Page 19 - Volume 14 Number 10

scan of the engine gauges, and then returned my scan to the flight instruments. What the ...? My airspeed was only 80 knots! I rechecked the torque gauges to ensure I was at takeoff power and rechecked my pitch attitude both visually and with the attitude indicator – all normal. Knowing “pitch plus power equals performance,” I was sure the airspeed indicator was faulty. But dang, so was the one on the co-pilot’s side! By then, the indicators were decreasing below 60 knots. I reached over and moved the alternate air selector lever to the alternate position; nothing happened. I leveled off at 1,500 feet AGL pattern altitude, pulled power back to the middle “magic number” (500 ft-lbs for this C90), and stared in disbelief at the airspeed indicators that now were reading extremely high ... since they had decreased so much that the needles were beginning to point to the numbers to the left of zero!
Suspecting the problem, I reached down to the Pressurization Control switch and moved it to the Dump position. The cabin was only about a thousand feet below the airplane with 0.5 psid differential pressure so the dumping was not at all drastic. Immediately, the airspeed indicators resumed normal operation. I announced on the Unicom frequency that I was on left downwind for landing, proceeded with a normal landing, and taxied back to the shop’s ramp. I knew what was likely wrong and, sure enough, they found the static air line drains were all open. This allowed cabin air to enter the static system since the drains are behind an access panel low on the right sidewall of the cockpit, inside the pressure vessel. The entrance of cabin air, being at a greater pressure than ambient, was what led the airspeed indicators to read low, since they sensed less of a difference between pitot and static pressure. It absolutely blew my mind that a half of a psi error could lead to a negative airspeed indication!
Once the drains were closed, the next takeoff was, of course, normal and I departed on my way. In the shop’s defense I will mention that the drain valve blade “handles” were unusual. When the metal blade was parallel with the drain line, the drain was closed. Vice versa, when the blade was perpendicular to the line, it was open. In fact, as we examined the drains we noted that someone previously had used a Sharpie to draw a picture on the back side of the access panel, showing clearly the proper open and closed orientations of the blade. The mechanic obviously did not notice that picture.
Could this mistake have been caught before takeoff? I doubt it, except by observing the airspeed indicators during a ground pressurization test. The main takeaway here is the advantage of making that first flight in day, visual, conditions. I hope I could have handled this abnormality successfully even at night or departing into a low overcast, but I am thankful that those conditions did not exist. I would encourage strongly that the first post-maintenance flight be VFR.
OCTOBER 2020
KING AIR MAGAZINE • 17