As others have said, final fuel reserves are typically at least half an hour, and you shouldn't really be cutting into them. What if their first approach into MAN had led to another go around?
With a major storm heading north-easterly across the UK, the planning should have reasonably foreseen that an airport 56 miles east may also be unavailable, and should've further diverted prior to that point.
They likely used the majority of their final fuel reserve on the secondary diversion from EDI to MAN, presumably having planned to land at their alternate (EDI) around the time they reached the final fuel reserve.
Any CAA report into this, if there is one produced, is going to be interesting, because there's multiple people having made multiple decisions that led to this.
Suspect they were IFR. All your points stand. First time flying things with a jet engine, I was shocked how much more fuel gets burned at low altitude. It almost always works out better to max climb to altitude and descend than to fly low and level. On a small jet, things can get spicy fast when ATC route you around at 5000' for 15 minutes or so. Three aborted landings would gobble gas like crazy.
§ 91.167 Fuel requirements for flight in IFR conditions.
(a) No person may operate a civil aircraft in IFR conditions unless it carries enough fuel (considering weather reports and forecasts and weather conditions) to—
(1) Complete the flight to the first airport of intended landing;
(2) Except as provided in paragraph (b) of this section, fly from that airport to the alternate airport; and
(3) Fly after that for 45 minutes at normal cruising speed
They were most definitely IFR. Not because of the weather but because IFR is required above certain altitude 18,000 ft in the U.S. and typically lower in Europe (depends on a country). Jets including small private jets are almost always on IFR. Airliners with passengers - always.
Why does it burn fuel so fast?
My guess is higher air density means more wind resistance, which acts as negative forward acceleration.
Not just that. Jet engines are efficient at higher speeds because the exhaust of the jet engine is fast.
If the plane is going fast as well, that exhaust is more or less stationary relative to the ground. The engine works to exchange the position of the plane with the position of the air in front of it.
If the plane is going slow, it's accelerating the air backwards. That's where the work is going, making the engine less efficient.
Think about it this way: if the jet airplane is tied to the ground, its engines are running at 0% efficiency, working hard to blow the air backwards. You wouldn't want to stand behind a jet engine when the plane is about to take off, when that's effectively the case.
The same applies to propeller-driven planes, of course. But those can vary the prop speed as well as propeller pitch, having more control on how fast the air is being pushed backwards. This allows the engine to be efficient at a wider ranger of speeds, particularly, at the slower range.
But the propeller has a limit of how fast it can push the air back. When the prop blades start reaching the speed of sound, weird shit starts happening [1]. So propeller-driven aircraft have a limit on speeds at which they can go efficiently.
Jet engines (turbofans when it comes to airliners) trade off low efficiency at low speed / low altitude (where the airplane is spending a small percentage of flight time) for higher efficiency at high speed / high altitude.
Variable pitch turbine fans[2] aim to address this tradeoff, but the tech has yet to catch on.
[1] https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech
[2] https://en.wikipedia.org/wiki/Variable_pitch_fan
That sounds like Oberth effect in rocketry, where the faster you go the more efficient your rocket be: https://en.wikipedia.org/wiki/Oberth_effect
they have nothing to do with each other.
I think about it like this:
Jet needs to suck air from front. If air is stopped, sucking is hard. If air is already being thrown at you, you don't even need to suck, just let it come in.
You are right that accelerating the air backwards more reduces efficiency but I think it should be mentioned that the jet engine has to accelerate the air backwards to do any work pushing the plane forward. Picking it up and setting it back down affects the air with a net force of zero and therefore the force pushing the plane forward is also zero.
So perhaps the differential air speed between the intake and exhaust is a big factor in the efficiency equation? The bigger the difference the more work is needed..
Variable pitch turbine fans sound very interesting! Perhaps in the future as tech improves and fuel efficiency incentives continue to increase.
So, newton's first law?
Just reaching altitude again to make it to the first and later second alternate are mostly likely the biggest factors in the extra fuel consumption. That's very expensive.
The 30 min reserve is on top of the fuel needed to reach the alternate and do a landing there, so only the flight to the second alternate, plus the 2nd and 3rd landings at the initial destination would have cut into the reserve.
