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A Pilot’s Diversion Strategy

Posted in air travel, air travel humor, air traveler, airline, airline cartoon, airline delays, airline passenger, airline pilot, airline pilot blog, airport, flight attendant, flight crew, flight delays with tags , , , , , , , , , , on July 13, 2018 by Chris Manno

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“Everyone’s got a plan until they get punched in the face.”

–Iron Mike Tyson

The factors that shape a diversion strategy are both quantifiable and variable. Variables include weather, traffic density, airport crowding, holding pattern location, and what I call wild cards, which I’ll discuss later.

Quantifiable factors include altitude, speed, distance, time and fuel. Though there are many foundational points upon which to anchor a diversion strategy, I center mine on the one controlling factor common to every other facet of the decision: fuel.

That’s one very simple and unifying parameter that is reliably quantifiable and easily revised accurately. So, let’s start at the beginning with known fuel numbers. First, what is your required (you, the pilot, determine this) minimum on-deck fuel? Set that number in stone. For example, in a 737-800, I want 6.0 as a minimum in a divert situation. 5.0 is adequate, but in a divert situation, you really need to pad everything.

Next, the added fuel for holding. Dispatch adds holding fuel to a release when you and they agree on how much you’ll likely need based on destination variables like weather, construction, traffic density and more.

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The reserve fuel added for holding presumes you can hold for a specified time, fly an approach to a missed approach, then proceed to a landing at your alternate with your agreed upon (6.0, in my example) fuel.

But in the fuel planning phase, no one knows exactly where you’ll hold nor at what altitude. So, you have to work backwards from your approach to your holding fix: how much fuel will you need from holding to missed approach? Flight planning systems figure an estimate, because again, you can’t know for sure where you’ll be sent to hold.

And here’s where we encounter a few wild cards: we’re not sure where or at what altitude we’ll hold, so we really can’t confirm a fuel flow or total enroute burn. Also, we can’t rule out spacing and delay vectors enroute to the initial approach fix, nor non-optimum speed and altitude assignments along the way. Same thing with the missed approach: there could be multiple vectors, speed restrictions and worst of all, a much lower altitude for the cruise leg to your designated alternate.

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Throw in another factor: priority. If you’re planned to land at the same time as international flights that have been airborne for eight or more hours, you may find—contrary to what ATC might say—that you are vectored around or behind these flights. All of these factors can radically affect your fuel burn for the worse.

Step one: as you enter holding, you can and must determine the probable fuel burn from holding to your destination. Add that to your on-deck minimum. In my example:

Holding at an intermediate STAR fix:

6.0 + 2.5 = 8.5

Now we need to add the planned enroute burn from missed approach to your designated alternate. From DFW to TUL, a typical alternate, that would be an unadjusted 4.5. But we need to adjust that, adding a significant pad for the wild cards I cited above. I’ll add another 2.0. We have:

8.5 + 6.5 = 15.0

So, 15.0 is your bingo fuel: at 15.0 we’ve lost the ability to go missed approach and divert. What’s the next step? Figure the minimum fuel from your holding position to your alternate. Add a pad for the wild cards, and you have your bingo from holding.

If it takes me 2.5 to get from holding to TUL, I add a pad (1.0) and add that to my on-deck minimum:

3.0 + 6.5 = 9.5.

So, 9.5 is my bingo from holding direct to my alternate.

Why the difference? Simple: if you’re holding for a variable that may clear (weather, closed runway, traffic sequence, runway change, and more) you may have an option besides divert. As long as my enroute burn from holding to destination is equal to or less than the burn to my alternate, once the holding cause is eliminated (“DFW is now VFR”) we can proceed safely to our destination and land with 6.5 or more.

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And here’s another wild card you can play: ask for a different holding fix. Chances are, in major airport areas (ATL, JFK, MIA, ORD, LAX) the answer will be no. But in many other places, ATC may grant your request. You can even fudge it by adding “for weather” to your request, which sounds the same as what it really is: “For whether or not we can hold longer.”

A different fix offers one more good option: if you’re not in a stack, you won’t have your fuel burn incrementally increased as you’re assigned a lower altitude. Add to this asset an EFC synergy. Some pilots like to “Go Ugly Early” and divert if their EFC exceeds their planned holding time. But, if you’re sitting high at a comparable fix away from the descending stack, you can safely loiter till your EFC just to see if the EFC shortens, as it often does.

For example, if you’re given a 50 minute EFC and have planned for 35 minutes of holding, chances are decent that if you’re still holding at 20 minutes, that EFC might be cancelled or revised to within your holding fuel.

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A second wild card is to find a different suitable alternate, possibly close-in. For example, instead of OKC or AUS for DFW, if the weather movement allows, AFW, DAL and even SPS (daytime only) maximize your loiter fuel and when you’re refueled and outbound, you’ll be cleared from tower to approach, rather than tower to departure then center, which may get you slapped with a traffic flow wheels-up delay.

Other good reasons for choosing a different alternate include crowding (you don’t want to be the last jet in line for fuel or a gate) and even time of day for staffing requirements. Finally, if you’re going illegal as a crew as soon as you divert, do you really want to spend the night in Abilene, or would Austin be more “sensible?”

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Regardless, the bottom line is this: your minimum on-deck fuel. Add generously padded buffers for the wild cards, plus figure both from missed approach and direct divert. Monitor both figures. The 737-FMS will display both direct and missed approach fuel predictions—monitor both. We also often put in three or four different potential divert airports and monitor both figures for all of them.

As captain, I ask two things of the first officer. First, “What am I missing?” Not, what do you think of my plan—I really want to know what I’m not thinking of and what would be better. And second, I have the FO pick a divert alternate, and monitor both numbers. That keeps us both in the loop, keeps both sets of eyes on all the fuel numbers and finally, two heads are better than one. They both need to be fully in the game.

And that, fellow aviators, is one pilot’s strategy. Good luck and fly safe.

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