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Ceilings, Visibility, & Icing


Know the Rules, then Set Your Limits

by Crista Videriksen Worthy

It’s surprising when you suddenly discover you had the wrong idea about something all along. In this case my misunderstanding came from creating personal minimums to match regulations that didn’t exist. In the past when contemplating an IFR flight my husband and I would pull out the approach plates and compare them to the METAR or TAF. If the reported ceiling was below the published approach minimums we’d cancel the flight or go somewhere else. We thought that was required for the airlines so we figured we’d emulate them. As it turns out that wasn’t necessary not even if we had been an airline!

Many years ago, the FAA did set operating minimums of ceilings and visibility. But with improvements in the technology of navigational aids, the FAA removed ceiling requirements in 1966 and replaced them with Decision Height (DH) and Minimum Descent Altitude (MDA). FAA Order 8900.1 Vol. 4, Chap. 2, Sec. 1 reads: “Specifying the Operating Minimums. A major change in the method of specifying the operating minimums for approaches with vertical guidance evolved with the introduction of the DH and Runway Visual Range (RVR) concepts. These changes were finalized by the publication of U.S. TERPS criteria in 1966. This conceptual change eliminated the ceiling requirement by introducing a DH.”

How was this done? Every instrument pilot is familiar with 14 CFR Part 91.175(c) (2), which reads, “No pilot may operate an aircraft [...] below the authorized MDA or continue an approach below the authorized DA/DH unless [...] the flight visibility is not less than the visibility prescribed in the standard instrument approach being used...” So, ceiling isn’t even mentioned, just visibility.

Part 91 pilots can attempt an approach with both ceiling and visibility below minimums, but Part 121 flights are held to a higher standard. They don’t need the ceiling, but they must have the controlling visibility to begin an approach. Indeed, they can’t even launch for the destination if the visibility is below the minimum specified for that approach. If the visibility is above minimums and the Part 121 pilot learns that the visibility has dropped below minimums before crossing the FAF, the approach must be aborted. However, if visibility drops and is reported after the pilot has passed the FAF, the approach may continue. In any case, any pilot who reaches the Decision Altitude (DA) or MDA without having the necessary visual references (e.g., runway, runway markings, runway lights) must go missed. In this way, visibility is controlling for all operators with respect to an actual landing.

When it comes to alternates, however, ceilings are required. That’s because the FAA wants to be sure you have a solid out if the weather tanks. Precision approaches require 600–2 (600 feet and two statute miles visibility) while non-precision approach alternates require 800–2.

You might say (as I did, mostly without really thinking it through) that if the reported ceiling is below minimums and you launch anyway, you won’t see the runway when you get to the DA or MDA. But then again, you just might, because conditions may change before you arrive, or the ASOS may be positioned so that it reports weather that is worse than what you would actually see. An ASOS reports weather directly above it, but if a cloud or patch of fog is parked over the instrument, that’s the report you’ll get, even if the rest of the area could be well above what’s reported. For example, when we lived in California and operated out of a coastal airport, it was routine for half the airport to be socked in and the other half to be completely clear.

I’m not advocating flying an approach when reported ceilings are below minimums, but legally speaking, they’re irrelevant. Of course, ceilings are definitely a way to gauge your chances of breaking out. So it turns out my personal rules were even more conservative than those for Part 121 flyers.

What about icing? Our rule has always been not to fly in clouds below freezing, and most of the pilots I’ve spoken to here in Idaho have the same personal rule, even a retired FedEx pilot. Our rule has grounded us a few times, but we’ve never had an icing problem. But what does the FAA actually say about launching when you might get ice? While there isn’t any specific provision in FAR Part 91 that bans flight into known icing conditions for small private aircraft, FAR 91.9(a) requires compliance with operating limitations specified in the approved Flight Manual. If the manual states that the aircraft is not approved for flight into known icing, that would apply. So, what does the FAA consider to be “known icing”?

The FAA defines icing in several different publications. FAR 121.321 Operations in Icing states: “conditions conductive to airframe icing means visible moisture at or below a static air temperature of 5° C or a total air temperature of 10° C.” The FAA also published guidance in the Instrument Flying Handbook (FAA-H-8083-15B), stating: “A pilot can expect icing when flying in visible precipitation, such as rain or cloud droplets, and the temperature is between +02° and -10°.” According to the Aeronautical Information Manual (AIM) 7-1-13, icing is implied by the presence of precipitation at temperatures at or below zero degrees Celsius. The message is clear: flying into known icing conditions without the proper anti-icing equipment can be perilous! When icing is detected in flight, you should either leave the area of precipitation or go to an altitude where the temperature is above freezing. It is important to note that the warmer altitude may be above you!

With regard to enforcement, the FAA said it will evaluate the actions taken by the pilot both before and during the flight to determine if the pilot’s actions were reasonable in light of FAR 91.9(a), operating limitations in the flight manual, 91.13(a) careless or reckless flying, and 91.103 becoming familiar with all available information about the flight before taking off. If ice did form on the aircraft, the FAA will look at what actions the pilot took to exit the conditions.

The FAA provides several sources of information, including Parts 121 and 135, and the Aeronautical Information Manual, which complement Part 91 for the GA pilot. It’s up to you to gather all available information, form your own personal minimums and, most importantly, stick to them when the chips are down.