Page 21 - Volume 13 Number 11

Ask the Expert
Takeoff OAT Restrictions
by Tom Clements
At the fourth King Air Gathering – KAG IV, the sold- out event held in September 2019, at the classic Hangar Hotel complex at the Fredericksburg, Texas airport (T82) – a question was asked about engine ice vane usage on the ground. Specifically, a concern was expressed about a temperature restriction stated in the Pilot’s Operating Handbook (POH) for the model 200-series. Is a limit being violated at times when ice vanes are being used?
A follow-up question asked about a similar concern for the 300-series. I plan to review and discuss these questions and more in this article.
Realize that all King Airs have an OAT limit above which they are not allowed to operate. In almost all cases this is expressed as ISA + 37°C. “Golly, 37°C is only 98.6°F, so there’ll be lots of times that we cannot fly!” many folks think. Wrong! The 37°C temperature is not the same as “ISA + 37°C.” ISA stands for “International Standard Atmosphere,” the engineering-accepted model of the average worldwide atmosphere. This is the one with a Sea Level temperature of 15°C or 59°F and a lapse rate of 2°C for each 1,000 feet up to the stratosphere that starts at FL350. ISA + 37°C is a shorthand way of saying, “The OAT that is 37°C above the standard temperature for that altitude.”
Therefore, at Sea Level, the King Air’s limiting OAT for operation is 52°C (15 + 37). This equates to about 125°F. Does it ever hit that sweltering temp? Sure, but it’s quite rare. Can you figure what the limit is at 10,000 feet? Since the standard atmosphere experiences a drop of 2°C for every thousand feet, we would have decreased 20° from Sea Level to 10,000. That puts ISA at -5°C. Adding 37 more gives 32°C, or about 90°F ... mighty warm at 10K!
I heard from experimental flight test colleagues at Beech that the limiting factor for hot weather operation is the size and capability of the engine oil cooler. We all know that performance decreases as temperature increases. Although performance would definitely degrade, the actual reason why there is an OAT limit is based on the ability of the oil cooler to keep oil temperature from exceeding its limit. As a side note, the fact that the Blackhawk XP67A engine modification to the 350 adds a fixed “cowl flap” at the oil cooler’s outlet yet still has an OAT limit 3° cooler than before – ISA + 34°C now – lends
NOVEMBER 2019
support to the assumption that oil cooling is the reason for the OAT limit. (If that poses an operational problem for XP67 airplanes based in hotter climates, a larger oil cooler is available that brings the OAT limit back up to the original value.)
When the model 200 first appeared, its POH stated that engine ice vanes could not be extended when the OAT exceeds 15°C. This applied for all operating conditions, ground and flight. Again, we return to oil cooling considerations. Unlike the King Airs that preceded the 200, as well as those that came later with the “Pitot Cowl” design, the cowling used for the 200-series is unique. When the ice vanes are extended, the “bypass door” also opens to allow the deflected ice particles to harmlessly leave the cowling. Oil cooling suffers now because the bypassing air is no longer able to flow across the oil cooler’s fins. From its market introduction in 1974 up until the 1993 model year, the +15°C ice vane limitation was heeded with no operational difficulty experienced.
In 1993, beginning with serial number BB-1444, the B200 incorporated many welcome improvements. Among these were the advent of four-blade propellers as standard equipment, replacing the three-blade Hartzells and McCauleys of the past. The higher low idle compressor speeds and flatter low pitch stop blade angles – required to ensure that propeller speed remained above the new minimum propeller speed limit, a limit imposed to avoid the “reactionless vibration” mode that may lead to propeller damage – conspired to make FOD (foreign object damage) much more likely. Soon after the 300 model made its appearance in 1984, reports began arriving at Beech of numerous cases of first-stage compressor FOD on the PT6A-60A engines used on this new model. The distance from propeller tip to the ground is less in a 300 than in a 200. Combining that fact with the 300’s pitot cowl and four-blade standard propellers with their higher idle speeds, FOD became much too common!
The easy solution was to change the procedure such that ice vanes – now correctly called “engine anti-ice” on the later King Air models – were deployed for all ground operation. The location of the oil cooler in the pitot cowl causes oil cooling to not be negatively impacted due to engine anti-ice activation. Thus, there really was no downside risk associated with this new procedure of “Ice vanes extended for all ground ops.”
KING AIR MAGAZINE • 19