The Pitot Tube is an essential part of an essential aircraft system and failure to understand exactly how that system functions or assure its proper operation has made for catastrophic results. When you inspect the Pitot Tube don’t forget that there are two — not just one — holes in that thing that must be open for the system to work properly … three if you count the static port.
MORE THAN A FEW PINHOLES
We all look into the ram air opening of the Pitot Tube before every flight. We want to make sure that there is no debris in the hole that could block the incoming rush of air into the tube. But the Pitot Tube also has a drain hole that must be clear as well. The air that enters the tube is not always dry — humidity and visible moisture (clouds, rain, etc) will enter the tube in the slipstream, so a drain hole is included to get rid of the water. The drain hole is located on the underside of the Pitot Tube and should be checked prior to every flight — especially an IFR flight.
The ram air from the Pitot Tube is led through a flexible line into the Airspeed Indicator. The Airspeed Indicator gets its reading by comparing the ram air that comes from the Pitot Tube to the outside static air that is coming from the static port(s).
How it works: You can understand what is taking place easier by imagining a closed cylinder with a diaphragm across the center. On one side of the diaphragm is the ram air and on the other is the static air. Ram air is like what your hand would feel if you stuck it out the window of a moving car — static air is like what your hand feels inside a closed car. The ram air will be much stronger and therefore will ‘inflate’ the side of the cylinder that is linked to the Pitot Tube. That pressure will cause the diaphragm to bulge from the ram air side toward the weaker static air side. The greater the bulge, the greater the airspeed reading.
SYSTEM FAILURES AND WHAT THEY MEAN
THE ZERO READING
If the ram air opening of the Pitot Tube became blocked, the drain hole would then act like another static port. For the diaphragm, it would be like letting the air out of a balloon. The pressure in the system would equalize on both sides of the diaphragm as the pent-up air pressure in the ram air side ‘drained’ out the drain hole. Without a pressure difference in the diaphragm, there would be no bulge. Without a bulge, there is no airspeed indication. The Airspeed Indicator would read zero.
HIGHER OR FASTER?
If both the ram air opening and drain hole both become sealed in flight, the ram air side of the cylinder is ‘pressurized’ and this force pushes back the diaphragm and against the static pressure. If both ram and drain holes become blocked while the diaphragm is bulging, the pressure will become trapped. It would be like blowing up a balloon and then tying it off. The Airspeed Indicator would read correctly — but only for the airspeed and altitude at which the Pitot Tube holes became clogged! This is the condition that is sometimes referred to as the ‘airspeed acting like an altimeter.’
Example: If the airplane climbs to a higher altitude, the outside static pressure will reduce. This means that inside the Airspeed Indicator there will be less static pressure available to oppose the trapped, higher-pressure ram air. The diaphragm will bulge even more, because less static air is there to hold it back. This greater bulge will show up on the Airspeed Indicator as a faster speed. Like an altimeter, the airspeed indicator will now indicate a larger number when you climb. When the airplane descends, the static pressure will increase. The greater static pressure will provide more resistance to the sealed up ram air, the diaphragm will bulge less and the airspeed indicator will indicate a slower airspeed.
FATAL READINGS: A CASE STUDY
The ‘airspeed acting like an altimeter’ phenomenon (described above) caused the fatal crash of a Boeing 727 in 1974. The airplane was enroute to Buffalo, New York, to pick up the Buffalo Bills football team. The crew of three was alone on the airplane and was climbing through freezing rain. At some point, the Pitot Tubes and drain holes became covered over with ice. From the Cockpit Voice Recorder (CVR) investigators later heard the conversation among the pilots. The key conversation began as the aircraft climbed through 16,000 feet:
- 1st Officer: Do you believe we are going 340 knots and I’m climbing at 5,000 feet per minute?
- Captain: That’s because we are light.
[Remember, it was just the three of them on a 727. When the airplane got to 23,000 feet the airspeed was indicating 405 knots.]
- Captain: Would you believe that #%@*&!
- 1st Officer: I believe it. I just can’t do anything about it!
- Captain: Pull her back and let her climb.
[At this point the Flight Data Recorder and the CVR indicated that the stall warning system went off.]
- 1st Officer: There’s the Mach Buffet, I guess we’ll have to pull it up.
- Captain: Pull it up!
REALITY VS. THE INSTRUMENT
Of course, it was not the ‘Mach’ buffet that they felt — it was the stall buffet. When the First Officer pulled it up, as the Captain requested, the airplane went into an aggravated stall. The airplane had reached 24,800 feet before entering into a stall/spin that put the airplane into a 15,000-fpm descent. The crew was able to send out a MayDay call, but, misled by their instruments to fight the wrong problem, they were unable to recover before impact.
The airplane was not going 405 knots in a climb — the airspeed indicator was acting like an altimeter. The higher they climbed, the faster the airspeed reading became and that reading was completely false. The crew believed the false reading so much that they concluded that the buffet they felt must have been a product of approaching the speed of sound, rather than the stall buffet it actually was.
BOTTOM LINE: The same problem can easily develop in a smaller airplane — the Pitot-Static systems in a 727 and a Cessna 172 are essentially the same. However (and in one of the rare cases that this is actually fortunate for you) you can bet that a Cessna 172 will not accelerate in a climb. So, be suspicious of the system if you see something that can’t possibly make sense. Prior to that, make sure to do a thorough, non-complacent, investigative, preflight inspection and know your systems.