Page 23 - August 2015 Volume 9, Number 8

Ask the Expert Pitot-Static System
Abnormalities
by Tom Clements
When we were taking our first ground school lessons, we learned – among many other things – how static air pressure was sensed and used to operate the altimeter, vertical velocity indicator (VVI), and airspeed indicator. Of these three instruments, airspeed was the only one that required an additional input: ram air pressure as sensed by the pitot tube.
Although we have now advanced in our flying endeavors to the pressurized, turboprop world of King Airs, the knowledge we learned initially about the pitot- static system remains basically the same as it applies to King Airs as it did to our simple, single-engine trainers. Our altimeter and VVI still work by measuring static pressure and its rate-of-change, and our airspeed is still derived by measuring the difference between pitot and static pressure.
What has changed, in some of the more-sophisticated and modern King Air installations, is that the actual indicators are no longer mechanical devices but instead are electronic. These electronic displays may look and act almost the same as three of our old “six-pack” friends – with the exception of having a red, “Off” flag of some sort – or they may be part of a complete glass cockpit. In these situations, no longer do pitot and static lines connect directly to the appropriate instrument but instead they connect to an Air Data Computer, or ADC. The ADC then sends the appropriate electronic information to the displays to make them depict the proper information.
Think back to that initial ground school discussion of the pitot-static system. What did you learn about failures? I bet you were taught about the wasp building its nest in the pitot tube, right? How about the tube being blocked with ice? Did your instructor mention the importance of assuring that the cleaning crew did not leave the static ports covered with tape after their wash job? Did you learn about the need for an alternate static system, where it was located, and how it was selected? Yes, I thought so.
Let’s do a quick review of these failures. If the pitot tube is blocked before takeoff, we would hope that the
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lack of an increasing airspeed indication would be noted early in the takeoff roll and lead to an uneventful aborted takeoff and a taxi back to the ramp to find and fix the problem. (Pilot to copilot, copilot to pilot: “I thought you had taken off the pitot covers!”)
If the pitot tube became blocked in flight, during the climb – quite likely due to icing, with either the pitot tube’s heating element inoperative or the pitot heat switch inadvertently not activated – now what will happen? That’s correct: With trapped ram pressure but decreasing static pressure caused by the airplane’s ascent, the difference between the two increases, causes the indicated airspeed to increase. Perhaps the most infamous case of this was the Northwest B727 with just the crew on board, on its way to pick up a sports team in the northeast. All on board were killed when the airplane stalled, spun, and crashed due to the huge angle-of- attack that came from a massive nose-up attitude. And why was the nose so high? Because the pilots fixated on the increasing indicated airspeed and kept pulling the nose higher and higher in a futile attempt to slow the plane down. It was this accident that led the FAA to mandate that pitot heat be on in flight at all times in jet airplanes and why annunciators tell the crew if they forget to activate the system. The King Air 350 has this warning system also.
On the other hand, if the static ports were blocked with wax or covered with tape prior to takeoff, that mistake will not likely be noted during the takeoff ground roll. However, as the airplane begins its climb, it will become obvious quickly that something is amiss. The altitude and vertical speed won’t show any change! Airspeed? It will be reading less than it should due to the trapped static pressure being greater than the actual static pressure at the current altitude.
Here is the time that the selection of alternate air is called for and will immediately solve the problem. The alternate air source’s actual location can and is different in different types of flying machines, but for all King Airs the source is the unpressurized aft fuselage or tail. If you are not familiar with it, peak up at the aft side of the aft pressure bulkhead when your aircraft is in the
SEPTEMBER 2015