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Home » John E. McLain

This and That

By John E. McLain (January 2002)

After 14 years of writing this column, it sometimes becomes difficult to come up with new subjects. However, there are some topics I keep wanting to talk about, but I always seem to find they are not material for a complete article. So, to start the New Year, I’m going to meander around a bit with a variety of short subjects.

Recently, I read an article in a trade magazine which I can only describe as “how to scud run safely.” Although that was not the article’s actual title, it was the subject of discussion, and it made me concerned. In my opinion, the phrase “scud run” and the word “safety” do not mix. Any writer who tries to combine them loses all credibility with me. The appropriate combination with “scud run” is “unsafe.” Putting the concept of the article aside, the writer completely lost credibility when he described basic VFR weather minimums as a 1,000 foot ceiling and three miles visibility.

I would refer the reader to the table in FAR 91.155 (a) describing VFR weather minimums. Nowhere is there any mention of a ceiling, although there are 10 classifications of Basic VFR Weather Minimums. If we eliminate the airspace at or above 10,000 feet MSL and more than 1,200 feet AGL, then there are only eight classifications and, of these eight, five are the same. These five are three statute miles visibility, 500 feet below, 1,000 feet above and 2,000 feet horizontally from clouds. This includes class E airspace which, I can assure you, accounts for at least 80% of the airspace in the United States below 10,000 feet MSL.

It becomes fairly obvious that a proper description of basic VFR weather minimums, for the vast majority of U.S. airspace below 10,000 feet MSL, would be three miles visibility, 500 below, 1,000 above and 2000 feet horizontally from clouds; not the simplistic 1,000 foot ceiling and three miles visibility.

So where does the misconception of a 1,000-foot ceiling and three miles visibility being basic VFR weather minimums come from, when a ceiling is not even mentioned in the table? The answer is contained in FAR 91.155 (c), which states that VFR operations are not permitted in surface based controlled airspace beneath a ceiling less than 1,000 feet AGL, controlled airspace being Class A, B, C, D or E. Since surface based controlled airspace constitutes, by my estimate, less than 1% of the airspace in the United States, it seems ridiculous to refer to a 1,000 foot ceiling as a part of “basic VFR weather minimums.” Incidentally, it is this same surface based controlled airspace where special VFR applies.

The alert reader may note that when the ceiling in this airspace goes above 1,000 feet, the other VFR weather minimums apply. This creates a real problem that few pilots consider. Say you want to land at an airport in surface-based class E airspace. The weather is 900 feet, overcast and eight miles visibility. Normal VFR access is denied, but you could land by requesting and receiving a special VFR clearance from the appropriate ATC facility. Then, let’s say, the weather increases to 1,000 feet, overcast and eight miles visibility. The airport now supports normal VFR arrivals and departures, but special VFR no longer applies. Everything seems fine, except that the normal cloud clearance of 500 feet below clouds is still in effect. Therefore, you would have to enter or depart the airspace at 500 feet AGL or below, in order to meet the cloud clearance criteria. Not too many pilots think about or realize this.

The pilot who considers a 1,000 foot ceiling and three miles as basic VFR lacks understanding of VFR weather minimums and could easily violate a very important regulation.

Next I would like to discuss touchdown speeds. Let me first say that my discussion here refers only to piston powered airplanes. Turbine powered airplanes have operational and equipment considerations that change the equation. From my experience in conducting flight tests, and just my observations, I’ve concluded that fully 80% of all landings result in the airplane touching down at an excessive speed.

Each day, I go to the FAA web site for the Office of Accident Investigation (www.faa.gov/avr/aai/iirform.htm). This is a compilation of all accidents and incidents received by the office the previous day. It is the incidents that interest me the most, because they include the “fender benders” such as collapsed landing gear, aircraft flipped, or aircraft departed the runway on landing and struck a parked airplane. The obvious conclusion, for most of these accidents, is that if the airplane had touched down at a slower airspeed the accident would have not occurred or would have been less severe. Outside of stalling and dropping it in from 10 feet or more, I have never seen or heard of an aircraft accident or incident where touching down at the slowest possible speed led to a more serious consequence.

What, then, should the touchdown speed be? The Practical Test Standards are specific. They state that touchdown should be at the approximate stall speed. Keeping in mind that the Practical Test Standards are just that, test standards, the word approximate becomes a judgment call by the examiner and is not absolute. I can offer a more absolute number: the minimum hydroplaning speed of the airplane, a speed every pilot should know.

Hydroplaning is the condition where the tires on a wet runway no longer contribute to directional control, and braking action is nil. Tests have shown the minimum dynamic hydroplaning speed of a tire is 8.6 times the square root of the tire pressure in pounds per square inch (PSI). To make life easier, the general application is nine times the square root of the tire pressure. As an example, if the tire pressure is 36 psi, its square root is six. Multiply six by nine and we get a minimum hydroplaning speed of approximately 54 knots. If you touch down at a speed above 54 knots on a wet runway, you may encounter hydroplaning.

Once it begins, hydroplaning can continue below your 54 knots. If your normal landing technique produces a touchdown speed below the hydroplaning speed, then wet runways will not require you to change your technique.

There are several good reasons why touchdown should be at the lowest possible speed. First and foremost are those related to safety. First, keep in mind that an airplane is an air machine, not a ground machine, by which I mean that it is easier to control in the air than on the ground. In the air, speed is essential for positive control. On the ground, excess speed can be detrimental. Suppose something like a blown tire, collapsed strut or just a plain loss of control caused you to swerve off the runway. Would you rather be going slowly or fast? I hope I do not have to expound on this further.

Why do so many pilots land too fast? It is simply that the fast landing is easier to teach and to do, and can result in a smoother landing. The full- or semi-stall touchdown is a little more difficult to do properly and will not always result in smoothness. To me, this concept is almost criminal. The idea that any pilot would not use the safest procedure, simply because a less safe procedure is easier, is repugnant. Safety never should be compromised because a less safe procedure is easier.

I sometimes play a game when I am giving instruction to a pilot who lands too fast. Immediately after touchdown, without adding any power, I take control, lift off again, and land another 500 to 1,000 feet down the runway. This usually makes a strong impression.

Let me leave you, this month, with one last observation. Once you cross the runway threshold, without using power you no longer have control over how far down the runway you will touch down. In a proper landing, you will touch down when the airplane runs out of flying speed. If it touches down before that, you have forced it onto the runway at too high a speed, and airplanes are like people in that they don’t like being forced to do something, especially when they know it is wrong.

Next month I will discuss some more ideas from the “This and That”  file.