A common problem on all flight tests, but especially for the instrument rating and CFI-I evaluation, is understanding the need for alternate airports, and how to analyze conditions for their suitability. The basic premise behind all the FAA regulations is that every “pilot in command” is required to research and plan an alternate airport. That is for all VFR flights “not in the vicinity” but especially IFR (with limited allowable exceptions).
Alternate airport means an airport at which an aircraft may land if a landing at the intended airport becomes inadvisable.
§91.103 (that preflight “all available information” reg.) requires alternates for all flights; VFR “not in the vicinity” and in controlled airspace under IFR (except in certain exceptions):
This information must include — For a flight under IFR or a flight not in the vicinity of an airport, weather reports and forecasts, fuel requirements, alternatives available if the planned flight cannot be completed, and any known traffic delays of which the pilot in command has been advised by ATC;
Basically, in §91.103, the FAA is requiring due diligence on all the popular ways to kill yourself in an aircraft. Every well-trained pilot should pay attention and comply with this basic guidance. But DPEs still get applicants showing up for flight tests without calculating their take-off and landing performance – and sometimes they have never done it ever. A recent flight test candidate for an instrument rating had over 200 take-offs and landings at his “home field” and did not even know the length of the runway – guessed 3K when it was actually over 5K! (“I thought I only needed that was just for the private test…”)
IFR Alternate Requirements
For planning an IFR flight in controlled airspace, a flight plan is required under §91.169 and an alternate must be specified (except with limited allowable conditions).
§91.169 IFR flight plan: Information required.
(a) Information required. Unless otherwise authorized by ATC, each person filing an IFR flight plan must include in it the following information:
(1) Information required under §91.153(a) of this part;
(2) Except as provided in paragraph (b) of this section, an alternate airport.
The default FAA position for IFR planning (just like VFR “not in the vicinity”) is to research and select a suitable alternate airport. In IFR regulations, relief is found in §61.169 (c): “IFR alternate airport weather minima.” If weather forecasts predict a pilot can descend from the MEA in forecast VFR weather conditions – no alternate airport (or even instrument approach procedure) is required. Also if the airport of intended landing is forecasting “pretty good MVFR conditions” (that famous 1-2-3 guide), no alternate needs to be filed. But how do we know what the weather will be down the road an hour or two from now? What are the “legal forecasts” for my destination and for my alternate airports?
Legal Weather For IFR Planning
Certainly, if it is “clear blue sky” at the destination and an official weather product says so, you are legal (and safe) to proceed. But if the weather is low and crappy with widespread instrument conditions forecast, a Terminal Area Forecast (TAF) is required and/or a pilot must plan a legal alternate (depending on conditions). (For Part 91 operations, the newer MOS is legally acceptable for your weather decision also). Remember, you do not have to fly to your filed alternate, this is a required “escape plan” for the IFR flight plan. Traditional area forecasts, do not go below a 1000 ft ceiling, so forecast IFR conditions at your destination with no TAF require an alternate! MOS data, properly derived and interpreted, can provide a legal remedy for Part 91 operators in these cases. Remember, the FAA default position for filing IFR is the specification of an alternate airport (this also means extra fuel; see this FAA Legal Opinion on what those 45 minutes include, Also NTSB).
Don’t Be Fooled By EFB
ForeFlight (and many other EFBs) are very helpful in providing an amazing array of weather products. But it is a serious mistake to assume that because you see a TAF listed on the app this is legal for your flight planning (check TAF limitations). In the example below, KMMU is 13 miles from KEWR. If the weather in the area is forecast to be IFR, the TAF you see on your app. is not legal for determining an alternate (no TAF at KMMU, no way to determine 1-2-3). Using traditional weather products, an IFR alternate is required, and that planned alternate airport also needs some way to determine legal weather, with acceptable conditions for the IFR runway in use. (and watch for alternate “NA” and non-standard alternate minimums). More on LAMP/MOS and the newer GFA below.
Flight test applicants (even CFI-I candidates) really struggle with “legal weather” for IFR. Merely pointing to the EFB and parroting “800/2 or 600/2” is not really enough (you will not be safe in the clouds). Also assuming any mobile app is correctly importing and displaying LAMP data may be hazardous. (there are many different algorithms at work here). Without a reliable weather forecast at your destination in forecast IFR conditions, it’s a good idea to pick a pretty big airport (usually a Delta with a TAF) for your planned alternate. You want to reliably determine the required 600/2 800/2 and you also want some “big city” services (see “3Rs” below)
TAF vs “MOS Magic”
There is a new forecast option (but only legal for Part 91) called the Localized Aviation MOS Program (LAMP) point forecast. Many of you might be familiar with the Model Output Statistics (MOS) seen on ForeFlight and other EFBs. Since the FAA does not specify which forecast you must use to determine the conditions in 91.169, you can pick your options. The FAA requirement is only to obtain “appropriate weather reports and forecasts, or a combination of them” for naming your destinations and determining an alternate (not available to Part 135 and 121 yet). Computational 
models are valid and have a pretty good level of detail to determine the local weather accurately. The ceilings and viz, as currently specified, need a little massaging to align with FAA requirements but GFAs are updated hourly and very accurate. But just like any single source news, too much reliance on one data stream may be dangerous. Always start with the larger synoptic picture and build a thorough weather picture for safety.
LAMP forecasts are computationally derived (no human hands) and recently became available on the 1800wxbrief website from over 1800 locations. By contrast, there are only 750 airports with TAFs. Studies have indicated that LAMP forecasts are more accurate than human-developed TAFs. IT seems TAFs tend to err more frequently on the side of caution due to human intervention. But maybe there is an advantage to the many years of meteorological experience behind every TAF. MOS forecasts are more frequent and prolific but are not approved yet for Part 135/121 operations (barely out of “clinical trials.”) Another current issue you may detect is that each mobile App might import computational data differently. Raw LAMP data direct from the NWS will be more accurate until a standard algorithm is decided for mobile app. imports.
What are the “3 Rs” for Alternate Planning?
I tell my flight students that realistically (and in the interest of safety) a required alternate airport should meet the “3 R” rule. Remember a miss in low IFR for a GA pilot is “almost an emergency” and you really did not want to end up at this alternate. Your “sure bet alternate” should have Radar, Restaurant, and Rental Cars (in addition to those legal weather requirements)! “Big-city services” are essential for your personal (and passenger) comfort. I am continually amazed that pilots pick some small part-time rural airports, with a non-precision approach and no weather forecasting as a viable alternate airport when they are flight planning. What exactly are you going to do now that you (maybe) successfully landed (the family or customers in the back are *not* going to be happy)? Fly safely out there (and often)!
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