With 100mph winds I could easily see the 30 min reserve being eaten up by the flight from Edinburgh to Manchester. It's 178 miles! It takes a good 15-20 minutes to cross that distance when flying normally, add ascent & descent time and the landing pattern and you're easily at 24 minutes.
Edit: in other comments here, it seems like Edinburgh to Manchester is a 45 minute flight. So yeah, they could easily have been outside of reserves when they did the go-around at Edinburgh and still had only 6 minutes left at Manchester.
Yeah, although it depends what the alternate was in the flight plan. It may have been Manchester. Although I think its more likely it was Edinburgh, which in the circumstances was too optimistic. Too much concern about the minimal costs of fuel tankering to add a bit more gas? Or saving time by not refuelling?
Ive never flown on Ryanair and dont intend to.
As far as I’ve heard, Ryanair will cut into literally everything (including comfort and decency) for the sake of efficiency – other than safety. Even if they wanted to, they're subject to the same commercial aviation regulations as everybody else.
Do you have anything other than this single incident to back up your insinuation that they’re less safe than a full service airline?
I don't know how true this is but I have heard Ryanair will use the absolute legal minimum amount of fuel whenever possible whereas other airlines might fly with a bit more.
In theory though that shouldn't matter because as you say, the legal minimum should really be enough.
That seems like a cost/convenience tradeoff: The implication of only carrying minimum fuel is that the pilots can't hold for long to see if conditions improve and instead have to immediately go for the alternate destination airport.
The consequence of that is everybody ending up in the wrong place, but not in an unsafe way.
The flight plans I've seen accounted for two alternates, not one, a significant time in a holding pattern and up to three go-arounds. This was for cargo 747s and a while ago so chances are the regulations have changed by now, also, it may have been due to the kind of cargo.
From what I can tell, that only seems to apply to EASA since 2022. As it took off from an EU airport and landed in the UK, I don't know if that rule would apply.
You get that energy back on descent, no?
4 replies and 3 are dismissing even the idea..
Yes, you get "some" back, and its not negligible amount. Typical modern airliner can descend on 15-20:1, giving you over 150-200km (90-120mi) range from typical cruising altitude of 33 000 feet even with engines off. Most everyday descents are actually done by maintaining altitude as long as possible, and then iddling the engines fully for as long as clearance allows. (Ofc you then use engines as you geat nearer, because its safer to be a little low when stabilizing on approach, than a little high)
Thanks to turbofans(edited from turboprops) better efficiency + less drag at higher altitude its actually more fuel economical to command full thrust and gain altitude quickly, than slower climb, or maintaining altitude (which goes against our intuition from cars, where if you wanna get far, you never give full throttle).
But theres still some drag, so you dont get everything back, so you generally want to avoid murking in low altitudes as long as possible. Full thrust repeatedly at lowest altitudes (from failed go arounds) is the least economical part of flight, so you want to avoid those if possible. But its true that the altitude you gain is equivalent to "banking" the energy, just not all of it.
(1) this was a jet, not a turboprop
Edit: changed turbofan into turbprop, which is what I meant.
(2) fuel burned stays burned, you don't 'get it back'
(3) the altitude gained may have been adjusted to account for the low fuel situation
(4) the winds are a major factor here, far larger than the fact that 'what goes up must come down', something that is already taken into account when computing the fuel reserve in the first place.
So it seems. But because you want to land you then want to shed all that velocity. So you 'get it back' only to have to waste the bigger fraction of it. A go around is much like a mini take-off, you just miss the runway portion of it.
Nah. You want to land, but you are really not shedding most of your velocity until after touchdown. What you gain by burning fuel is energy, and you can either bank it into altitude, or velocity. You must shed both to land, but not so for go-around. There you shed almost all of your altitude, but you keep most of your velocity -> you still have a lot of energy left. That's why on go-around you spool your engines and start climbing basically right away, unlike typical takeoff, where after spooling up the engines you are still earth-bound until you build enough velocity.
So you only ever really lost your "altitude" component of energy, not "velocity" one. You run your engines at TOGA (Take Off / Go Around = maximum thrust), thrust to gain mainly altitude, only increasing speed a little bit. Then on another approach attempt you use both the altitude and excess velocity bank again.
