flight plan. It's the FAA's way of making sure that pilots have an out. This helps to make what might be a tense situation a simple matter of flying to the alternate and landing there.

However, when the ceilings and visibility really come down significantly over a widespread area, finding an acceptable alternate can become a difficult task. When things get that bad, even the most proficient instrument pilots ought to reconsider the importance of launching such a trip in the first place.

Alternates as easy as 1-2-3

The regulations regarding alternates are rather simple if you can remember 1-2-3. If the forecast weather at your destination, from one hour before to one hour after your estimated time of arrival, is at least a 2,000-foot ceiling and three statute miles' visibility, then no alternate is required to be filed. Beyond the 1-2-3 rule, things get a little more complicated.

Standard IFR alternate minimums state that if the alternate airport has a precision approach (ILS or PAR), it can be filed as an alternate if, at the time of arrival, the forecast is no worse than 600 feet agl and two miles. If the airport is served by a nonprecision approach (NDB, VOR, LOC, GPS, etc.), the ceiling rises to 800 feet agl and visibility remains at two miles.

But the planning doesn't end there; the 600-2 and 800-2 rules are basic minimums that apply when there are no specific alternate minimums listed in the beginning of the National Ocean Service's Terminal Procedures Publication (TPP) or on Jeppesen Airway Manuals' airport layout pages. If alternate minimums for a specific airport are not published, they are assumed to be standard minimums. Jeppesen publishes the minimums regardless of whether they're standard or not, to avoid any possible confusion. You'll find that airports in mountainous areas or those that have obstructions near them generally have higher-than-standard alternate minimums. It is imperative to check these before filing an airport as the alternate, especially in mountainous areas. It is also worth noting here that alternate minimums apply for planning purposes alone. If you're airborne and heading to the alternate, published minimums will now apply.

If the destination has no instrument approach, then weather must be good enough to make a VFR approach from the IFR minimum en route altitude (MEA). If you're under radar surveillance, however, you may be able to descend to the minimum vectoring altitude (MVA), which will sometimes be lower than the MEA. MVAs aren't printed on any chart; they're known only by the controller and vary from sector to sector, based on radar coverage. If you are flying near a large airport in flat country, the controllers could probably get you down pretty low, given the close proximity of the radar array. Airports located far from the radar transmitter and controlled by a center controller probably wouldn't have a vectoring altitude any lower than the MEA.

Wherefore art thou, alternates?

Flight service briefers are generally the best source when fishing for an alternate, since they have all of the terminal forecasts in front of them. If you receive a DUATS weather briefing, you can manually find airports with acceptable weather minimums in those reports as well.

In general, large airports are your best bet for an alternate without requiring you to dig through several charts to see what airports meet the criteria for an alternate. Large airports have weather reporting facilities right on the field, so they issue the necessary terminal forecasts. These airports also have towers that are open all or most of the day, superior approach lighting systems, and multiple ILSs (or possibly even precision approach radar) to cover the precision approach criteria. If the weather is marginal for one, pick a second large airport, just in case, and plan your fuel accordingly. Remember, though, that as convenient as large airports are for filing purposes, should you head to one as an alternate, you may be faced with long delays (holding patterns, vectors, etc.), which may require more fuel.

Some computer flight planning programs, when interfaced with a DUATS provider, make finding an alternate easier, but they may not know whether an airport has a precision or nonprecision approach. Again, this leaves the flight service briefer or a DUATS printout as the best sources for finding alternates.

Plan B

Flexibility is key when flying in poor weather. Hours and hours of flight planning are worthless if, after you get going, the winds and destination weather aren't as forecast. In these situations plan B should be in your mind before you even leave for the airport. In fact, it's best to have several alternatives in mind.

Your first clue that alternate plans will play an active role in your flight will come during the preflight briefing. Is there a widespread area of low ceilings and poor visibility in the destination area? Were several reporting stations issuing "specials" to their METARs? This could be the clue that the weather isn't behaving as forecast and that weather predictions are likely to be off all day.

