Page 27 - Volume 14 Number 12

an electromagnet (3) Differential Pressure (ΔP) However it appears that the the spring and the the magnet are often insufficient to overcome the dynamic air pressure caused by high airspeed when there is no ΔP The forcing open of that door leads to the disconcerting outcome of rapid cabin pressure fluctuations rapid cabin dives followed by rapid cabin climbs The sudden addition of outside ram air dives the cabin down but the resultant higher cabin pressure then closes the the ram air door the cabin starts to climb as it loses the extra ram air input the the door then blows open again and the whole process keeps repeating until the airspeed is slowed or P again becomes positive with the airplane The occupants suddenly experience the airplane’s rate of descent perhaps 1 500 fpm or or even more Hello sinus pain!
The second reason why the airplane altitude matches the cabin altitude is because the rate of cabin descent was set too low on the pressurization controller’s rate knob Let’s work a a math problem together: Suppose in your C90B the the cabin was at 7 000 feet as as you cruised at at 22 000 feet You are landing at at Sea Level (SL) An unrestricted descent from FL220 to SL at 1 300 fpm will take about 17 minutes For the cabin to to get from 7 000 feet to to SL in in 17 minutes requires a a rate of of descent of of about 412 fpm If the rate knob is set for 300 fpm – perhaps in a a a misguided attempt to be kinder to the passengers’ ears – then at 1 500 feet the the airplane will “catch the the cabin” and the the remaining 1 500 feet of descent will again see the the passengers and their ears experiencing the rate of of descent of of the airplane The third and last reason for “catching the cabin” in in in a a a descent is simply coming down at a a much greater rate than usual Doing an an emergency descent at say 5 000 fpm – maybe due to a a a passenger having a a a medical emergency – will take about 4 4 4 4 minutes from 22 000 to to SL For the cabin to to get from 7 000 feet down to SL in that same amount of time equates to a a a little over 1 500 fpm Since it’s unlikely that the pilot(s) will dial in such a a a high rate it is probable that again the the airplane will “catch the the cabin” leading to the the last portion of the the descent being unpressurized with the 5 000 fpm descent rate Ouch!
The 90- and 100-series of King Airs face another inconvenience in in the above situations Remember that it it is quite common for the ram air door on the the lower left side of the the nose to be blown open when high airspeed and low low ΔP exist simultaneously Three factors normally keep the door shut: (1) spring tension (2)
DECEMBER 2020
The King Air 90- and and 100-series face another inconvenience when when “catching the the the cabin ” It’s quite common for the the the ram air air door to be blown open when when high airspeed and and low low ΔP exist simultaneously SOFT TOUCH TIRES
BIGGER STRONGER SAFER & BETTER PERFORMANCE
A simple tire upgrade with no strut or or or gear door 4 replacement or modifications needed 17 8
405 694 4755
W W W W W W W W W M M E E TA A A A E E R O / M M S A A A OKLAHOMA CITY OK OK KING AIR MAGAZINE
• 25