Nice in depth post. The default Asobo A320 Neo FMC does barely any profile management. I climb and decend with pilot managed speeds.
I’m pretty much controlling speed manually based on the typical performance of the default A320Neo aircraft and its refusal to increase N1 to around 90-92% near top of climb. I’m almost always with 15tons of payload and 3.5 to 4 tons of fuel. Giving just under 60t gross. Above FL280 it will have very poor climb above mach 0.72 hence why I use that. In real life climb is still very satisfactory at 0.75 because the N1 increases as it’s meant to.
Further regarding speed, even in econ mode, the variation in cruise mach is only 0.756 with tailwinds to 0.79 headwinds. Not much different. The arbitrary 10kts I use is just to recognise a notable headwind has slowed GS by 10 to 120kts so I make an upward correction to climb speed and sacrifice some climb for speed. Conversely if there is a notable tailwind of at least 10kts I can trade some airspeed for quicker climb as the groundspeed will still be greater than or equal to still air. Of course this is not an exact science in the same way when climbing in a piston you lean the mixture for peak cht and then add back a “little bit of mixture” for a touch of engine cooling just to see that temperature needle move back down “slightly”.
Many airlines have different SOPs. In Europe it seems most Easyjet flights are cruising at 0.756 regardless of winds which is interesting; whereas the flag carriers keep the speed up. Some airlines want to maximise flight time in order to sell more snacks; usually the schedules are more than long enough to fly slow into headwinds and still arrive early, even if departing late
Its not really an SOP, its just different cost index used per airline and / or route. Most fuel efficient would be to not correct for headwind or tailwind at all during climb, rather fly the speed for max. climb rate (which does not change with wind), then at cruise altitude adjust the cruise speed as wind affects the long range cruise speed. If you use ECON speeds and COST INDEX it will optimize fuel consumption over flight time for all phases of flight. Usually when (for whatever reason) ECON speeds are unavailable, you don’t correct for wind during climb.
I’ve noticed recently that FBW hasn’t truly implemented cost indexing. Put it on 5 or 95 and you get the same managed speeds. I know they’re not done, and that’s picking at nits, but at the same time you would think that would be quite easy to implement, as it’s simple math. “IF CI=‘75’ THEN SET Vcr=‘m0.80’” for example. Only using the right number, I just made that one up. I also made up the programming syntax, but the meaning should be clear…
The most fuel efficient outcome is minimum fuel per nautical ground mile. If you had a Cessna 172 climbing at Vy into a headwind of 40kts it will reach optimum altitude quicker but a cruise climb at 90kts will get you to destination with less fuel used. In any case a huge amount will be consumed vs normal and 3/4 of the time winds are stronger the higher you climb, there are occasional exceptions 1/4 of the time though (sea breezes, katabatic winds, no jetstream in situ etc). Still air, tailwinds or light headwinds fine get me up there quick, but on a day with strong headwinds up in the strong zone isn’t somewhere we need to rush to.
If the winds are less favourable at higher levels surely the true optimum level is that which gives the highest groundspeed for a given IAS combined with engine efficiency. In a piston there are many situations where you save fuel flying out at 2000ft agl and back at 11000ft amsl vs both ways at high level. The slower the aircraft the more wind matters.
Thats not really how it works on a jet though. Its almost always more fuel efficient to climb to altitude as fast as possible. Not the most time efficient maybe, that I agree. I have been working as ops manager, I’ve run the numbers for numerous aircraft. Same with de-rating (flex) for take-off, great for reducing engine wear but increases overall fuel consumption.
Agree with you regarding FLX not being optimum efficiency, but it’s saving turbine life with the associated fuel penalty being the cheaper option.
Yes, jets not affected as much by winds and the engines not that efficient at anything much below 60 percent; for that reason, when I fly the Beech King Air 350, I always keep torque above 50% even in decent, in fact descent is the fastest part of the flight in that aircraft for me.
