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when to use aux fuel pump in circuit?

Guest skydemon

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Guest skydemon

I only recently got my license and I have always been told to switch of


the aux fuel pump after taking off at 400ft whilst doing circuits and


then to switch back on doing your FETCH checks on the downwind leg.


However I read in the latest copy of the RAA mag (whatever it's called)


that it shouldnt be switched off at all as in certain conditions the


mechanical fuel pump may end up only pumping fuel vapour through which


will obviously cause a loss of power, can anyone confirm what is the


'standard' procedure for this?


Also on the same note I have always been taught to switch fuel pump on


in any climb as well, however I had my check flight done by a CFI from


a different school who said there is no need to do it (only on take off or landing), again can anyone advise the normal procedure for this?






PS I should mention this for flying Jabirus...



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Hi Mick


Firstly, welcome to the forums!


I was also trained to turn the fuel pump off (in a Gazelle) at 500ft AGL on take off and back on when doing your down wind checks.


I had a problem with my training that I started at one school (school A) for only a couple of lessons, went to another for nearly all of my training (school B)


and back to school A for 1 more lesson. At school B I was trained that


after takeoff and at 500ft AGL to turn the fuel pump off and bring the


throttle back from 5,800 to 5,500 to finish your climb and then further


back to 5,200 for the cruisewhich is what you do with Rotax's. When I


pulled the throttle back at school A I was hauled over the coals and


told off - my instructor said never do that again. I have since learnt


that this is what they do in GA with suitable engines but the Rotax has


a maximum of 5mins at full throttle. The point is it is hard when


different schools teach you different things but it did make me do my


own research, like you are doing, educating myself to the different


right and wrong ways of flying. This only makes you more knowledgable


when you fly.



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Guest TOSGcentral

OK – this is what I understand, and consequently used to teach as standard:




The auxiliary fuel pump is a safety back up to the mechanical fuel pump


on the engine. Use of the auxiliary is therefore to ensure that in any


potential situation of hazard occasioned by failure of the main pump


does not hazard the aircraft’s operation.


There is a secondary function that is associated with engines mounted


above the fuel tank supply and therefore lack a natural gravity feed of


fuel to the motor where you will get some supply despite any pump


working or not. Failure of the main pump would cause an immediate


problem that the auxiliary will fix!


The auxiliary is also used to prime the fuel system as fuel tends to drain back to the main tank(s) on some aircraft leaving empty fuel lines and avoidable use of battery cranking to re-prime the system.




The main fuel pump on a two stroke engine (eg Rotax)


is normally driven by pressure differences in the crankcase and


operates the pump accordingly by giving constantly reversing pressures


within the pump – much like ourselves breathing by our rib cages


expanding and contracting.


These pumps are highly reliable but their weakness is that they depend


upon two thin plastic diaphragms. If the pump is not serviced,


re-kitted or a material weakness in the plastic develops – then the


pump will stop working even though the engine is still turning and the


variable pressure is still there.




Once you understand the fuel system then the actual purpose of the


auxiliary pump makes sense – it is a BACK-UP! Having the pump on all


the time ensures that you will never know if the main pump has failed


until the auxiliary also fails – then you have nothing!


This may be translated into a sensible, and practical, operating


procedure – that should be standard practice. We will go through this


stage by stage. However, your primary and dominant source of reference


is the aircraft’s Operating Handbook that should dictate how the


auxiliary pump is used.




If applicable to the type the auxiliary should be switched on to prime


the fuel system. On some types the pump is then switched off, or, if


you have sensible carburettors that do not allow engine flooding, it


may remain on for starting.


Start the engine. Switch off the auxiliary and you can incorporate this


as part of your start procedure along with a dead-cut mag check on dual


magneto engines. What you are doing here is proving that the main lift


pump is working correctly – before you go anywhere! If it is not then


get out and fix it without taxiing all over the airfield and getting to


pre-take off checks!




The taxi period should give you proof that the main pump is doing it’s


job without the auxiliary running and masking any failure. Do your


pre-take off checks normally and incorporate switching the auxiliary


back on as part of the fuel check portion.


This is to ensure that if the main pump fails on take-off then the


auxiliary will keep you going while height and decisions are critical.


You will find out later that the main pump has failed, but that will be


at a height where re-application of the auxiliary will get you going


again and you are in a frame of mind where you can think at least a bit






Normal procedure is to turn the auxiliary OFF when transiting 500’ agl.


This is OK if you are going somewhere but for circuits in the aircraft


I normally fly I leave the pump on as any failure below 1000’ is going


to claim a lot of your attention!




The auxiliary pump may remain off – unless you have reason to use it – but should be switched on for aerobatics (prohibited in ultralights) or stalling exercises. It should then be switched back off again when the airwork is completed.


The pump must be re-engaged at any time the aircraft is within 500’ agl (eg high ground) or higher depending on the glide/sink performance of the type in use.




The auxiliary must be turned on again as part of your pre-landing


checks, or positively checked that it is on if it is already running


for some reason (do not just flip a switch and turn the bloody thing off! Use eyes for a positive position check before engaging hands!)


Once the aircraft is down and at normal taxi speed then the pump should


be turned off again to prove the main pump is still working while the


aircraft is taxiing. The ‘key words’ I used with students was “Flaps up


(if applicable), Pump off, turn off (meaning vacate the runway)â€Â.




If you are flying repeat circuits then turning off the auxiliary


demonstrates that the main fuel pump is still functional – you should


not fly with either pump disabled!


If turning off the auxilary causes the engine to stop then you have a


main fuel pump failure and can get on with doing something about it,


rather than waiting until next time you want to fly and finding it is


not working!


The above procedures and logic are quite safe and time proven. They


have served me well enough over 26,000 flights! I have never (outside of test flying) had a fuel pump failure that incapacitated me!


What is far more relevant is fuel hygiene! You are far more likely to


get fuel supply problems with contaminated fuel and blocked filters!


Ensure your are religious about fuel drain checks and visual checks for


clogging filters!


Hope the above has helped







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Guest howard

Personal views only.


Switch the aux pump on pre-start to prove that it works and to save


your battery from having to make the mech pump prime the system during


engine crank, but turn it off once started and when taxying and during


ground checks - that way you have a good idea that the mech pump is at


least pumping something.


I always switch the aux pump on on take off and when operating below


what I consider to be my reaction/action altitude; ie. noise stops due


mech pump failure -> expletive -> SOPs incl. aux pump on ->


prime up time -> restart time. Translate that into your


reaction/action altitude and I believe you should never have the aux


pump off below that altitude.



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Thanks for the detailed and proper answer Tony...As i see it the


discipline of turning the Aux pump off at 500' agl and back on again


with downwind checks is as important as confirming that the Primary


pump is servicable as well!







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My 2c worth - if I am below gliding range to my departure field, and in Normal Ops, I touch nothing (v.v. engine configuration) until I am.


This includes my Aux fuel pump - and may mean that it is not switched


off until at sufficient altitude on late x-wind or even start of


downwind leg.



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Guest TOSGcentral

Quite right Paul!


The bottom line is not slavish obedience to rote practice but undestanding why the practice is there in the first place!


Until the aircraft is at an altitude where the pilot is able to


responsibly deal with a total engine failure then any systems the


aircraft has to support flight must remain ON!


In practical terms, and in training in the likes of a Thruster on hot


summer days, my aux pump stays on all the time in repeat circuit work.


But it always goes OFF after a full stop to check the mechanical pump





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