Ask the Expert: Pitot Tube-Static System Abnormalities

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 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 shop next with its “hell hole” door open. See that round fitting with the hole in the middle, below the outflow valve, almost touching the bottom fuselage skin? Yes, that’s the end of the alternate static air line.

I now want you to think about a failure that was never discussed back in private pilot ground school and, unfortunately, perhaps has also been overlooked during your Initial and recurrent King Air training sessions. As you probably know, down near floor level on the right sidewall in the cockpit, there is an upholstered panel providing maintenance access to two or three static air line moisture drains. Although I have yet to find a mechanic who has ever seen water drip out when they are opened for one of the phase inspections, nevertheless that is their purpose and they are located at the low points in the lines. Suppose that the mechanic got distracted or called away and forgets to close the drains. Now what?

This exact scenario happened not once, but two years in a row, when I went to pick up my company’s old C90 from a well-respected shop in California! In their defense (They still should not have made the mistake!), the drain valves on this 1972 model were rather unusual in that the valve “handle” was positioned parallel with the tube when the drain was closed and perpendicular to the tube when open … just opposite of what one would normally expect.

So there I am, thorough post-maintenance pre-flight complete, all system run-up checks complete, departing for the short hop to my home base. All seems
normal … rotate, gear up, and pitch for the normal 10° initial attitude.

Hmmm, why is my airspeed so low?! Yes, the copilot’s indicator is also reading low. We’re down to 80 KIAS and it is decreasing rapidly. Let’s recheck attitude – 10°, check – and recheck power – torque near redline, propellers still a maximum speed, check – so the IAS has got to be reading incorrectly. It was about now that my scan revealed that both altimeters were reading just a little above the departure airport’s elevation. I estimated I was about at pattern altitude, pulled power back, and turned to remain in the pattern. Now I had time to select alternate air – reaching over to the lever on the right side panel – and no change took place! You see, the open drains still permitted cabin air pressure to enter the instruments since the drains were downstream of the source. I figured that depressurizing the cabin would be my only solution and since I was only at pattern altitude I used the Dump switch instead of the more gentle method of turning off both left and right bleed air switches. Immediately, as the cabin ascended to aircraft altitude, all instruments returned to normal operation, allowing for an uneventful approach and landing. (Followed by a rather eventful meeting with the shop supervisor!)

Here is the part of this story that absolutely “blew my mind.” Before I dumped, the airspeed had dropped so much that it was actually reading below zero! A quick glance at it, without observing its previous slow demise, would have left an observer believing we were overspeeding, past VMO. I am quite happy that this whole event transpired in clear weather conditions and that I had not departed into low clouds.

It had never occurred to me previously just how little the actual difference is between pitot and static pressure that leads to our normal IAS indications. Merely having the static pressure about one thousand feet lower than it should have been led to more than a 150 KIAS difference!

Imagine what would happen if those drains were opened in normal cruise flight, while fully pressurized. Instead of simply sensing an incorrect static pressure from the beginning – as happened to me – now the system would instantaneously experience a huge pressure increase, with the “felt” altitude going from, say, 24,000 feet to 8,000 feet in a heartbeat. A colleague said this happened while he was flying a Convair 580 and not only were six instruments ruined, but it included having the glass faces of the VVIs ejected from the panel!

In conclusion then, two things: First, add one more pitot-static malfunction into your storage bin of facts: The effect of introducing pressurized cabin air into the static lines. Second, know your magic numbers. If and when your airspeed indication is malfunctioning, it certainly is comforting to have solid torque and airframe configuration combinations in mind that you know will yield safe and appropriate speeds.

If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at kblonigen@cox.net.