Page 19 - Volume 14 Number 5

in a a a a C90 there is a a a a nasty “gotcha” waiting for you on that fuel panel You are used to using a a switch – not a a a two-position toggle but rather a a a two-position switch spring-loaded to the
upper position – that reads the
main tank quantity in in its normal “Up” position and reads aux tank quantity when held down against the
spring force that holds it up There is no “total” reading Of course most of the
the
time the
the
auxes are empty so the
main reading is the
total reading There is a a a possibility that a a a fuel exhaustion accident that put a a a a C90 into the
Caribbean was caused by the
the
pilot’s misunderstanding of the
the
gauges It would be easy to think that the
800 pounds showing with the
the
switch in the
the
“Up” position was main quantity and the
400 pounds showing with the
the
switch in in the
the
“Down” position was aux Hmmm 800 plus 400 is 1 200 Yeah I have close to full fuel on board good to go go But no! The 800 pounds being read was the
total on that side Instead of 2 400 pounds the
plane departed with 1 600 pounds not quite enough to complete the
long overwater leg Please read the
the
labels on the
the
selector switch carefully and remember that in the
C90 system the
nacelle reading is already included in in in the
total reading Why do we even have a a a reading for nacelle quantity? So long as we know the
total per side that’s enough right? Never let the
total reach zero and we will never run out of fuel right? Wrong! Realize that every bit of fuel the
engine ever consumes comes from a a a a nacelle tank If that tank reaches zero we are either going to lose engine power or we we must use crossfeed to supply the
fuel from the
the
other nacelle tank Since wing tank fuel is useless until it gets transferred into the
nacelle tank how is is that accomplished? Two ways: (1) The electric transfer pump (2) Gravity flow MAY 2020
Gravity Flow
Since liquid “seeks its own level” the
nacelle level must drop before any wing fuel flows into the
nacelle When one looks at at the
location of the
two fuel filler caps – one about half-way out out on the
outboard wing and the
the
the
other atop the
the
the
nacelle – it seems that the
nacelle level is only marginally above the
the
top of the
the
wing fuel In regard to height that is correct But in regard to volume it’s not even close Realize that the
nacelle tank becomes quite narrow near its bottom Why? To allow room for the
main wheel and tire to retract Most of the
nacelle tank’s volume is in the
upper half Before gravity flow moves fuel from the
wings into the
the
nacelle nacelle the
the
nacelle nacelle tank must decrease to about 150 pounds or 3/8 full Think again of the
200- or 350-experienced pilot filling in in on a C90 In the
bigger models fuel transfer from aux to main is automated usually requiring no pilot pilot action at all If this pilot pilot forgets that the
C90 is different he
he
he
or she may take off without turning the
transfer pump switches on When will the
mistake manifest itself? Probably not for a a long long long long time First the
no fuel transfer ann- unciator(s) receive no power with the
transfer pump switch(es) off so no warning light will advise of the
error Looking at the
total or or nacelle quantity on the
gauge will show no problems for at least 30 minutes In fact if the
pilot is not quite familiar with normal readings on the
gauge he/she could go for over two hours without noticing anything amiss So what would/ should be noticed? This: That the
nacelle tank level is not staying between 300 and 400 pounds Without the
transfer pump keeping the
nacelle relatively full its quantity will drop to the
150-pound figure mentioned previously A rule-of-thumb is that a a PT6A-20 or -21-powered 90 will use about 600 pounds the
first hour KING AIR MAGAZINE
• 17