dkul

Well...under normal circumstances the shorter route is better due to shorter flight time and lower fuel burn. If you are asking about rejecting the shorter route in a weather situation, then that is also not prefered because as pilots we are trying to stay as close to schedule and ontime performance while still being safe. In the specific situation discussed here there was weather issues involved. Normally DAL-IAH is a straight shot down to College Station and then into IAH however if there is bad weather along that route (i.e. thunderstorms) then a different flight plan is needed but it is not as simple as just flying a little more to the West because that is where the arrivals into DAL/DFW fly. So then the flight might have to go as far West as Abilene so the pilot fuels for that. Where things get interesting is when once airborne it appears that there is enough of a space in the weather to fly basically the normal path. That is where the fuel burn (or lack there of) comes into play.

OPNLguy

That assumption at the end of your post is a common one, and one often promulgated by movies and misinformation from the media, so you're not alone in having it by any means.

Airliners are not like automobiles, i.e. one "fills them up" just before an aircraft's first flight of the day, and refuels once the needle gets closer towards the big "E" later in the day. Airline flight fuel loads are mission-specific, and a big factor in that is the duration of the flight (mission). I'll explain why that comes into play in as minute, but I first have to briefly discuss weights.

The concept of an aircraft's max takeoff weight is pretty easily understood, and it involves runway length, temperature, airport elevation above sea level, among other items. Rarely understood is another aircraft weight limitation called max structural landing weight, and it's lower than the aircraft's max takeoff weight. I'm not conversant in the specs of the aircraft that CoEx flies (since I don't dispatch for them), so I'll use some other numbers for an aircraft that I'm quite familar with.

Let's assume that a 737-500 has a max takeoff from DAL of 130.0 (that's 130,000 lbs., and all subquent weight references will likewise be expressed in hundreds of lbs.), and also has a max structural landing weight of 110.0 applicable no matter where it lands. One doesn't restrict the takeoff weight to a max of 110.0 for the departure out of DAL, since the aircraft will obviously be burning fuel for the DAL-XYZ flight, and would thus be that much below 110.0 upon arrival at XYZ. What we do then is to take just the amount of fuel it takes to get from DAL-XYZ (and that doesn't include alternate, holding, or FAR reserve fuel), and then add that amount (the "burn") to the aircraft's max landing weight.

For example, if the burn for the "normal" route for DAL-IAH was 4.0, we'd add that to 110.0 and come up with 114.0, which then becomes the max takeoff weight limited by landing weight. The aircraft could be loaded (with everything, folks, bags, freight, and fuel) right up to that 114.0 limit, fly the normal route to IAH, and arrive at or below the 110.0 max landing weight.

The curve that Mother Nature often throws at us involves thunderstorms and ATC's response to them, and it's commonly so dynamic and unpredictable an environment that I hardly know how to adequately describe it. If there are storms on the normal route out of DAL, or inbound to IAH, a common re-route is to head west to Abilene (ABI), then to San Antonio (SAT), and into IAH from the west side. Sometimes the re-route is even further west (like to Midland (MAF)) but we'll stick with DAL..ABI..SAT..IAH and further assume that the fuel burn on that re-route is 6.0 instead of the normal route's 4.0. That makes the max takeoff weght limited by landing 116.0, and again, if the flight is loaded right up to that amount, and flies it's full re-route, it'll still arrive at IAH at 110.0 as it should.

If the OP's DAL-IAH flight was planned/fueled based upon the "normal" route and they're issued a longer one by ATC once off the gate, the captain and dispatcher communicate as to whether there is sufficient fuel onboard to accomplish the re-route, and if not, returning to the gate for additional fuel is obviously prudent. If the station folks have other IAH passengers they can get on the flight (maybe early passengers from a later flight that they're getting out of town early so they can better make their own connections), that's also prudent, espectially since the same weather would have been delaying later flights in/out of DAL or IAH.

The proverbial fly in the ointment here is when a flight gets airborne and ATC is either able to clear the aircraft back on it's "normal" shorter route, or (more likely) cut the re-route short (i.e. turning the flight south towards SAT when it gets halfway between Fort Worth and Abilene.) That shortens the distance, and the fuel burn. Back to using my numbers, let's assume this shortcut from the re-route will be a 5.0 burn instead of a 6.0 burn, now meaning that the flight, having taken off at 116.0, will now arrive at IAH at 111.0, or overweight by 1.0. The captain has two choices at this point, the first being to decline ATC's offer of the shortcut and fly the longer re-route to burn the 6.0 fuel as planned, or to accept the shortcut but start operating the aircraft inefficently (i.e. creating drag with spoilers, prematurely extending the landing gear, and other fuel-wasting techniques) to consume that 1.0 fuel overage by the time they land at IAH.