In flight, ~all your energy losses go to drag. Doesn't matter if you bank it into speed or altitude, both is exchanged to be at minimums (0 altitude above ground, lowest safe landing speed) at touchdown. If you produce extra energy in your engines, it has to go to either speed or altitude, which you then pull out again, usually by maintaining speed while lowering altitude while having engines at idle.
(1) The turbofan category of jet engine seems to inspire a lot of very pretty animated technical diagrams—here’s one set from a German manufacturer [0]. Now if only we could convince Bartozs Ciechanowski to take on such a subject… [1]
(2) I know glider pilots who fly without any fuel at all, once aloft… sounds not unlike the 150-200km glide range that @MaxikCZ mentions at idle from cruising altitude.
[0] https://aeroreport.de/en/good-to-know/how-does-a-turbofan-en...
[1] e.g. https://ciechanow.ski/airfoil/
Aircraft that are designed as gliders are much lighter and thus have much longer glide range than aircraft that aren't. They're so lightweight that they can climb on thermals. A 737 is not going to be able to do that, but a regular glider can't fly at 400 knots.
> thus have much longer glide range
Im gonna be a little pedantic, but the weight has surprisingly small effect on glide range, actually none of the weight affect the range directly, its all from secondary effects.
The glide is given mainly by drag and lift (so body and wing geometry), correlated to certain speed. The weight isnt in the equation at all. What weight does, is increases the speed in which the aircraft achieves this maximum glide ratio, and in higher speed you have higher drag, which finally reduces the range.
Thats why many modern gliders have water tanks in wings, to increase the weight of the glider, moving planes speed of best glide ratio higher, allowing for more efficiency at higher speeds. Its worth it if the atmospheric condition provide strong lifts. Pilot can then dump the water in flight to reduce the wing load, allowing them to land with less speed, or just keep in the air longer as thermals get weaker in the afternoon/evening
(source, I used to be a glider pilot)
It should also be noted that gliders have crazy aspect ratios. Airliner wings are designed for completely different flight envelopes than gliders, it’s all a game of what you optimize for and what trade offs you are willing and/or required to make.
But of course that doesn’t mean that airliners can’t glide well, the Gimly Glider and Air Transat flight come to mind. But gliders can definitely beat an airliner in terms of performance.
You are, of course, correct, and thanks for clarifying.
Re: (2): There's a difference between sailplanes and gliders. Sailplanes are gliders that can “soar”, i.e. gain altitude just from the air that is moving up for some reason. Your friends have licence that says „Sailplane Pilot Licence”, not „Glider”.
The distinction is less pronounced nowadays, because there is no mondern aircraft designed as gliders-but-not-sailplanes, but historically there were planes that fit this niche, mostly military transport of WW1 and WW2 vintage.
Passenger jets (with engines turned off) are relatively decent gliders, but incapable of soaring. So no, you can't get more that about 20:1 glide ratio no matter how good is the weather (for sailplanes).
Regarding the turbofan and [0], above...if you're communicating to a non-engineer (me), how does the design get to the point of such complexity? I would love to learn the design story behind such an incredibly complex piece of machinery.
I am being serious, if you cannot tell.
For the same thrust it's more efficient to accelerate a large mass of air a small amount than t accelerate a small mass of air a large amount. The fan is what gives you that.
I rough guessed the cost of fuel over a 737's life as $150 million. Where the engines cost something around $30 million. That pushes the engineering economics towards maximizing the engines efficiency.
I'm suspicious that bypass ratio's for turbofans are close to maxed out. The diameter of the fan gets unwieldy. That was the design issue that the 737 Max was trying to get around. With bad results. Possible the future is hybrid designs with two engines and 4 or more electrically driven fans.
Yes, sorry, meant to write turboprop.
1 - a turbofan is a subset of jet engine, and there are no 738s running anything other than a turbofan.
Actually, nothing in civil aviation that has a "jet engine" has used anything but a turbofan (or turboprop) since the early 70s with the exception of Concorde and some older business jets.
(Turboprops are jet engines, too, to be precise, with the jet of exhaust gases powering the propeller.)
> Turboprops are jet engines
They are certainly turbine engines, but I thought "jet" was reserved for those engines that propel the vehicle solely by their exhaust stream and bypass air. I am willing to be told I'm wrong, though.