If preflight conditions are acceptable and the flight has commenced, check in periodically with flight watch for updates. Note if the forecasts are holding true. Is the weather trending better or worse? Are other airplanes making it in? Have the TAFs been amended? Again, if the forecasts have been blown for the worse and your alternates are no longer acceptable, it's time to start looking for more options or turning around. In this situation, fuel could be your savior.

Don't fuel around

A friend of mine has a motto: "You can never have too much fuel unless you're on fire." This holds true for IFR flights as long as you stay within the weight and balance and performance limits of the airplane. Fuel is cheap when you're heading to your second alternate after missing a few approaches.

Under IFR, the FARs require that you carry enough fuel to fly to your intended destination and fly for another 45 minutes at normal cruise power. It's up to the pilot to calculate the actual time en route, given the winds, weather deviations, and other conditions that may prolong or speed up the flight.

If the weather doesn't meet the 1-2-3 rule, you must carry enough fuel to fly to your destination, fly from there to the alternate, and fly after that for another 45 minutes at normal cruise power. If your alternates are few and distances between them are far, you may find that the 45-minute rule won't cut it. In densely populated areas, 45 minutes may be adequate; however, on the rare day when the weather is crummy in the desert Southwest, safe minimums may be a few hours away. Pilots who ferry airplanes across oceans carry at least three to four hours of extra fuel since alternates can be several hundred miles away from the intended destination.

Covering your posterior

Think of alternates and minimums as a way to cover your rear in more ways than one. First and foremost is the fact that adhering to the rules of alternates and minimums will keep pilots, passengers, and those on the ground safe. If all minimums and fuel reserves are adhered to, it would take a catastrophic event, such as a total power loss, to make an airplane come out of the sky.

Finally, careful planning of alternates and minimums covers your rear in the event that any type of incident or accident occurs during a particular IFR flight. For example, if you declare an emergency because of low fuel or weather and are given priority handling by ATC, the first thing investigators will want to see is how you planned this particular flight. Errors in judgment comprise a huge number of cases in which pilots were found guilty and had their certificates suspended or revoked. Careful planning, prudent decision making, and lots of options will keep you flying safely and legally.

Takeoff minimums

Those of us operating under Part 91 of the FARs are legally allowed to take off in zero-zero conditions. It's not particularly smart, but then again it's not prohibited by the regs. Commercial operators adhering to Parts 121 and 135, on the other hand, have prescribed takeoff minimums to adhere to. A pilot flying under Part 91 would be prudent to adhere to these regulations, as well. If nothing else, the GA pilot should consider having weather decent enough to allow a return to the airport in case of an emergency after takeoff.

Like alternate minimums, takeoff minimums are listed in the TPP volumes and on Jeppesen's airport diagram plate. For airports that don't have specific minimums, the FARs list criteria that must be met instead. For aircraft, other than helicopters, with two engines or less, a one-mile visibility is considered minimum. For airplanes with more than two engines, the minimum is one-half mile.

Instrument departure procedures may also have minimum climb gradients associated with them. These gradients are usually runway-specific and are attainable by most general aviation airplanes in most conditions. However, the pilot of a light single with a big load on a hot day at a high-altitude airport may find that the airplane cannot meet the minimum climb gradient. It may be necessary to use another runway with a lower (or no) minimum climb gradient. If that's not available, the pilot may have to wait for better weather or cooler temperatures. Twin pilots need to consider their airplane's engine-out climb gradient. Most likely, a piston twin flying on one engine isn't going to perform nearly as well as the climb gradient requires.

Climb gradients are always expressed in feet per nautical mile, requiring the pilot to translate that figure into a more usable rate-of-climb figure expressed in feet per minute. Charts are available in TPP and Jeppesen books but can be figured in your head by calculating your estimated groundspeed, dividing by 60, and multiplying by the required climb gradient. For example, 120 knots divided by 60 equals two. Multiply two times 300 (the required foot-per-nautical-mile gradient) and the result is a required rate of climb of 600 feet per minute. Other calculations are not nearly that easy and may require you to fish out the table in your approach books.


E-mail the author at pete.bedell@aopa.org.