The OP seemed to think the crew had an evil, ulterior motive in taking the shorter route to "save" fuel, but I think the above example info demonstrates that they'd have consumed that fuel one way or another (Burning it on the longer route, or wasting it on the shorter one). What they were really trying to do was save the time, which would have been shorter on the shorter route. That meant better chances of their passengers making connections, and less time holding those connecting flights in IAH. Maybe not for him in his individual case, but for others on the flight.

It also has to be noted that the perception of how "sound" these kinds of operational decisions are are largely in the eye of the beholder. For example, let's assume I have a LIT-PHX flight that's half-full and an OKC-PHX flight that's half-full. Both are scheduled to arrive in PHX about the same time, and offer the same connections from PHX. The LIT-PHX flight gets airborne first, and upon passing about 50 miles south of Tulsa, I get word that my OKC-PHX aircraft is out of service with a mechanical issue and will be for several hours. If I "flagstop" the LIT-PHX flight into OKC to "rescue" those passengers, some of the LIT-PHX passengers might not be thrilled about the additional stop and associated delay getting into PHX, but I dare say that most (if not all) of the OKC-PHX folks are going to be tickled pink. We'd probably be holding the connections in PHX anyway, and with such flagstops only delaying the flight 30-40 minutes, some of that could be made up enroute.

Back to the OP's post, weather causes numerous issues, and all airlines (CoEx, as well as mine) try he play the hand that Mother Nature and ATC deal us the best we can. Many of our behind-the-scenes actions are not easily understood by the general public, but our goal is to limit inconvenience(s) to the fewest number of passengers possible.

Syzygies

Specifically, you're fueled for the longer route, and in the air the shorter route opens up, but the longer route is still a viable option. If you fly the shorter route, you then must circle to burn fuel.

If it were your choice, would you rather fly the longer route, or fly the shorter route then circle to burn fuel?

I'm thinking that circling isn't as safe. On the other hand, the airport is nearer if something else happens, and landing with too much fuel becomes the lesser of two evils.

Or, will flying the longer route burn more fuel on average? I.e. if you circle then you burn the minimum needed to land. The longer route burns at least that much fuel, while staying within a safe fuel window?

I realize that I'm splitting hairs on small odds, but there were people standing on wings on the river by my apartment recently. That's also unlikely. If you were playing a video game involving a billion flights, for safety which is the better play?

OPNLguy

It's not quite the option that many think. Max structure landing weight is a limitation in the aircraft's Airplane Flight Manual (AFM), and compliance with AFM limitations is required by law, absent an emergency.

Steve M

I think that when Syzygies said "if something else happens," he was specifically referring to an emergency.

Steve M

I'm not sure that I agree with that. It is an issue for both, and the same issue, but not necessarily equally so. That's the crux of my question that remains unanswered.

I'm very much aware of that.

So, are you saying that on your previous XJT flights, after calculating your fuel load based on your intended route and all of the other factors (alternate airport, etc), that you load cargo up to the point where upon arrival you'll be at max landing weight, even on a route like DAL-IAH? Put another way, there was usually more cargo than a particular flight could accept (either in terms of size or weight) and the deciding factor as to how much cargo to accept was calculated max landing weight upon arrival?

OPNLguy

Perhaps that was his intent, but since the wording sounded a little ambiguous, I just wanted to clarify it...

Steve M

There's no false assumption on my part, at least in regard to anything you've said above. My question was within the context of already knowing what you describe above.

Perhaps another way to put what you said is that under normal conditions, an aircraft would be fueled with the mininum amount of fuel to accomplish its mission, including the required safety factors, rather than "filling up the tank." I guess another way to state my question is that under normal conditions, how much below maximum landing weight is a typical flight expected to arrive? I'd be surprised if it was 0 lbs (that is, that all flights are expected to arrive right at maximum weight) unless there is some other factor, such as a large variance in cargo that can be loaded.

OPNLguy

The CoEx dispatcher for the flight is the one calculating the fuel load, alternates, etc...