Turbofans are by your own definition jet engines. It's just that the bypass air is much larger.
I think you meant turboprop there, but the distinction I notice is that one has all propulsive airflow inside the nacelle, and one does not.
Agh. No, I meant turbofan, but I misread your post and actually completely agree with you - turboprobs are not jet engines.
Ha! It happens. Enjoy your weekend.
No, you don’t magically get the fuel back. But you do get a lot of the _kinetic energy_ back, and that energy keeps you flying without having to burn yet more fuel. You burn a lot of fuel while climbing, but then hardly any at all while descending. And that descent might cover 100 miles across the ground.
The 737-800 uses CFM56-7 turbofan engines.
[1] https://en.wikipedia.org/wiki/CFM_International_CFM56#CFM56-...
1) Yea, sorry, turbofan, not turboprop nor a jet.
2) It stays burned, but the energy is banked in potential energy of the aircraft, namely in a form of altitude. If you run out of fuel 5 feet above ground, you dont get to fly far. When you run out of fuel 35000 feet above ground, you can still choose where to land from multiple options.
3) huh? I dont get what you trying to say, but: Its always more economical to climb, and the faster the better. Ofc you cant climb too high when you intend to attempt to land in 5-10 mins, but nontheless, every feet gained is "banked", and the aircraft is more economical to run the higher you are.
4) I am not saying the winds arent a factor, and in no way I was arguing about how fuel reserves are calculated. My only claim is that: yes, by spending more fuel to gain altitude, you can then "glide" down almost for free later. Its not 1:1, because of constant losses like drag, but its being compensated by higher engine efficiency and less drag at altitude, that its always worth it to climb if you can.
There was a flight that was low on fuel diverting to alternate between 2 islands. The pilot panicked and chose slower climb to intuitively save fuel. They had to ditch the plane in water because of it - if they initiated full climb, they would have made the jump.
Wow this has a lot of replies!
Yes, you get a lot of the energy back, BUT there is a huge problem!
Large airliners incur a LOT of additional drag to slow down while landing. Some of that is entirely intentional, some is less intentional.
It is highly preferred to deploy the landing gear before touching down. Failure to do so may lead to a hard landing and additional paperwork, so airlines do not allow the captain to exercise their own discretion.
Extending the flaps maintains lift at lower speed, and higher flap settings allow even lower speed. The highest flap setting generally also deploys leading edge slats.
If the wheels of the airliner touch down and detect the weight of the plane then spoilers kill the lift of the wings, air brakes fully deploy, as well as thrust reversers.
All of these things add drag, which uses up all that energy you've been converting.
The upshot is that each landing attempt uses a LOT of energy, and you have to use fuel to replenish that energy after every attempt.
In other words, yes you get it back, but only for one landing attempt.
As someone who has ridden a bike up a big hill, and then down it, I don't think you get it back.
That is perplexing. Of course you get the potential energy back. It turns into kinetic energy as you descend. That is why you need not pedal downhill, and often even need to brake to prevent the bike from speeding up too dangerously.
> often even need to brake to prevent the bike from speeding up too dangerously.
Indeed, which is what the airplane would have done on its way down to land. So it's more like riding the brakes on your way down the hill, and now at the bottom when you realize you need to abort the landing, you are at low speed and it's quite an exercise to get back uphill to try again
100%. You are correct on that. You can’t use your kinetic energy to go around after a landing attempt.
But not because “you don’t get the energy back”. (As recursive suggested about a downhill bike ride which is the part i am disagreeing with.) You do get it back, but because you want to land you bleed it away to drag. And once it is bled away you don’t have it anymore.
So we don’t disagree about the practical implications for flying. I’m disagreeing with recursive’s particular statement about downhill cycling and what it implies about the physics of the problem.
The glider guys would always suggest a forward slip. It's a lot of fun to do. It's not taught often enough during primary training for powered airplanes.
Aren't low-speed slips something that makes planes flip upside-down when not used very carefully? (Inadvertent rudder changes corrected with opposite aileron resulting in a snap roll.)
A cross controlled stall can result in a spin (which is probably what you mean by flip upside down). The rudder changes aren't inadvertent, they're intentionally opposite the aileron input - the goal is essentially to fly somewhat sideways, so the fuselage induces drag.
