Monday, January 24, 2011

Aircraft tire pressure and hydroplaning

A couple of weeks ago, a friend of mine sent me an article on the importance of properly inflated aircraft tires, it may be found here. This got me thinking about the importance of properly inflated aircraft tires. While the article that my friend sent me talks about improperly inflated tires as a cause of the crash of a Lear Jet and a commercial airline in the Middle East, it is important for all aircraft, from my Cessna trainer to Lear Jets to airtankers to Jumbo jets, to have properly inflated tires. I am going to talk a little about what I am learning about the importance of properly inflated tires on single-engine aircraft such as the Cessna 172 that I take scenic rides in.

I have recently been working on pre-flight inspections of an aircraft with the pilots that take me on scenic rides. The tires are one of a number of items on our pre-flight checklist. So, I told him about the article that my friend sent me. He told me that he thought that the proper tire pressure on the Cessna 172P was about 30 pounds per square inch (psi). He went on to remind me that we had talked about the relationship between tire pressure and hydroplaning and aircraft speed in ground school.

Automobiles can hyrdoplane, as can aircraft. Not a situation you want to be in either vehicle. When you hydroplane, for example on a wet runway, your tire loses contact with the ground and you lose control of the aircraft.

Take a moment and go here and watch a two minute video on NASA's work on hyrdoplaning. You will see a formula in the video:

9 * (the square root of the tire pressure) = the speed in knots at which you will begin to hydroplane.

According to the above formula, a tire with pressure of 30 psi will start to hydroplane at about 49 kts.

I learned a quick and dirty version of this formula in ground school:

(tire pressure &#247 by 6) * 9 equals the speed that aircraft will start hydroplaning.

Using the quick and dirty formula, the tire will start hydroplaning at 45 kts. I like that the number is a little lower than the 49 kts in the NASA video because you have a target safe landing speed that is a little lower. Perhaps providing a safety margin? But it is an easy formula for to remember, and you can do the calculation in your head (or on the pad of paper on your knee board).

The key here is that the lower the tire pressure, the lower is the speed that the aircraft will start hydroplaning. Using the quick and dirty formula, the number comes out to be 42 knots. If you land at 45 knots with tires at 28 psi on a wet runway you will hydroplane.

I don't know about you, but that short conversation with the pilot made an impression on me. I"ll leave it to the folk at NASA to do their tests and research on hydroplaning aircraft tires.

Note - apologies for not using proper mathematical notation for the square root symbol. If I can figure out how to include a square root symbol in html code that will show up here, I'll revise the article later.


Anonymous said...

I have issues with this Dirty Formula given in this article. Please correct me if I'm wrong but the dirty formula with only work if you have an aircraft that requires a low PSI. I don't remember the last time I flew an aircraft with only 36 PSI. For example lets due this formula in a bigger aircraft. The last plane i flew had a PSI of 81. The square root of 81 is 9. 9*9=81kts this plane will hydroplane. now lets us the dirty formula. 81/6=13.5*9=121.5kts. That is a difference error of 40.5 kts. So the way I see it is if you plan on only staying in an aircraft with low PSI this dirty formula works. otherwise it could be very dangerous. If I did something wrong please correct my mistake.

Random Ramblings NJ said...

You did nothing wrong in questioning the quick and dirty formula that I originally included in my post. You are correct, and I regret that I was not more careful before making my original post. So, thank-you! You will see that I have used strikeout font to cross out the erroneous information.

Anonymous said...

Many years ago I was given the formula, the square root of the tire pressure times the tire width. This formula takes into account "where the rubber meets the road." It seems to work for most tires but I will leave the test flying to much younger pilots.

Random Ramblings NJ said...

Thank-you Anonymous for your interest and for your comment on using the formula, the square root of the tire pressure times the tire width.