Not the way songwriter Paul Simon intended, this week’s blog is about stopping on the available runway. The accident statistics show pilots are much attracted to the far end of a landing runway. There are multiple reasons: Excess speed is one, and water is another. Water is a universal lubricant and one of its less endearing qualities is that it makes stopping difficult or impossible.
A friend who’s in the automotive body shop business always smiles when it rains because it means the work will be steady in the near future. Unless you fly only when the runways are perfectly dry there’s one word to know—hydroplaning! There are multiple kinds of hydroplaning that make for good cocktail party talk, but what’s important is that when enough water gets between the rubber and the runway you’re not stopping anytime soon.
A short hangar story from this spring: The night IFR approach was routine, and the runway was in sight at least three miles out. I’ve learned over the years that excess speed on landing only leads to wasted runway behind me, so I had settled comfortably into about 80 knots on short final with just a little power on the Bonanza. The landing light had thoughtfully sacrificed itself on the approach, so a blackout touchdown was in the offing. (That’s only fair when remembering all the blackouts inflicted upon students during night checkouts to simulate just such a happening.)
It was one of those rare (for me) satisfying touchdowns where the wheels just kiss the pavement and you’re not really sure the aircraft is down. That’s because it wasn’t—the airplane made just the slightest of swishing movements and things felt “fluid.” A minor touch of rudder and brake revealed tentative pavement contact at best. Just let gravity work it’s miracle as the speed bleeds off and then get on the brakes, gently.
We rolled slowly to the end of the 3,700-foot ungrooved strip using aerodynamic braking until the tires had a solid grip. Learned the next day there’d been heavy rain just an hour or so before our arrival with some standing water on the pavement. Couldn’t have seen it in the dark even if the landing light had worked. Grooved runways dissipate water very well but small airports usually don’t have them. They also don’t have braking action reports unless somebody happened to land just before and said something on the CTAF.
The formula for dynamic hydroplaning is the square root of the main gear tire pressure in PSI multiplied by 8.6. Probably best to memorize the computed number for the aircraft you normally fly unless you’re really quick with a calculator or abacus on the rollout! The mains in my aircraft are inflated to 36 to 40 pounds, so that equates to 51.5 to 54.4 knots—call it 55 knots. That means dissipating roughly 20 knots before the brakes begin to work dependably on a watery runway.
We haven’t discussed reverted rubber or viscous hydroplaning—my hunch is that dynamic is the one that is most likely to be encountered. The others occur under special circumstances, but the bad news is that they can affect braking at significantly slower speeds.
Contaminated landing distance performance is a subject most light aircraft POHs avoid. We’ve spoken before about the “optimistic” nature of POH numbers. They are absolute performance numbers: The pros don’t use them, and why the FAA tests our ability to measure it to the nearest 10 feet, or calculate fuel burn to a gallon or three, is a mystery.
The Air Safety Institute recommends the “50-50 method” in computing takeoff and landing distance. Whatever the book says the test pilot could do over the 50′ obstacle—add 50 percent to that. Under the ambient conditions that evening, the POH landing distance for my aircraft on a paved, level, dry runway was about 1,500′. Add the margin and a more realistic dry runway number is 2,300′. Now factor in water’s lubricity and it’s smart to double the number to 3,000′. Any downslope to the runway, density altitude, clearing the obstacle by a few extra feet, or an extra five knots for comfort will add up quickly. Your mileage will vary so be conservative!
We’re only talking about the longitudinal aspect of landing—not what a blustering crosswind might do if the tires are not in full contact with the runway. The demonstrated crosswind component means nothing at that point. Then a really wide runway—or better, one where the wind is right down the slot—is much the desired option. Anyone else with a hydroplane story?
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