Another Fuel question

[petec]

I did my first paying flight last night in the Islander, and have a couple of questions relating to fuel:

When booking the flight it told me that the average fuel burn was 66kg EGJJ to EGJB, and that the aircraft (G-XAXA) had 99kg fuel aboard, so I set my fuel purchase as:

Av fuel burn JJ-JB....................... = 66 kg
Loiter 45 mins @186kg/hr ............ = 139 kg
Divert (back to JJ ?).................... = 66 kg
Fuel required ........................... = 271 kg
Less already loaded ................... = 99 kg
Fuel Purchase required ............... = 172 kg

As it turned out, all I used was 34 kg for the flight (No loiter and no diversion), which left the aircraft with 237 kg of fuel, 138 kg of which I'd paid for ( ie: 237 kg left in a/c less 99 kg left from previous flight).

Seeing as there was so much of MY fuel left in the a/craft, I quickly reserved it for a flight to Biggin Hill, but there's got to be something wrong there. Were my calculations correct ?

Note: I'm not sure that the above figures are 100%, but they're not far off, and the principle of the problem is explained.

[DaveB]

Hiya Pete

For one.. note that the freeware Islander you've been flying up to now burns approximately 29% more fuel than the payware model you now have. What is shown as an 'average' fuelburn in our stats is based largely on flights done in the freeware model. As more and more flights are done in the payware model, the 'average' shown will become more accurate
My last Jer-Gue was done in G-SSKY (all G-SSKY flights in my logbook are for the payware model) and I burned 13kg. I can't give you a comparison for my freeware flights as I don't have them handy but as a direct comparison for your next flight (Guernsey to Biggin) I burned 113kg in the freeware model and 80kg in the F1 model. The flight back was slightly different.. 107kg freeware and 79kg F1. Only a small difference between the payware flights and 6kg difference between the freeware flights. A HUGE difference between freeware and payware though.
My recent experience with the F1 model shows an optimum 'fuel remaining' figure of between 50 and 65kg is enough to get you out of trouble and I have yet to encounter a flight where I've had to drop either pax or cargo. This was certainly not the case with the freeware model!! As an example.. for the GUE-BIG flight, I had 55kg onboard and loaded 80kg for the flight. This put the TOW to 2952kg.. still 41kg short of MTOW and with easily enough to divert me to just about any airfield on the south coast should it have been required.

To get these figures, I'm taking off at max rpm and full throttle then when in a positive ROC, I pull the Pitch levers down to 80% and gently ease the throttles to around the 2500 mark.. perhaps a little above to maintain a climb speed of between 110 and 120kts then move the throttles TO 2500 for the final part of the climb and cruise. In level flight, the payware model will sit nicely at 135KIAS.. right at the upper end of the 'ECO' range and cruise alt for the Southern Region is no more than 2500ft. Sadly, 2500ft is too low up in Jockland as there are too many bumpy bits to run into Inverness to Glasgow saw me climb higher than I've had to go at 3732ft and this at times still only gave me 720ft clearance

Hope this helps

ATB

DaveB

[petec]

Thanks Dave,

Although i burnt only about half of the average burn for the JJ - JG flight, it's still lots more than you burnt for the same trip. I'll bear in mind your power settings and give them a go this evening during my flight to Biggin.

Did I get the formula for calculating fuel purchase right ?

BTW: On another note, winds were gusting to 19 kts last night, and at times I was showing well over 25kts IAS while taxying along at a real crawl. I was expecting a speeding fine, but nothing came.

[Garry Russell]

Is the taxy limit not ground speed rather than IAS???

[DaveB]

Hi Tom

Yup.. we fell into this trap a few client versions ago but it was sorted in short order. As Garry says.. the client reads groundspeed when on the deck though I have to confess.. it still makes me b1oody nervous and I'm too old for that degree of stress

Re fuel calc..
As a rule of thumb, you're pretty much on the ball. Whether an Islander would be expected to carry fuel for a 30min hold plus an hour for diversion is something else. The longest route I've done is around 130nm and that pretty much seems to be the outer limit of the routes we have. Cruising at 135kias (140kts GS) 2500ft, the old girl is burning +/- 26USgph/71kgh/158PPH so in theory.. we'd be looking at carrying somewhere in the region of 100kg+ as dead fuel which on the Islander is prohibitive. Running around the CI, if for some reason GUE were out due to adverse weather, you could (I guess) bet that both Alderney and Jersey would also be out due to the same weather. Dinard is some 56nm away and can be reached with 32kg and Cherbourg is around 46nm away and can be reached with 28kg. Using these as workable examples, my previous guestimate as an ideal 'fuel remaining' on the F1/Virtavia model of between 50 and 65kg should give a sufficient margin for most events and importantly, it isn't prohibitive in that we can still carry max cargo and pax plus the route fuel for any of our flights and maintain this 50-65kg safety margin. Of course, a headwind from hell on the longest routes (130nm) may make 50-65kg too marginal for comfort but in those situations.. it is down to the pilot to decide what to do. The Islander will carry up to a max of 353.84kg of fuel but at the expense of pax/cargo. The most I've carried thus far is around 135kg so to need anything like 353.84kg you'd need to be flying almost empty of pax/cargo and directly into a hurricane which, you wouldn't be doing (would you?) Fortunately, the Islander is capable of much greater distances than any of our routes

ATB

DaveB

[Garry Russell]

Dave

Do I recall in the back of my shattered memory area about some sort of ground speed guage (Fraser)??.......or have I gone completly doolally

This thread highlights one thing

There is no such thing as a fuelish question

[DaveB]

How true

Yes.. Fraser made a groundspeed gauge which is still available here.. somewhere However, the F1 Islander doesn't have a 2D panel so placement would be difficult but.. it does have a 'non-2D' panel (which is basically a blank/clear window with an invisible gauge fitted so the groundspeed gauge could be plonked somewhere on there should push come to shove. The GS gauge is in effect, a lamp that lights when you hit 20kts to give a warning that you're a short way off a ground overspeed. Pain in the @rse with using the 'non-2D' window is that should you change views, the 'clear' window cycles off and doesn't come back unless you hit shift+1.. not a problem if you stay in the same view but if you change views, you have to select shift+1 again.. if you put the gauge there.

Easy innit

ATB

DaveB

[petec]

Somewhere, going way back in the mists of time, I seem to remember a 'push back' and speed gauge which displayed as a small window which you could drag and drop anywhere on the screen, and if I remember correctly, it was a doddle to fit (must have been, cos I used it ).

Mustapha look to see if it's still out there somewhere

[Garry Russell]

I suppose.in a fairly short time, if you fly the same aircraft you will get a feel of how fast the worlds going past the window....a sort of virtual second sense.

Not having a 2D is a pain and that is one reason I wouldn't get the Islander and I won't fly VC...........Ever

[DaveB]

Yep.. this is 'one' of the stumbling blocks with FSX for me too. I can tolerate it in the Islander (just) but few others

Pete.. that gauge should be available at Allnodes (www.flightsim.com). Just plonk Pushback gauge into the search engine and see what pop's up

ATB

DaveB