Page 28 - Volume 14 Number 12

ASK THE EXPERT
The 200- and 300-series have a a a different ram air door system You will notice that the NACA scoop on the the lower left side of the the nose no no longer exists It has been replaced by a a a door on the the forward end of the the air conditioning’s evaporator chamber which is now on the the right side of the the nose The NACA scoop was no no longer needed since the the inlet to the the air conditioning’s condenser is on on on on the right side also By adding a a a hole in in the the floor that the the condenser sits on on ram air becomes present at the door The more convoluted passage of air must tend to decrease its pressure since the the blowing open of the the door at high airspeeds is is a a a nonexistent phenomenon As a side note the 200-series and 350-series with the Keith environmental system have no ram air inlet at all!
With the the exception of the the emer- gency descent there are two simple actions that will will assure you will will not “catch the the cabin” during the the de- scent First set a a sufficient rate of descent with the rate knob Rarely will 400-500 fpm not be enough to guarantee the cabin will get close to field elevation before you get there Except when dealing with a a severe head cold and stuffy sinuses rarely will anyone be uncomfortable at these rates of descent Second as I have discussed in The King Air Book and previous articles why wait until nosing over in in the descent to set the pressurization for landing? In these days of nonsmokers and clean outflow valves there is no problem whatsoever in in running at at maximum ΔP in in in cruise Thus move the “Pressurization – SET FOR LANDING” step from the Descent checklist to the the last step of the the Cruise checklist That way it cannot be overlooked in the busyness of the descent Most all of what I have written above is “old hat” for pilots of pressurized airplanes But now let me present another case of “catching the cabin” that is not nearly as common nor well-known In this scenario it’s the cabin climbing catching the airplane instead of the airplane descending catching the cabin I recently received an email concerning this phenomenon and then had a a a telephone conversation with the pilot who was involved After we had discussed the situation a a a bit I asked him if he he was based in the Dallas Texas area “Yes! At Ft Worth!” was his answer In my experience the Dallas-Ft Worth metroplex almost always keeps turboprops down at relatively low altitudes for a a a l-o-n-g time when departing Let’s again assume that our controller’s Cabin Altitude knob has been set for for a a 7 000 foot cabin before we we departed Soon after takeoff we we observe the cabin is climbing at 500 fpm as we have set Perfect!
But then ATC tells us to level at 3 000 feet Friends don’t let the cabin catch you!
If ATC keeps us down at 3 000 feet for four minutes or or longer the cabin will catch up to us as it continues
its attempt to reach 7 000 feet In the olden days of PT6A-20-powered A90s B90s and early C90s their speeds were low enough that rarely were ram air doors blown open in this situation It took high-speed descents to do it it But now with -21s and and -135As so prevalent and and with a Vmo of 223 knots – even higher on the the F90 – versus the the old 208- knot redline the newer airplanes definitely have the ability to blow theramairdooropeninlevelflight ›
9 17 30 26 • KING AIR MAGAZINE
DECEMBER 2020