Posted for " Richard Kidder" <Rkexecutiveair@aol.com>:
Marv, you are generally correct in your description of the design and
construction of the Propjet fuel system. Both wings should hold 55 gallons,
say with approx 3 gal. unusable. The belly tank should hold 35, with maybe 2
unusable. The fuel pickup tube should be located approx. .25" from the bottom
skin of the tank. All fuel bays of the belly tank are vented to each other at the
tops of the bays and the are additional slots cut to allow all fuel to move
between the bays in addition to the slosh doors. The slosh doors are closed
with a small dab of grease during tank closeout and the possibility of Hysol
drips is somewhat mitigated by removing most of the Hysol on the cap strip
just above each door just before the skin is set in place. The slosh doors will be
heard to bang open and closed when the airplane is fully fueled and rocked
slowly by hand in a quiet hanger.
The fuel selector provided and recommended is an ON-LEFT-RIGHT- OFF unit.
Left and right can be selected in flight to correct any fuel imbalance issues
and OFF is normaly only selected when the aircraft is parked on a side slope
with 100% fuel to prevent fuel transferring out of the lower wing vent. On
is for start and all other normal operations.
Now for some salt and pepper for your meat and potatoes........."Apparently a
number of accidents were caused by PILOTS FORGETTING TO TURN THE FUEL
ON" Can you say CHECKLIST!!!!!! , how can this be a design fault, if I forget
to put gas in my car and I run out in the middle of nowhere is the car to
blame? Perhaps someone could design a device to detect the presence of a
"PILOT" and render any mechanical device 100% safe and foolproof. But
seriously, the real answer to this possible scenario would be pilot training and strict
adherence to the operations manual and the checklist. As you pointed out,
this problem can occur on any number of aircraft types. I would also point
out that there is an emergency fuel shut off valve in the system that should be
safety wired open with breakaway wire and verified to be in the open
position. Additional check valves and such could be incorporated into the fuel
delivery system, but would also add additional failure points and weight.
Hypothesis # 2...........What if someone takes off without having the
wings and belly tank full...... Well, the whole purpose of having the belly
tank is to increase the range of the aircraft. The earlier Propjet belly tank was
10 gal and was merely a pickup point for the fuel, if you had say 104 gal
usable in the wings you must have a reserve for landing under VFR and more
for IFR, which would mean at a 33gal hr average fuel burn you would have to land
with approx 20 gal left in each wing. Now with the additional fuel in the belly
tank acting as the reserve fuel you can safely use all the fuel in each wing
thereby increasing range. To unport the belly tank with say 25 to 30 gal
inside and the pick up point located in the lowest point, it would take some
pretty severe pitch attitudes to unport the pickup. Now we are getting out of
the normal flight envelope and all bets are off.
Hypothesis # 3 ..........."There is already air in the belly because
last leg of flight was a long one and one wing emptied"...One wing would not be
emptied during normal operations, I do not think the aircraft would fly very
well with one wing full and one empty. That is what the BOTH position is
for. What if this, what if that what if , what if the solenoid sticks.. etc. If
one is smart enough to buy, build ,and fly one of these airplanes, one would
think that you could tell how much fuel you put in at the beginning of the
flight, record, and or monitor the fuel consumption and time flown and have
a pretty good idea of how much fuel you are putting in at the pump. Also, when
there is air in the belly tank, such as during initial fuel calibration
maintenance, you can actually hear the fuel slapping around in the belly if
you rock and shake the plane by hand.
In conclusion, there is always a way to make things safer, and I think a
frank and positive discussion of this subject would benefit all involved.
These really are amazing and wonderful machines and can be made as safe as
practical with the knowledge and talent of all the folks here on the
list....
Richard Kidder
.
[Thanks for jumping in with your well-reasoned comments, Richard. This is the sort
of discussion I was hoping would emerge in response to last night's post.
Regarding your comments about taking off with the fuel selector off, I totally agree. I
have always used the printed checklists and when flying with someone else I ask them
to read off each item and reply with a repetition of the item followed by a "check". I
also understand the attitude of many pilots who believe you ought to be able to locate
every control in your aircraft blindfolded as well as have every checklist memorized so
you don't need to read... however, if this always worked we would never hear about gear
up landings, or any number of other incidents which may and have been caused by
failure to read a checklist. All that being said, the possibility still exists that someone
_could_ attempt to start operations with the fuel selector off. There was an incident a
number of years ago where someone started the engine with the selector off, the vent
valves were closed (but I belive they were manually operated, not the electrically actuated
valves that are standard to the present system) and the belly tank was physically
ruptured from the resulting suction. Fortunately it happened during a maintenance check
and not during a flight operation. Obviously this was one of those cases where someone
was uncomfortable with the system as designed, chose to modify it according to their
perception of what would be better, only to discover that they'd ultimately made the wrong
decision. Nonetheless, perhaps a small vacuum sensor that would see a suction
condition developing in the tank and lighting off an annunciator would be a small price
to pay to provide a way to mitigate this situation. Personally, I'm with you, though, and
think adherence to the POH and use of the checklists is a much more reasonable
solution.
I also know that asking "what if" about a number of things can make people uncomfortable.
Heck, nobody said that we can't ask questions that have less than feel-good answers. You
mentioned "One wing would not be emptied during normal operations, I do not think the
aircraft would fly very well with one wing full and one empty." I never suggested that just
because one wing had been drained that the other one would necessarily be full. Is it so
inconceivable that someone might operate with zero g in the left, 20g in the right, and
whatever is assumed to be in the belly? If the belly had its full 33g as intended, as 33g/hr
burn there is supposedly still 1:40 of fuel on board... much more than a 45 min reserve.
Again... these are not unreasonable what if's, are they? Just asking.
What about this one: "To unport the belly tank with say 25 to 30 gal inside and the pick
up point located in the lowest point, it would take some pretty severe pitch attitudes to
unport the pickup. Now we are getting out of the normal flight envelope and all bets are off."
I am sure you are right, but you've changed the what if to suit your purposes. What if there
was only 15g in the belly (not supposed to happen, I know... but what if....) and you
assumed a full power climb attitude. Seems to me that you're going to be pretty nose
high when the VSI is reading out +4000fpm. Again, all I am asking is if this is a possibilty,
nothing more.
I am now removing my "devil's advocate" hat. I don't wear it very often and it even makes
_me_ uncomfortable. <Marv> ]
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