Technical

Blocked pitot tubes

We have the G500H TXi in a helicopter and noticed that there was always a tailwind whichever direction we flew in, including going in to land at an airfield.

Pupae in the tube

We assumed that the airspeed must be reading low, and thus the Garmin produces an incorrect wind direction.  We had the service agent blow through the pipes and some pupae shot out (see pic).

Pupae blocking the pitot tube

On the 2nd occasion about a year later I was flying solo in quite gusty conditions.  I took off from my home and felt uncomfortable in the bumps.  Having seen Tim Tucker’s video about “fly at a speed you personally are comfortable with”, I pootled along at 85 knots thinking gosh this is still pretty bumpy.  The interesting point here is the dangerous confirmation bias because the high ground speed I observed confirmed to me – in that microsecond – that there were high winds and not that there was an airspeed read error.  A few microseconds later, I did realise my mistake though but probably only with the benefit of experiencing this previously.

Here is why I think a blocked pitot is dangerous

1. The blockage causes just enough airspeed error as to be inconclusive.  If the airspeed read zero it would actually be safer as it would be immediately noticed.

2. The situation is basically undetectable until flying at cruise speed – you cannot detect this preflight.

3. Since the first time, we started to use a pitot cover and yet this STILL happened.

4. The diagnosis in flight is easily confused due to the confirmation bias

5. The analogue dial still matched the reading of the TAS in the Garmin but not exactly.  This the pilot is easily convinced that the 2nd instrument cannot be a slave and must therefore confirm the airspeed is 85.

6. Blockages happen a lot.  I have had one a year!

7. Just finding the blockage in the aft tube gives a false sense that the blockage is removed, you MUST also check the forward section (remove the G500X unit to get to the airspeed dial).

I have now concluded that it is actually quite easy to diagnose but I feel like other pilots won’t know. 

Diagnose a blocked pitot

If you suspect low airspeed (e.g. the ground speed less reported tailwind from the internet is much higher than the displayed airspeed), then a pitot blockage can be confirmed by changing direction 45 degrees whilst pulling a gentle N1 of say 60%.  If the tailwind arrow does not significantly rotate (by 45 degrees), then your pitot may be blocked.

And keep it covered!

Robinson R66 with covered pitot tube

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