Hi Bob,
I've been stumped with why Variable reserve calculation has changed with these 2 different questions.

Q1) An Echo has 300 litres of fuel on board at start up on a CHTR flight.
The trip fuel required is 120 litres.
The fixed reserve is 15 gallons.
The start up allowance is 3 gallons.
Fuel flow for holding is 17.2 gph.
The maximum time the aircraft could plan to hold at the destination aerodrome is -

Your Explanation : The fuel available for holding is simply the difference between what you need and what you've got.
Trip fuel is given as 120 litres which is 31.6 gallons.

The min fuel required is 31.6 gal x 1.1 + 15 + 3 = 52.76 gallons.
Fuel available 300 litres = 79 gallons.
Margin fuel for holding = 26.24 gallons.

Holding time is 26.24 gals @ 17.2 gph = 92 minutes.
The correct answer is: 92 min

Q2 Given that -

Trip fuel required to transit to and from the search area is 30 US gal
Take-off is with 130 US gal
Your operator requires you to allow a variable reserve on all flights
Fuel flow for the search is 17.2 gph
Fixed reserve is 15 gallons

Your Explanation: The maximum time that can be spent in the search area is closest to -
First you must find the Trip fuel available.
You don't need to allow for taxi because the fuel given is at take-off.

Trip fuel available = total fuel - fixed reserve ÷ 1.1
= 130 - 15 ÷ 1.1
= 105 gallons.
Less the transit fuel = 105 - 30
= 75 gallons

@ 17.2 gph = 75 ÷ 17.2 x 60 = 262 minutes.
The correct answer is: 262 min

So for Question 1 - You took the Trip Fuel given in the question and x1.1 to find the variable reserve.
For Question 2 however, You said first you must find the trip fuel, but the question states Trip fuel is 30USG, you then calculate the VR on the fuel remaining after you have removed Fixed Reserve.
I'm a little confused as to why 1 question calculates VR off just Trip Fuel, while the other calculates VR off Total remaining fuel once FR has been removed.
Any extra clarity or explanation would be most appreciated

In Question No 1 you are asked how long you can hold over the destination. You do not need to carry variable reserve for holding, so all you need to do is find how much fuel you need to do the trip to the destination, that will be trip fuel x 1.1 plus fixed reserve plus taxi = 52.76 gallons.

You can burn all of the additional fuel holding because you have already accounted for all of the reserves. That's 26.24 gallons.

In Question No 2
I think you will find that the question says that your company requires you to carry the variable reserve on all flights except for holding over an aerodrome. So you need to carry the variable reserve for the search PLUS the transit fuel. So the easiest way to deal with it is to take out the fixed reserve (no taxi because the fuel given is at take-off) then divide by 1.1.

That takes the variable reserve out of the transit fuel PLUS the search fuel which is what the question specified. 130 - 15 ÷ 1.1 = 105 gallons. That accounts for the transit fuel PLUS the search fuel. Take out the transit fuel and you have the fuel available for the search, 75 gallons.

The main difference between the two questions is that search fuel does require the variable reserve but holding fuel does not.