In general forward slips are safe, but yes you have to make sure you keep the nose down/speed up. There's little in aviation that isn't dangerous if you aren't careful.
Yes, being that one is cross-controlled they must be used very carefully. It's really obvious that one is cross-controlling. It's the only time outside of really powerful crosswinds that you see what's below and ahead of you out of the side window. That view is what makes it fun.
You're probably thinking of a skid, which is when you put too much rudder in the same direction as the ailerons. Then the lower (and slower because it's on the inside) wing stalls first (and goes lower still) and away you go. Often when turning to land, so there's not enough altitude to recover.
Yes, but that also doesn't get any energy back on descent, quite the opposite, that is "riding the brakes on your way down"
Well it's not all lost otherwise it'd be a stall spin accident caused by performing the maneuver with too little airspeed. And that's hard to do. It's a noisy maneuver, the air slamming against the fuselage makes itself heard. Once performed it's not easily forgotten.
More dangerous than inadvertently spinning with too little airspeed is the possibility of shock cooling when relying on a forward slip for too much altitude loss. It really does need to be well-controlled.
I thought this topic was about energy gained and lost during a go-around. If velocity was V and altitude was H before the go-around, and velocity and altitude are again the same V and H after the go-around, then it follows that all the potential energy that was accumulated during the go-around (from converting fuel into altitude) has been dissipated (lost). Otherwise V would be higher the second time.
The subtopic changed from energy gained during descents to descents in general
Imagine a hill with 500 feet of elevation descent, followed immediately by 500 feet of ascent. No curves.
If you coast all the way down the first part, you'll get about 20 feet up the other hill before you need to start pedaling. This is a direct analogy to "getting your energy back" by losing elevation.
That is exactly what a rollercoaster does and it doesn’t start “pedaling” after 20 feet. Of course real systems have losses and you can’t practically use all the energy.
But you don’t have to believe me. Look at the video of this glider doing an unlicensed airshow: https://youtu.be/QwK9wu8Cxeo?si=L-0Mfmu8wk1ZlQU7
It is a glider so it can’t “pedal”. You can see it steeply descending from 5:13 to 5:30 while it is speeding up and then the pilot picks up the nose and trades some of his speed for elevation again. And then he does it again around the 7 minutes mark.
You have two buckets of “water”. One bucket is kinetic energy and the other is potential energy. You can trade one for the other. You can also “lose” from the total volume of “water” due to drag (or friction in the case of the bike or roller coaster). Or you can add more “water” to your system by pedaling or thrusting with your engines. This is just simple physics 101. Also simple lived experience if you ever have the opportunity to fly an airplane.
The more water you put in your system the leakier your buckets get. Drag is not linear with speed. That was my point.
This is because bikes cost you about 50% more energy going uphill than walking[1]. You get back everything you don't lose from having to pedal too slowly, hunch over the front wheel, and maintain constant torque on the pedals.
1: https://pedalchile.com/blog/uphill
Just as with bikes, it will depend on how slow it is descending. On "right" trajectory engines could technically be basically idle, and you save fuel flying high so it might not be all that huge loss.
No, and you don't want it. You want to be on the ground and stopped. In the lowest energy state.
It's not currently feasible to harvest it into fuel. It's (very very nearly) all lost to drag, on purpose.
How? On descent you can trade some of your altitude (potential energy) for kinetic energy, but then you can’t land the plane. For descent on an approach you’re going from low energy to even lower energy. In emergencies and with enough runway you can futz around with this some, but wiggle room on an airliner is not great, negligible to what will be expended on a go around.
Some of it. The air density is an important part of efficiency at higher altitudes, so every moment spent under like FL320 is wasted fuel.
So the entire climb "up", you are also wasting energy fighting the thick air. On the way back "down", that air again fights you, even though you are basically at idle thrust.
Your fuel reserves are calculated for cruise flight, so time spent doing low altitude flying is already at a disadvantage. "Two hours of reserves" is significantly less than that spent holding at a few thousand feet. Fuel efficiency while climbing is yet again dramatically worse
The problem isn’t getting the energy back, it’s doing so more slowly than gravity. Planes are somewhat limited in their ability to glide.
