Un-usable Fuel

[cdoughty]

Hey guys--legal question here. My son went for his checkride and failed the oral because he said the unusable fuel was 2.5 gals per side. The DPE didn't believe this. He then asked him to do W&B for 190, 210 lb occupants and 90 lbs of baggage (already illegal) and didn't believe that you could only go about 20 miles legally.

I have seen various references between 1 and 3 but none that I think are official. Does anyone know of an official Cessna or FAA statement of the un-usable fuel?

[8474]

This is interesting. I have the Garmin Pilot app and although I enter total fuel capacity at 25 gals when I enter a flight from point A to point B the app only shows available fuel as 21 gals. Does Garmin somehow know that unusable is 4 gallons or are they just factoring in a safety margin based on percentage?

[simonlowther]

I believe that under the modern rules (FAR 23.something) it is 1/4 tank, even though that fuel is available in level flight unusable is defined as the first evidence of malfunction under all intended operations (ie includes Take-Off).

So sorry that your son missed his check-ride, hopefully it hasn't knocked him too much.
Si

[8342]

Hey guys--legal question here. My son went for his checkride and failed the oral because he said the unusable fuel was 2.5 gals per side. The DPE didn't believe this. He then asked him to do W&B for 190, 210 lb occupants and 90 lbs of baggage (already illegal) and didn't believe that you could only go about 20 miles legally.

I have seen various references between 1 and 3 but none that I think are official. Does anyone know of an official Cessna or FAA statement of the un-usable fuel?

Where did the Examiner get his information that proved your son wrong?

If he cannot prove your son wrong, I think your son should go to a higher authority.

Your son does not have to go to this same Examiner for the retest. If I were him I would seriously try to find another.

Rick

[cdoughty]

Thanks all,
Closest I could find was for the 140A two-page PoH which says approximately 2 gals unusable for normal maneuvers. But that's a technically different airplane with different wing (and tanks??) and different type cert. The early 150 says something similar with 22.5 usable of 26 total.

I'm pretty sure Cessna never said for the 140 and wasn't required to under CAR4a. I just wondered if was missing something obvious.

Simon I agree the red parts of the fuel gauge imply per FARs that fuel is unusable.

[8474]

Thanks all,
Closest I could find was for the 140A two-page PoH which says approximately 2 gals unusable for normal maneuvers. But that's a technically different airplane with different wing (and tanks??) and different type cert. The early 150 says something similar with 22.5 usable of 26 total.

I'm pretty sure Cessna never said for the 140 and wasn't required to under CAR4a. I just wondered if was missing something obvious.

Simon I agree the red parts of the fuel gauge imply per FARs that fuel is unusable.

Yes, the 140A POH says useable at 21 so it might seem thats why the Garmin Pilot app shows that, I'm surprised that Garmin would have that much info in their data base.

[6643]

It is my understanding that when the first 120/140s rolled off of the assembly line, there were no markings on the fuel gauges, so the whole concept of unusable fuel came later.

I agree that, as silly as it sounds, going by strict interpretation of the FAA definition, that 1/4 tank is unusable.

I'd throw the red flag and demand a review of the play. Also, if he goes again, preempt the problem by asking the examiner what he thinks!
Part 23 doesn't seem to mention unusable fuel. Part 25 says this:

§ 25.959 Unusable fuel supply.
The unusable fuel quantity for each fuel tank and its fuel system components must be established at not less than the quantity at which the first evidence of engine malfunction occurs under the most adverse fuel feed condition for all intended operations and flight maneuvers involving fuel feeding from that tank.

The 120/140 wasn't certified under either 23 or 25.

Here is what CAR 4a has to say:
§ 4a.605 Capacity and feed.
...
The system shall be so arranged that the entire fuel supply may be utilized in the steepest climb and at the best gliding angle and so that the feed ports will not be uncovered during normal maneuvers involving moderate rolling or side slipping. The system shall also feed fuel promptly after one tank has run dry and another tank is turned on.

I suspect that failure to meet the requirements of the first sentence is what lead to the no-takeoff markings.

