Takeoff OAT Restrictions

Takeoff OAT Restrictions

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 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.”

Therefore, when this same FOD worry started affecting B200s of 1993 and after design – as well as earlier 200s and B200s that were now being retrofitted with four-blade props – the solution was easy … copy the 300 technique and use ice vanes all the time while on the ground. Oops! What about that +15°C limit that applies to the 200-series but not the 300-series?

For a few years, the limitation was basically ignored. Personal observation has convinced me that it is extremely rare for oil temperature to hit the maximum redline even in Phoenix, Arizona, in the summer months with a lengthy ground delay. Whew, I am happy for that! Then Beech got around to revising the POH and removing the +15°C limit. Now there is a “Note” in the “Before Engine Starting” section of the normal checklist that reads as follows: “The engine ice vanes should be extended for all ground operations to minimize ingestion of ground debris. Turn engine anti-ice off, when required, to maintain oil temperature within limits.”

If you, unlike I, do indeed find that you must turn engine anti-ice off because of hot oil, then avoid using beta and reverse even if it means riding the brakes at times.

Under the title of “Icing Limitations” found in Section 2 of the B200’s POH it states: “ICE VANES, LEFT and RIGHT, shall be extended for operations in ambient temperatures of +5°C or below when flight free of visible moisture cannot be assured.” The next statement is: “ICE VANES, LEFT and RIGHT, shall be retracted for all takeoff and flight operations in ambient temperatures of above +15°C.”

It is obvious that FOD due to ground debris is not a problem in flight. It is also not a problem during takeoff unless the takeoff is aborted and reverse remains in use to too low of an airspeed. Hence, when doing the runway lineup procedure on warmer days, it is time to retract the vanes. Not only is better oil cooling assured but more takeoff power can now be achieved with less chance of being ITT-limited.

Now let’s examine the 300-series “Icing Limitations” found in Section 2 of its POH. This one is nearly identical to the 200, except for substituting “Engine Anti-Ice” for “Ice Vanes”: “ENGINE ANTI-ICE, LEFT and RIGHT, shall be ON for operations in ambient temperatures of +5°C or below when flight free of visible moisture cannot be assured.” The next statement is: “ENGINE ANTI-ICE, LEFT and RIGHT, shall be OFF for all takeoff and flight operations in ambient temperatures of above +10°C.”

Do you notice what is different between the 300 and 200 in the second limitation? The ambient temperature got dropped by 5°: +10°C for the 300 and +15°C for the 200. Why the difference?

Since the pitot cowl of the 300 negates any oil cooling worry, the reason has nothing to do with the oil cooler’s effectiveness. Rather, it comes from wanting to ensure proper takeoff performance. When there is no need for ice protection, why subject the engine to the slight power loss that goes hand-in-hand with ice vane deployment? The “Minimum Takeoff Power” numbers – from the graph in the Performance section of the POH – are based on the assumption that ice vanes will not be deployed during takeoff when unneeded.

In a similar manner, this helps explain the 300-series’ POH statement that, on first reading, makes no sense: “For takeoff, Generator Load must not exceed 30% with air conditioning on, nor 50% with air conditioning off.” Since the condenser blower operates whenever AC is operating with the nose gear extended, and since this blower uses about 50 amps, it seems that the generator load would be higher, not lower, with AC on. Right? Yes, that is correct … but it’s not what the restriction is addressing.

The engine is subject to three things that can cause available takeoff power to be less than optimal even though the engine itself is fine: (1) Cowling inefficiencies, caused by ice vane deployment; (2) Compressor shaft load or drag caused by the need to drive the AC’s compressor (on the right engine); and (3) Compressor shaft load caused by generator load (on both engines). If we have little electrical load – no electric heater or windshield heat in use – then we can abide the AC drag and still have sufficient power available to the propeller to meet takeoff power design criteria. However, if the generators are working their guts out, then we don’t have enough “leftover” power to load up the compressor with the AC’s compressor drag.

To summarize then, the 300’s requirement to not use engine anti-ice for takeoff when OAT is above +10°C is based not on oil cooling concerns but instead is based on eliminating the cowling inefficiencies that could lead to inability to meet the Minimum Takeoff Power target torque.

For all of the other King Air models – 90-series, 100-series – you, like the 300-series, have no tie-in between ice vane deployment and oil cooling. If you have a three-blade propeller, especially if it’s combined with the original “Chin” style of cowling, there is no concern about FOD due to ground debris even with the ice vanes retracted. On the other hand, four blade props combined with the pitot cowl – F90-1s, C90As and after – have enough FOD potential that engine anti-ice ON while on the ground is strongly recommended.

But consider this: Leaving the ice vanes up on a four-blade 200 or any member of the 300-series is asking for FOD … leading to a very expensive repair. But having the ice vanes down on your B90 causes no problems whatsoever. If you, like many pilots, fly a variety of King Air models, then there is absolutely nothing wrong with making “Ice Vanes down for all Ground Ops” your SOP (Standard Operating Practice). Are ITTs affected? Is engine starting affected? No! The only negative associated with this procedure is forgetting to retract them when taking the runway and hence not being able to attain your target minimum takeoff power.

I’ll leave you with this thought, readers: Forgetting to retract the ice vanes for takeoff may not be as bad as you think it is. Why? Because the ram air loss at takeoff speed – 100 knots? – is a whole lot less than what you are used to seeing when you pull those ice vane handles out (or activate the switches) before entering that cloud deck below you while in a descent doing 200+ knots. You dig? Groovy, man!

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