Page 21 - Volume 11 Number 4

temporary emergency Airworthiness Directive (AD) to be issued to prohibit icing flight – and two changes were made. First, the oil cooler was relocated to the scoop that was attached to the bottom of the nacelle, so that if and when it was bypassed and iced up there would be no effect on engine airflow. Second, engine auto-ignition was designed and installed.
Prior to this, King Airs had left and right ignition switches so that the ignitors (glow plugs, back then) could be activated without the starters being energized. This was for use while doing windmilling airstarts. But it appeared that too many early King Air pilots – who likely had almost no previous turbine experience – forgot that these switches had to be turned on to relight engines that had suffered a flameout due to ice. (“Heck, it didn’t work that way in the P-38 or C-47!”) So, both Beech and the FAA wanted a system that would automatically turn on the ignitors when power was lost. The system is comprised of a simple electrical relay that activates the ignitors whenever torque is below about 400 ft.-lbs. when the switches are in the “arm” position. To state the obvious: When an engine flames out in flight, torque immediately goes to zero (actually, a negative value), well below 400 ft.-lbs.
Then it happened. The Beech test airplane had a
double engine flameout due to ice ingestion. The crew
got the engines running at a lower altitude – although
with compressor damage – and landed safely. The line at their Salina, Kansas, facility and the airplanes investigation revealed the culprit.
With the clarity of 20-20 hindsight it is amazing that no one thought of the problem before, but here it is: The deflected ice particles came in contact with the oil cooler face, the warm oil melted the ice, and the subsequent water blew out the cowling from the aft side of the cooler. What was being overlooked, however, is the little device called the vernatherm valve, the gadget that regulates the flow of oil through the cooler to maintain the desired temperature. Under very cold OAT conditions, all of the oil is bypassing the cooler, so now the deflected ice particles coat the face of the cooler leaving absolutely no other path than directly into the engine intake. Damn! That explains why the problem was only showing up at 16,000 feet and above ... due to the cold winter OATs up that high causing the oil to bypass the cooler.
Back to the drawing boards the engineers went – with the FAA in close oversight, since King Airs losing engines in flight had definitely gotten their attention, causing a
APRIL 2017
In 1966, the second edition of the King Air series, the A90, replaced the “straight 90,” and one of its many large and desirable improvements was the incorporation of the ice vane system replacing the alcohol spray system. But then, a big problem raised its ugly head: Some of the A90s were experiencing loss of engine power due to ice ingestion! In fact, one of the first cases involved a Beech demonstrator A90 with the vice president of sales on board, at night over the Rockies!
“What’s wrong?! How can this be?! We tested the system thoroughly and the FAA certified the design! What’s going on?!” said the incredulous design engineers. Beech and Pratt & Whitney immediately started a new series of icing flight tests, trying to find why the problem was manifesting itself at this time. It was now the winter of 1966 - 1967. Closed circuit TV cameras were installed in the cowling to try to see what was actually taking place. Time and again Beech sought out icing conditions, flew in them extensively, and yet the airplane came through just fine. It was observed that all the engine flameouts related to them by the operators took place at 16,000 feet or above, so those conditions were emphasized. Weeks elapsed without any problems being discovered.
There were so few King Airs in existence at that time – probably less than 150 – Beech set up a modification
KING AIR MAGAZINE • 19