[8342]

Here is what I have found in the Glossary about "Unusable fuel", taken from the Aircraft Weight and Balance Handbook, FAA-H-8083-1B.

"The fuel remaining after a test has been completed in accordance with governmental regulations."

Now when you go to Chapter 3 which pertaining to Weighing the Aircraft and Determining the Empty Weight Center of Gravity, here is what it says,

" Drain the fuel from the tanks in the manner specified by the aircraft manufacturer. If there are no specific instructions, drain the fuel until the fuel quantity gauges read empty when the aircraft is in the level-flight attitude. Any fuel remaining in the system is considered residual or unusable fuel and is part of the aircraft empty weight."

It also goes on the say that residual fuel and arm are usually found in Note 1 of the TCDS. Unfortunately Note 1 of the Cessna 140 TCDS makes no mention of the residual fuel and arm.

With all of their being said and since I will be reweighing mine within the next month or so, I will follow the guidelines above to determine the unusable fuel for my airplane.

Rick

[6643]

Note 1 of the Cessna 140 TCDS makes no mention of the residual fuel and arm.

That's because a Cessna 120/140 does not have a TCDS. it was certified with an "Aircraft Specification". They predate TCDS's and have a slightly different format.

The whole exercise is academic since the type also predates the concept of unusable fuel. My personal rule is to always land with at least one tank at or above the "no takeoff" mark. I try to switch off a tank when the gauge stops registering, but before the engine quits. I once let it run out over the airport, and the engine restarted within seconds after switching to the other tank.

It is common practice to fill the tanks when weighing the plane, then subtract out 25 gallons from the results to determine the empty weight.

[maverick_fa]

Hey guys--legal question here. My son went for his checkride and failed the oral because he said the unusable fuel was 2.5 gals per side.

This is the Transport Canada versions (CARs)...

527.1553 Fuel Quantity Indicator
If the unusable fuel supply for any tank exceeds one gallon , or five percent of the tank capacity, whichever is greater, a red arc must be marked on its indicator extending from the calibrated zero reading to the lowest reading obtainable in level flight.

-> Unusable fuel = Red Marks (C140 = 1/4 tank = ~3g in 1 tank)

• I already performed some climbing with less than the ''Red Mark'' & it's working BUT the gauges lack of accuracy & it's fluctuating a lot.
  On the top of that we don't have any formal document that we can use for unusable fuel...

525.959 Unusable Fuel Supply
The unusable fuel quantity for each fuel tank and its fuel system components must be established at not less than the quantity at which the first evidence of engine malfunction occurs under the most adverse fuel feed condition for all intended operations and flight manoeuvres involving fuel feeding from that tank. Fuel system component failures need not be considered.

-> Normal Operation include Climbing at Vx (even if don't officially know Vx) & banking up to 30° for what we call normal attitude.

• I can climb at 60 mph with a 1/4 tank without any problem
  I never officially confirmed that I was able to do left/right turn @ 30° of bank with a 1/4 tank because I don't have an AI/T&B on my C140 but visually it looks ok to me

My safe answer to a flight test would be 6g of unusable fuel (minimum 3g on each side) due to lack of information but this is just my opinion & because I want to stay on the safe side of the operation...
My practical answer is 3g in 1 tank

2.5g per tank (5g total) can be considered as a critical error (due to safety) during the oral portion of the flight test for the reasons mentioned above
1 critical error = failure (in Canada)

He then asked him to do W&B for 190, 210 lb occupants and 90 lbs of baggage (already illegal) and didn't believe that you could only go about 20 miles legally.

-> No takeoff are allowed. Baggage are too heavy & in my case the MTOW is exceeded (for my C140)

Question for you Mr Doughty...
I know your are a tall man & I guess your son is even taller than you
What is the weight of your son & the DPE
Did you exceeded the MTOW with enough fuel for the flight exam... it's typically 1.3 to 1.6 hours for a private pilot license flight test. Add a minimum of 30 minutes of fuel for the reserve (~1 full tank)