Acting Your … Altitude

When flying a light single engine airplane you would not expect to accelerate while climbing, but what if the instruments told you just that – do you believe the instruments or is something else going on? While it is possible for the airspeed indicator to gradually show an increase in speed while the vertical speed indicator is showing a climb, chances are you will *not* see those results from your single engine trainer. More likely, it will happen because of a situation that develops in the pitot-static system…

When the airplane is not moving there is no ram air, so the airspeed indicator reads zero because the pitot and static pressure are the same. As the airplane increases speed the ram air starts to ‘inflate’ the indicator and pushes back the weaker static air. The greater the ‘push’ or differential pressure, the greater the speed will read on the face of the indicator. Note: The airspeed indicator is the only instrument that utilizes both ram air from the Pitot Tube and static air from the static ports.

The Pitot Tube has not one but two holes. The ram air port is turned so that on-rushing air goes right into the hole and then is routed through a tube to the airspeed indicator. The second hole is located on the bottom of the Pitot Tube and designed to allow water to drain out of the tube. This drain hole plays the pivotal role in the airspeed-acts-like-an-altimeter situation.

1) If the ram air hole alone ever becomes clogged in flight, the airspeed indicator will read zero. This is because there will be no more on-rushing air to inflate the airspeed indicator and the inflated, high-pressure air will equalize out through the drain hole — just like letting air flow out of a balloon.

2) If both the ram air hole and the drain hole become clogged, the high-pressure ram air becomes trapped in the airspeed indicator, because the drain hole can not vent the pressure. It is like blowing up a balloon and then tying it off. Now, with the high-pressure air trapped in the indicator, any change in altitude will change the airspeed reading.

A CLOSE LOOK AT CASE 2: The static air pressure will diminish if the airplane climbs (higher altitude = less air pressure). Remember: The airspeed indicator is reading a delicate balancing act between the high-pressure ram air and the lower pressure static air. Example: If you take a tied off balloon up with you in the airplane, the balloon will get bigger as you climb. This is because there is less outside pressure to contain the balloon. The pressure inside the balloon has less resistance and therefore is able to expand.

Translation: The same expansion would happen in a plugged pitot system and cause the airspeed indicator to display a faster speed. So as the airplane climbs the airspeed indicator will gradually display a faster and faster speed. A descent will indicate a slower speed. This is why it is referred to as ‘acting like an altimeter’ the airspeed reads higher the higher you go and visa versa. The airspeed will always read incorrectly except when the airplane is at the altitude where the blockage occurred.

This situation can easily confuse a pilot (accelerating in a climb or decelerating in a descent,) but the source of the blockage could indicate a larger problem. Most common ram and drain hole clog is ice. Use the pitot heat anytime there is visible moisture present in the air and properly inspect both the Pitot Tube ram and drain holes before every flight.