Checkboard's is right, but I was too inexperienced to check that at the time. It's not a normal pilot or navigator check. It Checkboard a flight engineer, by any chance?
Bumpfich, I'm so pleased that you would like to hear the story. I thought no one would ever ask!!
I was in a Canberra and we noticed that the fuel flows seemed a bit too high. We were flying the right indicated Mach No. I did note that the wind component seemed a bit lower than forecast, but I thought nothing of it. Met forecasts aren't always that accurate.
When we landed, I reported to the ground crew that the fuel flows seemed a bit higher than usual. No big deal. On the Avon, it was usually the P1/P3 switch. They said they'd have a look at it.
We had to fly the same aircraft the next day and when we signed for it it, we noted that the engines had been test-run and 'no fault found'. We did rather feel that the ground crew gave us a bit of an old-fashioned look that implied "I've been up half the night running bloody engines which were perfectly serviceable because the aircrew can't diagnose the fault properly", but we put it behind us and got airborne.
Exactly the same thing happpened on that trip as well, except that this time the headwind component was even lower than forecast than before. Fuel flows were high, just as before. This time the controls felt a bit stiffer than usual, too. And then it struck me! We knew what the aircraft felt like when you flew it at M.75 instead of .73, because we practiced it occasionally. And it felt just like that.
OK, so that must be the problem. The machmeter must be under-reading. That would explain why the W/Vs seemed wrong. You calculate your TAS from the machmeter - or ASI- it'll give the same result. If we used .73 instead of .75 to calculate a TAS, we would be calculating too low a value. Then when we compared it with the groundspeed from Doppler or from a succession of fixes, the headwind component would be too low. It all fitted. But it's the one fault where there is no independent confirmation. There is only one source of TAS - the pitot static system - whether you use CAS or Mach No to find it - you are still using a comparison of pitot and static.
So how to check it? You can't trust met forecasts - at least, not that accurately. And then it occurred to me. Our route took us back to base on the reciprocal of the outbound track. So I could get a groundspeed outbound (from the Doppler) and a groundspeed on the return). If we held the same Mach No and the wind hadn't changed, the TAS must be the average of the outbound and the return groundspeeds. So we tried it and - joy of joy! - we appeared to have a tailwind component in both directions! Obviously, it was our calculation of TAS which was out.
When we landed, I asked the groundcrew to have a look at the pitot-static system this time. We got a phone call about an hour later saying that they'd discovered a HUGE pitot leak.
That explained it all! The reason that the fuel-flows were too high was because we had set the power to fly at .75 instead of .73. Nobody really notices what the required rpm is - you just set whatever rpm you need to get the speed you need. Our TAS was higher - but we couldn't tell. because there is no independent check - and our Doppler groundspeeds were right. So we had been finding the wind and had found too low an apparent head-wind component, but- hey - so what?
Effectively, we had diagnosed a pitot leak from a fuel-flow indication. As I said earlier, it's all one subject!
Last edited by oxford blue; 26th Jun 2002 at 19:50.