Bell 206: JetRanger and LongRanger
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B2o6 Boost pumps
If one pump is weaker than the other it may be a check valve stuck in the open position. This will recirc fuel from the strong pump back thru the weak one. If the pump pressure difference is greater than 4 to 6 psi when the circuit breakers are pulled one at a time it's the likely culprit
Join Date: Mar 2004
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I'm expecting a call from Bell today, so will post the reply. The first guy I spoke to over there suggested the change in pressure with altitude.
Any chance you could borrow a boost pump from another operator to use on your aircraft to see if you can isolate a problem?
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Giovanni -
If the boost pumps operate at a constant RPM, and the pick-up pipe is always completeley submerged in the fuel, then in theory they should always transfer the same amount of fuel regardless of the atmospheric pressure. And if thats the case, then there should be no drop in pressure sensed by the transducers which feed the boost pump gauge, regardless of altitude.
Or am I missing something?
If the boost pumps operate at a constant RPM, and the pick-up pipe is always completeley submerged in the fuel, then in theory they should always transfer the same amount of fuel regardless of the atmospheric pressure. And if thats the case, then there should be no drop in pressure sensed by the transducers which feed the boost pump gauge, regardless of altitude.
Or am I missing something?
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Am I missing something................
Steve,
It would appear so. Without going into depth have a look here..........
Basic pump theory
Also read up on centrifugal pumps which are NOT positive displacement.
I don't know what values Vaqueroaero is getting I am just theorising as to why there is a change with altitude. I quoted a differential of psia, and in the aircraft you are looking at psig which does not account for changes in atmospheric pressure.
Hope this helps.
It would appear so. Without going into depth have a look here..........
Basic pump theory
Also read up on centrifugal pumps which are NOT positive displacement.
I don't know what values Vaqueroaero is getting I am just theorising as to why there is a change with altitude. I quoted a differential of psia, and in the aircraft you are looking at psig which does not account for changes in atmospheric pressure.
Hope this helps.
Join Date: Jul 2001
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B206 Boost Pumps Revisited
For all those who asked me to post Bells answer to the fuel pump pressure here it is.
According to the boffins at Bell the pressure drop was caused purely by the drop in atmospheric pressure. As long as the pressure is in the green there is no problem, if it drops out, then descend until the pressure rises again.
According to the boffins at Bell the pressure drop was caused purely by the drop in atmospheric pressure. As long as the pressure is in the green there is no problem, if it drops out, then descend until the pressure rises again.
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Since 1642..............
As Mr Torricelli said -
Got beer?
We have made many glass vessels ... with tubes two cubits long. These were filled with mercury, the open end was closed with the finger, and the tubes were then inverted in a vessel where there was mercury. .. We saw that an empty space was formed and that nothing happened in the vessel where this space was formed ... I claim that the force which keeps the mercury from falling is external and that the force comes from outside the tube. On the surface of the mercury which is in the bowl rests the weight of a column of fifty miles of air. Is it a surprise that into the vessel, in which the mercury has no inclination and no repugnance, not even the slightest, to being there, it should enter and should rise in a column high enough to make equilibrium with the weight of the external air which forces it up?
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Just to add some info on the TT strap issue, I found this snippet in the march edition of rotor breeze a bell published magazine.
{ Apart from a complete new rotor system design eliminating the tt strap altogether, the latest design consideration is out in the field today and uses a steel alloy that is more resistant to fretting corrosion. the first field straps have been returned for consideration and conditional and destructive testing, and the final phase of field evaluation has begun.
if this evaluation is succesful Bells intent is to introduce the new strap and extend the calendar life to 36 months and possibly more. }
it also went onto say that military OH-58 operate safely with no 24 month calendar life, bell advised the military of the 24 month life however the forces control the type certificate and elected not to impose the 24 month life limit.
i guess we will have to wait for the outcome of the testing phase!
cheers
BT
{ Apart from a complete new rotor system design eliminating the tt strap altogether, the latest design consideration is out in the field today and uses a steel alloy that is more resistant to fretting corrosion. the first field straps have been returned for consideration and conditional and destructive testing, and the final phase of field evaluation has begun.
if this evaluation is succesful Bells intent is to introduce the new strap and extend the calendar life to 36 months and possibly more. }
it also went onto say that military OH-58 operate safely with no 24 month calendar life, bell advised the military of the 24 month life however the forces control the type certificate and elected not to impose the 24 month life limit.
i guess we will have to wait for the outcome of the testing phase!
cheers
BT
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The calander life is the main draw back on the strap life. Been a while for me but the straps where about 2-3 grand each (x2) plus a day or so out of the air while the job is done.
It is a "narrow holding pattern" shaped part that takes a big bolt through either end. It consists of lots and lots of very fine wire windings. It anchors the rotor blade to the rotor head, absorbing the 9 tonnes (or so isn't it?) of outward force from the blade while still being flexible enough to allow it to twist as the blade changes pitch angle.
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Right Stuff: The coyote has described the TT straps pretty well, ie "it anchors the rotor blade to the head/hub." The TT stands for Tension Torsion. The outward forces (centrifugal) of the blade create a tension loading on the straps and when the blade pitch is changed the straps twist with it to allow blade movement, aka torsion.
When the helicopter is flying, the straps are working hard in both tension and torsional directions. The stress and constant movement of the windings would have to be the main factors in the life restrictions of the TT straps, although Bell stipulate that once a strap has been fitted to a helicopter the calendar life commences even if it does not fly.
When the helicopter is flying, the straps are working hard in both tension and torsional directions. The stress and constant movement of the windings would have to be the main factors in the life restrictions of the TT straps, although Bell stipulate that once a strap has been fitted to a helicopter the calendar life commences even if it does not fly.
I heard some years ago about 206 that lost one of the two blades in flight and then of course the remaining blade ribbed the main gearbox and the turbine out of the fuselage.
I think it was in N.Y. not sure though. Was it because of TT strap failure or what?? Does anyone remember?
I think it was in N.Y. not sure though. Was it because of TT strap failure or what?? Does anyone remember?
Join Date: Dec 2003
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Re T T Straps
The AAC lost a Gazelle Nov 01 due to a Tie-Bar failure(TT strap),At that time it had only done about half of it's life (5000hrs),it had failed due to corrosion leading to the breakdown of the wire windings.Since then,Tie-bars before a certain date were scrapped,new Tie-bars now have a life of 5yrs/1500hrs with a 2yr/600hr inspection,including the new redesigned Tie-bar...................i know people always ask why do they have to be replaced if they are only due on date......but i had to watch the agnuish that a work colleague put himself through,wondering if anything he could of done could of prevented the accident(He had carried out the flt servicing on th A/C the night before),plus the loss of the pilot.............something i would not wish on anyone.............
The AAC lost a Gazelle Nov 01 due to a Tie-Bar failure(TT strap),At that time it had only done about half of it's life (5000hrs),it had failed due to corrosion leading to the breakdown of the wire windings.Since then,Tie-bars before a certain date were scrapped,new Tie-bars now have a life of 5yrs/1500hrs with a 2yr/600hr inspection,including the new redesigned Tie-bar...................i know people always ask why do they have to be replaced if they are only due on date......but i had to watch the agnuish that a work colleague put himself through,wondering if anything he could of done could of prevented the accident(He had carried out the flt servicing on th A/C the night before),plus the loss of the pilot.............something i would not wish on anyone.............