Once the fuel burn and thus the max takeoff weight limited by landing are determined, subtracting the aircraft's operational empty weight (OEW) and the total fuel load results in the max payload (passengers, bags, freight, etc.) one carry, and yes, one can load the aircraft right up to that max takeoff weight limited by landing.

OPNLguy

Messages crossing in the mail here...

The answer is that it varies, not just between different types of aircraft, but also between different variants of the same type.

At one airline I used to dispatch for, we had numerous 737-200 variants that had max structural landing weights ranging from 98.0, 103.0, 105.0, and 107.0, and the resultant weight "spread" between fully-loaded flights and the aircraft's max landing weight was far less on the 98.0 birds than it was on the 107.0-capable birds. As a result, we had lots more payload-versus-fuel issues on the former than we did on the latter.

The "spread" could be as little as 1.0 to 2.0 up to 5.0 or 6.0, again depending upon the variant. If the destination required more fuel for longer holding, or a more-distant alternate, that additional fuel weight would come out of the "spread", and if that didn't fully cover it, payload would have to be reduced, meaning some stuff would have to stay behind. Unfortunately, that sometimes included passengers.

DragonPhish

Isn't 85 gallons about 5 minutes worth of fuel? Did I do the math right? Must have been 8500 lbs, 981 gallons, which would be almost an extra hour.

dkul

As a captain I would always fly the shorter route and worry about burning the fuel enroute. Why? Because we are going to burn the fuel either way so no saving there. But the flight time will be less saving wear and tear on plane and hopefully helping pax connect. Also...if I arrive near destination with extra fuel and end up holding then I have the fuel to do that and kill two birds with one stone (enough fuel to hold and burn the extra fuel)

OPNLguy's explanation about calculating max takeoff weight based on landing weight + fuel burn is basically what the RJ was always limited to. Very rarely on the DAL-IAH flight is the fuel burn so high that you actually get to the MAX structural takeoff weight. The older 145 ER's (I believe are all gone now from XJT) were very bad on weight limits...basically limiting to 45 pax. Don't have the books handy but seem to recall that 145LR was MAX take off of 48,501 and MAX landing of 42549. (Don't worry..don't fly for XJT anymore, so no need for me to know the numbers from memory ) So as you can see that is a difference of roughly 6000lbs. If burn is only 2000lbs then you have 4000lbs of "unusable" weight since the landing weight is more limiting. So yes...we try to max out landing weight with pax and cargo (mostly just suitcases) to get as much onboard as possible. If the flight is not full and we have the same situation happen with shorter route it is no problem because then the extra fuel weight can be "accomodated" below the landing weight.

85 gallons equates to about 600lbs of fuel. (1 gallon is calculated to 6.767lbs)
On an RJ that is about 15 minutes fuel burn in cruise (1000lbs per engine per hour). But as mentioned earlier in the thread...there is a "required" minimum fuel to be on board at brake release for take off..if the plane is even 10 lbs under that the captain has to take more fuel. (I say 10 lbs because the gauge does not register in single digits)

mendicantfriar

A pilot doesn't necessarily have to "circle" to burn excess fuel; he/she can fly at a lower cruising altitude, extend landing gear and/or flaps early, and/or extend a downwind leg during the approach phase of flight.


If weather is no longer an issue, I'll always take the shorter route and then figure out the most efficacious way to burn the fuel and arrive as close to on-time as possible. Holding is an option, but as noted, it's not mandatory.


This is the first time I've heard that circling isn't safe. And this idea is based on precisely what? The shorter route gets us to the airport faster, and with extra gas for contingencies. What's not to like?



You're spinning yourself into virtual incoherence.

Totoro

Passengers don't receive extra EQM/RDM, so why fly the longer route?

Bad for fuel economy, but great for VDB! That explains why I've had much less success with VDB these past few years.

Syzygies

Sorry I wasn't clear. I'm fascinated by pilot responses; I was trying to second-guess what an engineer's critique of my question might be. (I'm a mathematician.)

My thinking was this:

Flights on a longer route don't always also burn fuel intentionally on approach. Therefore, sometimes they have naturally used more fuel than necessary, from the point of view of safe landing rules.

Ideally, flights diverted back to the shorter route will burn essentially the minimum amount of fuel, from the point of view of safe landing rules.

If this consideration is the dominant effect, then the planes diverted back to shorter routes will consume less fuel on average. So a bean counter, or an environmentalist, will want the plane on the shorter route whenever possible, to conserve fuel.

We don't have to consider this, but anyone trying to keep an airline solvent does.