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Ice-Covered Pitot Tube Results In Low-Altitude Alert From ATC

You're flying an ILS through icing conditions and your airspeed suddenly begins decreasing. Your pitot tube is covered in ice. What would you do? Here's what happened to this pilot...


The Report

We found the following NASA ASRS report from November of 2018. An ATP rated pilot with over 22,000 hours of flight experience received a low altitude alert from ATC after becoming disoriented due to a blocked pitot/static system...

On approach crossing the ILS FAF, the airspeed indicator started to decrease as I continued my descent. I pushed the nose over to increase speed and realized that the pitot tube must have frozen over. I quickly reached for the circuit breakers and switched them on and off to re-energize them, and was quickly blown off the localizer course and descended below minimums. I tried to activate my iPad map function to verify position, adding to my overall distraction.

I was quickly alerted by the tower that they had a low altitude alarm and to climb immediately. I made visual contact with the ground and proceeded to the airport for landing. During taxi, I was told to call the tower and was informed I would have to report to the acting supervisor.

Pitot/Static Blockages Can Be Complex

This sounds like a worst-case scenario, right? Having an instrumentation failure in IMC on the final stages of an approach is a bad day, especially when it comes to airspeed indications.

Airspeed failures are a very real possibility, especially if you inadvertently enter icing conditions.

So what exactly can happen, and how will your airspeed indicator react?

In the flight scenario above, the entire pitot tube became covered in ice, yet the static ports remained open. The ram pressure was trapped in the pitot tube.


As long as this pilot remained at the same altitude, the airspeed was frozen, regardless of power setting. But as the pilot descended on the ILS, the static pressure started to increase. Because the trapped ram pressure in the pitot tube was constant, but the static pressure was increasing, and the airspeed indicator started to slow down.

The diagram below is a great example of this. If you start adding pressure to the "static ports" side of the scale, it will tip left, and airspeed will decrease.


There's a lot to take into account here, and being close to the ground on an ILS is just about the worst time to deal with an instrumentation failure like this. There are multiple types of pitot/static system blockages, each with its own unique hazards.

Click here to learn everything you need to know about Pitot/Static System Blockages.


Would You Do Anything Differently?

How would you have handled this emergency? Would you have done anything differently? Tell us in the comments below.

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