Blocked Pitot Tube


A likely cause if airspeed becomes erratic

By Dan Namowitz

A certain kind of aviation accident occurs with unfortunate regularity. Commonly, the victim is an inexperienced pilot, operating a rarely flown airplane from a small airport, but the mishap can occur in other circumstances, too. What's sad is that just a few minutes of preventive training can avoid it altogether, under any conditions.

Here's how it usually goes: The pilot fires up the airplane and taxis out for departure. Maybe there was a preflight inspection, maybe not. In either case, the throttle is advanced, and the steed begins galloping down the runway. At about the usual place the pilot eases back on the stick and casts a glance over at the airspeed indicator (ASI) for confirmation of having reached rotation speed.

Something's wrong. The needle of the ASI is erratic and says the airplane has only achieved 35 knots indicated airspeed — fast enough for performing mushing slow flight but surely not a good speed for topping the trees at the end of the field. The airplane feels like it is doing better than that — in fact, it is now airborne and climbing.

The pilot is confused. Torn between what he sees outside and what he sees inside his airplane, he doesn't know what to believe. He is a safety-conscious pilot at heart so, not wanting to take undue chances, he pokes the nose down and tries to land on the remaining runway. He touches down fast and realizes immediately that it's too late for that; he's not going to get stopped in time. So he changes his mind again and reaches for the sky, but time has run out for decision making. Off the end he goes in a cloud of dust and flying grass.

The cause of this kind of mishap is some kind of obstruction in the pitot-static system: an insect, some ice, or perhaps even a pitot-tube cover may be the culprit. Sometimes a preflight inspection could have averted the incident, but other times the problem is hidden inside and would have avoided detection despite a careful inspection. That is why, after a student has flown a very few hours in primary training, it is a good idea to demonstrate in flight — and then have your student practice — establishing the pitch/power relationship for various airspeeds without the use of the airspeed indicator. Not only is this a cheap insurance policy against accidents, but it has the added benefit of showing the student how much he or she already knows about the airplane after only a few hours of experience.

A good time to do this is when starting work on flight at speeds slower than cruise. This is when flight at new angles of attack, with different control pressures, will be encountered for the first time. First we do some preliminary work in the slow-flight realm, getting used to the pitch/power combinations that result in various airspeeds from cruise down to minimum-controllable airspeed. When the student has gotten used to the idea that he or she can achieve a particular speed at will — simply by setting the required pitch/power combination — it's time for the next step. I pull out one of those floppy rubber instrument covers, slap it over the ASI, and ask the student to set up level flight at various airspeeds, based on the estimate we have made of the appropriate power/pitch combinations required. When the student has the airplane set up, and he or she says, "Ready," I peel back a corner of the instrument cover and we take a peek at the indication. Usually they succeed within five knots or so, and are both surprised and delighted at this result.

Later in the training program I will repeat the drill a few times, to make sure that the lesson has been remembered. During at least one of those reviews I will not reach over and remove the cover until we are taxiing back to the ramp at the end of the lesson. By then, in addition to the airwork described, we have worked on stall recognition and recovery, and performed several takeoffs and landings, and perhaps a go-around or two. Often, as I lean over and remove the cover from the ASI, the student laughs and says, "You know, I forgot all about that!" Which gives me the opportunity to point out again that other information besides the ASI — control pressure, angle of attack, power setting, and the feel of control forces — can be used with fair precision to estimate airspeed, should the ASI ever malfunction.

I explain that numerous people have run airplanes off the end of runways for lack of this simple and effective training, especially at short strips where the time to recognize and react to a problem on takeoff is especially compressed. The student now agrees, based on the knowledge acquired, that such an accident cause is entirely unnecessary. He or she is happy to know that one of aviation's pitfalls for the unwary has been avoided. For you, the flight instructor, this may be one the easiest "saves" you ever make.


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