Old King Coal, I'm afraid I'm not a great fan of the FMC when going in to Dubai, for the same reason that some of the naysayers here are complaining about the new procedures – for thanks to the crazy, unpredictable vectoring we get at Dubai (see above), the figure the FMC will calculate from exiting the hold to touchdown is totally unrealistic.
I could probably write my real name to this, as quite a few of my ex-FOs will recognise the rant to follow, so from here on, I’ll consider ‘Wiley’s’ cover as totally blown.
The FMC will tell you it’ll cost you 600-700kgs to touchdown. On the 773, at Dubai, you’ll need 2 tonnes – considerably more than at EVERY other EK destination. (I’ve burnt more – 2.3 tonnes – on one occasion when the vectoring was particularly long because of Cat II conditions, but that was a ‘one off’. However, I could see quite early in the piece, (when asked to reduce to 180 knots – i.e., requiring 15 flap extension - at about 8,000’), that I wasn’t going to make it touchdown with min. divert fuel, so had to make the decision then and there to commit and continue the approach or divert to AEN at that point, with still about 400kgs more in the tanks than the min. divert fuel. After getting my FO’s agreement, I elected to continue the approach. We landed – as expected - 200kgs under the min divert fuel.)
So if committing isn’t an option and the min divert to AUH is (say) 6.2 tonnes, you simply MUST leave the hold with 8.2 tonnes to still have the minimum required 6.2 tonnes at the MAP.
The ‘up’ side to this is that if you reach that figure and ATC hasn’t cleared you out of the hold, you bug out straight to your alternate – having stayed right up to the last minute you possibly can (keeping both the commercial and flight ops departments happy) – but because you won’t then burn the 2 tonnes it would have taken to get from the hold to the MAP and you haven’t had to do a high power and ‘dirty’ missed approach, you’ll get to your alternate quite probably with 2 to 3 tonnes (or even) more fuel than the CMR, giving you lots of ‘fat’ should there be a delay or another missed approach at your alternate.
The only time you’ll find yourself arriving at the alternate with minimum fuel is if everything goes wrong – you’re not released from the hold until you’re right on min fuel, you burn every bit of that 2 tonnes you allowed for the approach and then you get to the minima and miss out.
If that was to happen, you still have enough fuel to reach your alternate, but depending upon why you were forced to go around, you might want to seriously consider declaring an emergency, requesting an immediate turn onto downwind and a 1500’ missed approach altitude, then vectors for a five mile final - and plugging in the autoland system and putting the machine down whatever the weather conditions. But to get into that situation, quite a few things will have had to wrong and maybe you shouldn’t have got up that morning.
Committing to destination involves exactly the same calculations. If the final reserve is 3.1 tonnes, you simply MUST depart the hold upon reaching 5.1 tonnes – and if ATC delay you beyond that point, it’s time – then and there - to declare an emergency, as you will be landing below your final reserve fuel. The only other thing to consider in this case is that you’re going to get a LOW FUEL warning, so do you pre-empt the EICAS and prepare – and brief - for a Flap 20 landing and complete the checklist in the relatively unstressed environment of the holding pattern, or wait for the EICAS message and deal with the checklist, possibly while on final approach? I know what the book says about pre-empting EICAS messages and checklists, but I also know what I would do.
I find that writing these figures down on my flight plan – for all the available alternates, giving me easily seen multiple options that (importantly) I’ve briefed for before top of descent – takes an enormous amount of pressure off the whole exercise. At any time, you know what options are still available and which ones are not – and most important of all, when you get to the ‘trigger’ figure for whatever option you’ve decided upon way back in the cruise, you divert.