Some of it, but much is lost to drag. They do have to limit speed at all times.
Not really. While you have a large potential energy buildup at a higher altitude, you cannot "bank it" / "save it" on descent. There is no way to store it in batteries or convert it back into fuel.
One of the challenges of aeronautics is the efficient disposition of the potential energy without converting it all into kinetic energy (ie speed) so that the landing happens at an optimally low speed - thus giving you a chance to brake and slow down at the end.
> "While you have a large potential energy buildup at a higher altitude, you cannot "bank it" / "save it" on descent. There is no way to store it in batteries or convert it back into fuel."
An electric fan aircraft absolutely can recharge it's batteries on descent. The fans simply act as turbines, creating drag to slow the aircraft and electricity to charge the batteries. Large commercial airliners already have a small turbine that works this way, the Ram Air Turbine (RAT) which is used to generate electrical power in emergencies.
You can use a turbine to generate electricity, so yes, you are converting potential energy into electrical potential. However, no real mass produced passenger plane today can use that electricity for flight (thrust).
RAT is only used when sh*t hits the fan. Even then, it can help you power some hydraulics / electrical, not “store” energy for further flight.
The OP asked - in a low fuel situation, can the energy spent on a climb get effectively recovered - and the answer is not really. We convert as much as we can into unpowered (low-powered) descent. But once you are at a spot where you make a final decision to land or not, you are by design low and slow - and all that energy you had 15m ago is gone.
If you need to keep flying, those engines need to spool back up. And that takes fuel.
> "no real mass produced passenger plane today can use that electricity for flight (thrust)"
Such aircraft do exist. For example, the Pipistrel Velis Electro trainer. And more recently, the Rhyxeon RX4E became the first electric aircraft to be type-certified for commercial passenger operations.
It's likely that we'll see many more electric fan aircraft in the coming years/decades, whether powered by batteries and/or hydrogen fuel cells, or hybrids with both conventional turbofan and electric propulsion in order to improve efficiency and environmental performance.
> RAT is only used when sh*t hits the fan.
Isn't it when air hits the fan, technically?
(Sorry.)
> As others have said, final fuel reserves are typically at least half an hour, and you shouldn't really be cutting into them.
This is one of the multiple layers of defense that airlines employ. In theory, no one single failure should cause a major incident because of redundancies and planning. Airlines rely on the "Swiss-cheese" model of safety. Each layer has its own risks and "holes" but by layering enough layers together there should be no clear path between all of the layers. In theory this prevents major incidents and given the commercial airline's safety records I'd say it works pretty fucking well. Landing with minutes of fuel left should be exceptional. But it also shouldn't be fatal or a major risk due to the other layers of the system. ATC will move heaven and earth to land a plane low on fuel or with engine trouble safely. And everyone else in the system having 30+ minutes of extra fuel gives the space for this sort of emergency sorting.
I think this also reflects on the "efficiency" that MBA types bring to companies that they ruin. If an MBA sees a dozen landings with an extra hour of fuel, their mind starts churning at saving money. Surely an hour of extra fuel is too much and just wasted. Wasted because every extra gallon of fuel you take off with is extra weight you have to carry throughout the flight. Surely things would be more efficient if we could make sure planes only carry enough fuel to make their trip with very minimal overhead. And when everything goes perfectly according to plan, these decisions work out fine. Money is saved. Bonuses are paid. But the inevitable always happens. That's why it's called inevitable. Lives are lost. Wrists are slapped. Some people at the bottom lose their jobs. The world moves on.
[dead]
I thought a lot of airlines had rules to limit the number of attempts you could make at a single airfield in an attempt to prevent this exact kind of situation.
It sounds to me like they tried harder at their intended destination than maybe they should have, followed by going to an alternate airport that probably wasn’t a good choice in the first place, and then having to divert to the final airport where luckily they could land in time.
Interesting. To me it does not really make sense to think in terms of fuel left because, no matter the reserves, there can always be a situation so unlikely, so outside the ordinary, that it will drain all fuel reserves before you make it to the planned destination.
I have no clue how else to think about it though.
So maybe the thing we can improve is an understanding of likelihood?
I.e. prevent the journey from occurring if weather conditions are likely to be adverse above a certain threshold?