UnusableFuel R22 Question

[VikingSteele]

Besides it being a " Gov Regulation"

does anyone have a more thorough answer to why there is the left over fuel..I couldnt find it specifically anywhere...

[VikingSteele]

Found soemthing, burried in a CFR,,,never mind

[heliflyknow]

It has to do with the way the fuel strainers are positioned in the tanks, they aren't flush with the bottom so there is some fuel that can't flow into them.

[aeroscout]

You want the fuel on the very bottom of the tank to be close to the fuel drains. You don't want fuel to be taken from the same area. Plus in fuel systems that are more complex there is fuel in crossover plumbing that often is unusable. Another reason fuel can be unusable is if the pickups ever become partially uncovered, you can get air in the fuel lines. That's not too good either.

[HighCountry]

Found soemthing, burried in a CFR,,,never mind

I'd like to know the CFR reference. As Aeroscout indicated, water and other contaminants are intended to go to the bottom where the sumps are located so that contamination can be detected. Fuel lines to the carburetor (or fuel manifold or burner can) are plumbed slightly higher to allow contamination to hopefully settle to the bottom and not interrupt the normal combustion process. If you run the engine to the point that it shuts off due to fuel starvation there will still be residual fuel left in the lines and bottom of the tanks. Also as you maneuver and change attitude you are sloshing around fuel and porting/unporting the fuel pickups. What fuel remains is considered "unburnable" or "unuseable"

[avbug]

§23.959 Unusable fuel supply.

(a) The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank. Fuel system component failures need not be considered.

( b ) The effect on the usable fuel quantity as a result of a failure of any pump shall be determined.

§23.971 Fuel tank sump.

(a) Each fuel tank must have a drainable sump with an effective capacity, in the normal ground and flight attitudes, of 0.25 percent of the tank capacity, or 1⁄16 gallon, whichever is greater.

( b ) Each fuel tank must allow drainage of any hazardous quantity of water from any part of the tank to its sump with the airplane in the normal ground attitude.

© Each reciprocating engine fuel system must have a sediment bowl or chamber that is accessible for drainage; has a capacity of 1 ounce for every 20 gallons of fuel tank capacity; and each fuel tank outlet is located so that, in the normal flight attitude, water will drain from all parts of the tank except the sump to the sediment bowl or chamber.

(d) Each sump, sediment bowl, and sediment chamber drain required by paragraphs (a), ( b ), and © of this section must comply with the drain provisions of §23.999( b )(1) and ( b )(2).

§23.999 Fuel system drains.

(a) There must be at least one drain to allow safe drainage of the entire fuel system with the airplane in its normal ground attitude.

( b ) Each drain required by paragraph (a) of this section and §23.971 must—

(1) Discharge clear of all parts of the airplane;

(2) Have a drain valve—

(i) That has manual or automatic means for positive locking in the closed position;

(ii) That is readily accessible;

(iii) That can be easily opened and closed;

(iv) That allows the fuel to be caught for examination;

(v) That can be observed for proper closing; and

(vi) That is either located or protected to prevent fuel spillage in the event of a landing with landing gear retracted.

Edited November 11, 2014 by avbug

[avbug]

§27.959 Unusable fuel supply.

 

The unusable fuel supply for each tank must be established as not less than the quantity at which the first evidence of malfunction occurs under the most adverse fuel feed condition occurring under any intended operations and flight maneuvers involving that tank.

§27.971 Fuel tank sump.

(a) Each fuel tank must have a drainable sump with an effective capacity in any ground attitude to be expected in service of 0.25 percent of the tank capacity or 1⁄16 gallon, whichever is greater, unless—

(1) The fuel system has a sediment bowl or chamber that is accessible for preflight drainage and has a minimum capacity of 1 ounce for every 20 gallons of fuel tank capacity; and

(2) Each fuel tank drain is located so that in any ground attitude to be expected in service, water will drain from all parts of the tank to the sediment bowl or chamber.

( b ) Each sump, sediment bowl, and sediment chamber drain required by this section must comply with the drain provisions of §27.999( b ).

§27.999 Fuel system drains.

(a) There must be at least one accessible drain at the lowest point in each fuel system to completely drain the system with the rotorcraft in any ground attitude to be expected in service.

( b ) Each drain required by paragraph (a) of this section must—

(1) Discharge clear of all parts of the rotorcraft;

(2) Have manual or automatic means to assure positive closure in the off position; and

(3) Have a drain valve—

(i) That is readily accessible and which can be easily opened and closed; and

(ii) That is either located or protected to prevent fuel spillage in the event of a landing with landing gear retracted.

[iChris]

Besides it being a " Gov Regulation"

does anyone have a more thorough answer to why there is the left over fuel..I couldnt find it specifically anywhere...

 

Aeroscout, HighCountry, & Avbug gave it to you straight.

 

A sump or sediment bowl is required to trap water and other contaminants below the fuel inlet port. This area of unusable fuel is thus isolates from the usable fuel drawn at the fuel inlet port. This area is required to be a specific size (§27.971; §29.971).

 

They also have to account for fuel sloshing in unusual attitude or out of trim conditions and any adverse fuel feed condition occurring under any intended operations and flight maneuvers (§27.959; §29.959).

 

An example is the Bell 206BIII with around 1 gallons unusable fuel under normal conditions. However, the unusable fuel increases to 10 gallons with one or both electrical fuel boost pumps inoperative.

 

The reason is the position of the electrical fuel boost pumps. In certain nose down attitudes with less than 10 gallon, the aft boost pumps could uncover. As long as one electrical pump is supplying positive pressure to the engine driven mechanical fuel pump there’s no problem; however, if both pumps fail and either pump uncovers or the forward pump fails and the aft pump uncovers, there is a possibility of engine flameout. In flight, when the fuel pump caution light illuminates, you don’t know initially which pump has failed.

 

With the failure of either or both electrical fuel boost pumps, the engine driven mechanical fuel pump is still able to draw its required fuel from the tank through an inoperative boost pump as long as that boost pumps inlet is fuel covered. Unlike boost pump failures, a complete failure of the engine driven mechanical fuel pump, game over, the engine will flameout irrespective of any operating boost pump.

 

Some may note during external load work in the Bell 206 (fuel less than 12 gallons) during quick changes in attitude and out of trim pedal turns, a flicker of the fuel pump caution light, that’s the aft pump uncovering and taking a little air. Not a problem since the forward one is still supplying positive pressure to the engine driven mechanical fuel pump.

 

BELL 206BIII FUEL CELL

 

 

 

R22 FUEL TANK

Edited November 11, 2014 by iChris