In something like an Atr72-500 winds are more significant and a large consideration for slow cruising pistons capable of the higher altitudes above 12,000ft such as DA62 and C182T. In Europe at the moment there are plenty of days when the wind above FL100 is above 40kts.
Obviously yes, the point I’m trying to make is, although fuel flow is lower when performing a de-rated take-off, the overall fuel consumption increases as you spend more time at lower altitude. And that is the big difference between pistons and jets, it is almost always more fuel efficient to climb at best rate to high altitude where the engines run more efficient rather than fly faster with an headwind to spend less time in that headwind.
On the ATR we used 170 kts as normal climb speed (160 for the 42) as per manufacturer. With a headwind we usually went 5 degrees PITCH HOLD after passing MSA and accelerate a bit further, not an exact science. The Operational Flightplan usually shows trip fuel and trip time for the filed level and then has a little table showing a couple of levels above and below with trip fuel and trip time so you can choose the fastest or more fuel efficient level.
On a turboprop, shortest time is always cheaper (at least the ones I flew). I didn’t understand why we flew maximum speed cruise (MSC) instead of maximum range cruise (MRC) when we were empty and ETA was not important (ferry flights, repositioning etc.). When calculating the difference, it is always cheaper to fly shortest trip time, the lower fuel burn does not offset the maintenance costs for the extra time in the air (unless fuel price is insanely high). For jets fuel burn is much more important factor.
I had a 80 kt crosswind yesterday at FL100 . Reminds me of a question I used to ask when I taught General Navigation to ATPL students. You have to fly from A to B, then back from B to A. What is the quickest?
A. Tailwind from A to B, headwind from B to A.
B. Headwind from A to B, tailwind from B to A.
C. A direct crosswind.
D. No wind.
E. Doesn’t matter.
Answer
Headwind on one sector and tailwind on the next sector (or vice versa) don’t cancel each other out as you spend more time in the headwind and less time having benefit of the tailwind, overall the total trip time is longer than in still air. The wind correction angle required to correct track for a direct crosswind also reduces the groundspeed and therefore increases trip time.
Although climbing to peak efficiency altitude at max rate is most fuel efficient outcome in nil wind, if the headwind is increasingly punitive at high level maybe climb close to max rate to FL250 or similar and then cruise climb from there up. I think most of the efficiency gains with altitude come early on. Especially the first 10,000ft air density halves.
Maybe you gain 10% less fuel burn per hour at mach 0.78 at FL 380 than FL320. But if the wind at that level lowers groundspeed by more than 10% then sitting lower would still be advantageous.
Have noticed on Flightradar24 several flight recently have maxed out at FL390-400 but then dropped to FL350 for the remaining couple of hours even when traffic management does not seem to be at play. A very strong headwind over 100kts was at high level bit was only 70kts at the lower level.
Not sure exactly what the score of fuel burn vs maintenance is with LCCs but it seems extended flight times and increasing pax hunger allow more sales opportunities from the in-flight magazine/menu. Cargo airlines seem to go full speed as there are no trolley sales to be derived from a leisurely pace.
Apparently most LCCs are happy to make £1 profit on the ticket sale knowing full well that 1 can of beer will triple their profit on that seat. Same number of cabin crew would still need to be there if they were selling beers and hot wraps or not. Ryanair will always run a route even if they only ever break even on ticket sales alone, so long as the airport doesn’t allow a McDonald’s or Burger King to set up in the terminal
This is never a consideration, at least I’ve never heard of it. The cost index is purely a time versus fuel consideration, fly slower and burn less fuel or fly faster and maybe be able to squeeze out an extra flight which makes up for the higher fuel burn. And of course its more efficient to cruise at a lower (or higher) altitude than that for optimum fuel burn depending on the wind. Our Operational Flightplan usually shows the fuel burn and difference in time for a range of flight levels + / - 4000 ft from the filed level so we choose the most efficient one.
. Reminds me of a question I used to ask when I taught General Navigation to ATPL students. You have to fly from A to B, then back from B to A. What is the quickest?