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When to divert

Riccardo Masia

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Riccardo Masia

So today I did a flight to Nice. It didn't all go smoothly over there as I was rerouted to the back of the line twice due to emergency and technical issues. This caused my fuel to quite drop below desirable levels. My question now, when looking at the FUEL PRED page, at what fuel level must a diversion be conducted? At this particular flight my route reserve was all used up, an I was left with about 1.8 tons on touchdown where my diversion required 0.9 tons with an extra 1.1 tons final reserve.

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Guy Owen

What you need to look at is look at your ALTN airport and how much fuel that requires to make it from your intended arrival airfield. SimBrief and PFPX will do this for you and factor it in to your block fuel. 

Another thing to consider is your airlines SOP on minimum landing fuel required and how many go arounds or missed approaches you can conduct before you have to decide. 

As the commander though, your judgment is needed and some communication with ATC on how long the estimated wait time is will normally help you decide pretty quickly!


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Julian Bittinger

By legal definition you must land at your alternate airport with your final reserve, which is the fuel required in a 30 minute hold 1.500ft above the field elevation of this alternate. 

In short that means minimum diversion fuel=final reserve + alternate fuel. 

If you might get low on fuel you can declare „minimum fuel“. That means if you follow your filed flightplan (including STAR and transition) you will arrive with exactly the legaly required fuel at your destination and you therefore can‘t accept any delay or rerouting (except any shortcuts of course). If you are about to land below your final reserve fuel or even expect that you are required to declare mayday fuel. 

„Minimum fuel“ is no mayday/emergency call, but mayday fuel definitely is.


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Simon Kelsey

The law (EU-OPS) says:

"The flight shall be conducted so that the expected usable fuel remaining on arrival at the destination aerodrome is not less than the required alternate fuel plus final reserve fuel, or the final reserve fuel if no alternate aerodrome is required."

The final reserve fuel, as Julian says, is equivalent 30 minutes holding at 1,500 ft above the alternate at the expected weight (roughly 1100kg for an A320).

The alternate fuel is the fuel required to get from your destination to the alternate and will be stated on your flight plan. So as Julian says, in the first instance you should add this to your final reserve figure and this will give you the minimum amount of fuel you need to arrive at your destination with. This is also the calculation that is done on the MCDU FUEL PRED page to give you the "EXTRA" fuel figure -- it adds the final reserve (30 mins) and the calculated alternate fuel, and the difference between the calculated arrival fuel and the reserve + alternate figure is shown in the EXTRA field... however, in my experience the MCDU tends to be EXTREMELY optimistic about the alternate fuel and you would be better off going from the OFP.

But what if, whilst we are preparing for our approach, we look at our estimated fuel on arrival and it is less than the reserve + alternate figure above? Do we have to initiate a diversion?

Not necessarily. The law goes on to say....

"If an in-flight fuel check shows that the expected usable fuel remaining on arrival at the destination aerodrome is less than the required alternate fuel plus final reserve fuel, the commander shall take in to account the traffic and operational conditions prevailing at the destination aerodrome, at the destination alternate aerodrome and any other adequate aerodrome, in deciding whether to proceed to the destination aerodrome or to divert so as to perform a safe landing with not less than final reserve fuel."

The reason for this is to provide sufficient flexibility to prevent ridiculous situations -- for example, imagine that our alternate fuel is 1400 kg and our final reserve is 1100 kg, for a 'reserves + alternate' figure of 2500 kg. We calculate that we are going to have 2400 kg in the tanks on arrival.

If we were required to divert every time in this situation, we're now burning in to our alternate fuel and we might be turning up with something very close to 1100 kg in the tanks when we touch down at our alternate, even assuming everything goes perfectly to plan, our route to our alternate is perfectly smooth and expeditious and hopefully we don't have to go around when we get there. Plainly, this would be a far more dicey situation than simply landing at our original destination with 2,400 kg in the tanks.

However, what it does mean is that, as always, the decision comes down to the Commander's judgement (that's you!) and company policy.

So what can we do in this situation?

