ATPL Systems/AGK question

[alpha_papa_lima]

Hi everyone,

Debate in-class about the following question split between two answers:

"The fuel quantity indicating system on a modern jet transport aircraft may use capacitance type probes with compensators. The compensators are designed to correct for variations in fuel.."

a) Density
b) Temperature

Any votes as to which answer CASA wants?
Cheers:ok:

[michael36]

temperature I believe, because at a certain temperature the fuel will have a certain density. So the sensor detects changes in temperature to adjust the density which the gauge can then indicate as a weight (kg or lb) of fuel on board

[TOUCH-AND-GO]

The Compensator changes capacitance as the dielectric constant of
the fuel changes. The dielectric constant is directly related to fuel
density.

Hope this helps! :ok:

[The Green Goblin]

Density.

The compensators compensate for temperature, calculate an accurate density and give you a reading on your fuel gauge.

Typical tricky CASA BS.

Just read the gauge, check it is within reasonable limits from the previous sectors burn allowing for APU use in the turn around, order the fuel and check the addition of the two is within 3% :hmm:

[NGsim]

Stop it GG, you're being practical which is the sort of thinking that will probably save your life.....or possibly kill you :ugh:

[alpha_papa_lima]

Thanks for the replies guys. Have been doing more research and found a few good sources.

"NTSB REPORT - FQIS Compensator Probe Testing" (http://www.twa800.com/ntsb/8-15-00/docket/Ex_20M.pdf)

(Page 6)... "This system uses submerged fuel probes that convert voltage capacitance to the volume of fuel within the aircraft’s fuel tanks. The compensator is used to adjust, or compensate, the fuel probe’s readings for fuel capacitance variations caused by variations in fuel TEMPERATURE."

also...

"JAR Professional Pilot Studies - Phil Croucher"

(Page 5-8)... "On their own, the detectors can only measure the height of fuel (volume), so a datum or REFERENCE CAPACITOR compensates for density to ensure that weight is indicated correctly... To ensure that only permittivity is measured, the COMPENSATOR UNIT is placed at the bottom of the tank so that it is always covered with fuel, which means that capacitance is not measured by fuel quantity. When temperature increases, permittivity will increase and so will capacitance."


Referring to the question in the first post, am I correct in saying that the wording implies the purpose specifically of the compensator? (The compensators are designed to correct for variations in fuel..")

In that case it would be temperature.:ok:
Thoughts?

[drpixie]

Hi APL,

A couple of thoughts - first, there are quite a few "bad" questions in the CASA exams, questions which are ambiguous or (occasionally) where the "correct" answer is just plain wrong. Always be very careful about the wording.

Unfortunately, it is almost impossible to correct these problem questions because you are not permitted to take anything from the exam, so you have no evidence of the problem. (I have had plenty of students query things where I know they misread the question, especially in the heat of the exam, but I've seen some very unclear questions myself.)

Second, this looks like a rather ambiguous question, the answer based on the stress of the wording, rather than understanding the ideas. The compensator is adjusting for fuel density (which is hard to measure electronically) - it does this by measuring temperature (which is easy) and deriving a density correction from temperature. So it's using temperature to correct for assumed density changes!

[xxRED BARONxx]

Hey APL just thought id let you know im studying for systems exam and came across this exact question/answer for this question in an AFT practice exam and the answer was TEMPERATURE.

[Checkboard]

Second, this looks like a rather ambiguous question, the answer based on the stress of the wording, rather than understanding the ideas

Actually it is all about the understanding! If the refuelled makes an error and fuels you with Avgas instead of Avtur, will you be able to pick up the error with the litres loaded/weight on gages fuel check?

Answer (based on understanding the principle of operation of the fuel probes) "No" - as the compensator cannot measure density directly, it assumes the tank is filled with Avtur. The weight shown on the gages will be incorrect, and you won't pick up the error that way.