Drain hole blockage?

popaviator

Well-Known Member
I should probably know this, but I'd reeeaaaly tired (still a lame excuse...I know) irate: But what errors would occur if just the drain hole is blocked in the pitot tube?

I would say none until you fly into visiable moisture. Then it would act just like both the drain hole and pitot is blocked because the moisture has no place to drain because the drain hole is blocked.

I'd have to go with what Brian wrote.

What Brian said....although it's possible that even in normal flight the airspeed would read a bit higher than normal. Having that opening at the back probably lets a small amount of the pitot pressure bleed out.

Drain hole blockage affects the behavior of the system when the pitot port is also blocked. If the pitot is blocked and the drain hole is not the airspeed will read zero because the pitot reads static pressure. If the drain hole AND the pitot is blocked at least the static pressure of the current altitude is trapped inside of the system. Climbs will indicate an increase in airspeed because the static pressure at the trapped altitude is greater than the static pressure at the new, higher altitude. Descents have the opposite effect. Changes in actual airspeed have no effect.

The way I describe it to my students is what it's function os for, and that it's important it's not blocked because you can think of it kinda like a VSI, as in it has a "calibrated leak." Some of the pressure will leak out through the drain hole, and cause a correct reading from the airspeed. If that "leak" is not present, the airspeed will be inacurate(higher than usual).

The way I describe it to my students is what it's function os for, and that it's important it's not blocked because you can think of it kinda like a VSI, as in it has a "calibrated leak." Some of the pressure will leak out through the drain hole, and cause a correct reading from the airspeed. If that "leak" is not present, the airspeed will be inacurate(higher than usual).

Hey we agree! Does anyone have an exact definition for the drain hole? I mean the engineering reason for its presence, I was under the impression it was to relieve some pressure from the tube (which if blocked would yield a higher pressure) but no clue if thats actually why it is there.

as stated earlier, the drain allows excess moisture/water to drain from the pitot system so the lines don't become blocked with water

This simulator is one of the best tools for teaching the pitot static system. It does an amazing job of visually showing what happens in any possible scenario, as well as the ability to change altitude and airspeed to see exactly how the instruments will react. Way better than a textbook explanation I have found. Good job Luiz!

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