Well firstly -- as with most things in aviation, we should be looking to anticipate this situation well in advance by doing regular fuel checks in the cruise, calculating and cross-checking our expected fuel on arrival. One easy way to do this is to note your actual fuel remaining from the ECAM as you pass over a waypoint and subtract from that figure the fuel required from that waypoint to touchdown (which should be shown on the OFP) -- doing this regularly will help you identify any trend early on as well as acting as a good cross-check of the MCDU figure (and indeed to confirm that you don't have a fuel leak!).

If it appears that we may be arriving with less than the reserve + alternate figure, then if we have detected this early on we can take some action to try and remedy the situation -- slowing down (to CI 0 speed for instance), negotiating a more direct route from ATC, flying closer to the optimum flight level, etc and hopefully this will allow us to conserve some fuel and arrive with more on board.

Even if we are expecting to have more than the final reserve + alternate figure on arrival, we should still compare the expected fuel on arrival with the final reserve + alternate figure as part of our approach briefing and preparation as this will give us time (during a quiet, low-workload period) to assess what we are capable of and set some bottom lines. It is good practice to think about fuel in terms of time; as a rough rule of thumb, 200 kg is about 5 minutes of flight time. For a go around and second approach you can bank on needing at least 600-700 kg, so this will give you a sense of how long you could hold for if required, whether you have enough fuel to go around and try again without eating in to your alternate fuel if you cannot land at your first attempt and so on. Would we even want to go around -- if the weather is rubbish, for instance, the definition of madness might be trying the same thing again and expecting a different result so if we are tight on fuel we might be better off going straight to our alternate if we can't get in off the first attempt -- but equally if we have assessed that we have some 800 kg of 'extra' we could plausibly say as part of our briefing "we've got enough fuel for two attempts and if we still can't get in or there's a deterioration in the weather we'll go straight to our alternate".

Now... if for whatever reason we are expecting to arrive with less than reserve + alternate for whatever reason then we have to decide whether we can 'commit' to our destination or divert. In doing so, the law is clear: we have to take in to account the traffic and operational conditions at the destination AND the alternate AND any other aerodromes where we might be able to land. Company policy may also be more restrictive than this. Once example might be something along the lines of "if the maximum delay is not known, the flight may continue toward the destination provided that it is possible to reach at least two aerodromes at which landing is assured with at least final reserve fuel remaining at touchdown; if the maximum delay is known the flight may continue to destination or to hold as long as landing at the destination is assured and it is possible to reach the destination with at least final reserve fuel remaining at touchdown."

What do we mean by 'landing is assured'? One yardstick might be if it could be completed in the event of any forecast deterioration in the weather and a plausible single failure of either ground or airborne facilities (such as CAT III to CAT I, for example).

So some questions we might want to ask ourselves is: is there likely to be any delay to our arrival? Are other aircraft holding? If so, do we know what the maximum delay we can expect is? Could we ask ATC how much holding is expected? Does the destination have two independent runways, or is there a possible alternate nearby with two independent runways (if so, these can count as 'two aerodromes' if we take in to account any likely ATC delay).

Ultimately therefore it is up to you to judge what is the safest course of action that is in line with the law and your airline's policy.... just be prepared to back up your decision!

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Aidan le Gras

I've had a circumstance where, following extensive unforseen holding delays into Sydney we barely had out alternate intact with fixed reserve after a planned single attempt at landing on a CAVOK day.

Next thing we heard two aircraft doing "windshear escapes", although they were 73's which are more sensitive than the bus to a windshear alert, we elected to divert at that point because we knew then that we would be re-sequenced for a later slot time than originally advised and therefore we could potentially end up in a situation where we were out of options. Had it been calm winds with no reasonable suspicion that we could end up out of options then we could disregarded the requirement for an alternate and carried on to our destination. You have to be very comfortable that nothing else is likely to cause you further delays. 

It's rare you find yourself in that position because even on a gin clear day, the company has a requirement to arrive at any destination (as opposed to alternate) airield with less than 60 minutes total endurance. But sometimes things don't go to plan. 

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