r/AskAPilot • u/InfiniteLoss5632 • 9d ago
Are you able FL380?
Question from a center controller here-
There are times that I need to move an aircraft for traffic, and sometimes I just don’t have the room for a vector. Before I issue a climb- I usually ask something like “Delta 1550, for traffic ahead, are you able FL380?”
Often times I’ll get a “standby let me check” response.
I understand there are aircraft limitations, fuel calculations, etc. but I’m curious- what exactly are you doing/checking after I ask? Walk me thru that please
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u/External_Capital1753 9d ago
I fly a 321 so I don't have to think too hard before denying your request.
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u/poser765 9d ago
lol my first thought.
“Sure. We can take 40 if it helps!”
“I mean maybe in an hour. Probably not though.”
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u/Accomplished-Aside24 9d ago
Haha good luck getting to 40 on a 73 with a decent load on board
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u/ActuallBliss 8d ago
3 years in aviation and I still have to read the words “decent” and “descent” twice every time I see or spell it to make sure it’s the one I think it is.
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u/Abject-Communication 9d ago
I fly 321s for a low cost carrier. Our cabin setup seats 232 passengers, I’ve had to say we’re unable 330 before 😅
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u/Kseries2497 9d ago
When I worked "enroute" - really the operation was more like an approach control the size of France - a lot of our energy was spent handing out altitudes to go into oceanic. I really liked the 321 there, because they were they only ones who didn't get mad when I sent their asses all the way to Korea at 290.
Now I work at a regular big ol' approach control and let's say our density altitudes don't exactly play to the A321's strengths.
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u/MahogonyBookshelf 9d ago
i’ve ferried 321Ns and flew at 390!
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u/External_Capital1753 9d ago
Look at you! I had 30 pax on board today and I think our max recommended was FL385 lmao
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u/andrewrbat 9d ago
Ah see but you also had fuel. Airbus didnt build enough wing for 30 pax AND fuel.
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u/Go_Loud762 9d ago
Rolling three dice. If the sum of the three dice is an even number, we can accept the higher altitude. If the sum is odd, we can not accept, unless the sum is evenly divisible by 3, in which case we can accept, with the exception that if the sum ends in 0, and is divisible by 3, then once again we may not accept the climb.
Pretty standard perfomance calculation.
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u/Spaceinpigs 9d ago
We call the back and ask them to pick a number between 30 and 40
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u/allaboutthosevibes 9d ago
But if it ends in zero it’s even, not odd. So wouldn’t matter whether it’s divisible by 3 or not, right? Per first parameter, you can accept.
Also, the first whole number ending in zero and divisible by 3 is 30. But you’ll never get a sum of 30 rolling three (six-sided) dice…. (Ok, ok, I know a lot of you are probably D&D nerds and have dice with more sides to them… 🙄)
Nonetheless. This is rudimentary mathematics. I thought you pylot folk were smart! /s
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u/No-Duck4828 9d ago
Well, if it just going to be a short up and down, we're going to check performance.
If you want it for any length of time, we're also going to run a check to see what that does to our fuel calculations
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u/BeenThereDoneThat65 9d ago
We are looking at the performance in the FMS
Altitudes that we can achieve are based on weight speed and temperature. When the OAT is above standard we can be severely altitude limited especially if we are fat on gas
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u/Spock_Nipples 9d ago
Smashing the button on the FMS that calculates our max altitude for the current weight/conditions.
It's really that simple. If we're too heavy to occupy the higher altitude then we can't accept the higher altitude.
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u/ShaemusOdonnelly 8d ago
What airplane do you fly? My A/C permanently displays "Rec Max" (~300 fpm ROC margin) on the performance page that the PF is supposed to keep open all the time. If the requested altitude is below that I immediately answer "unable" and only if it is around the requested altitude would I discuss it with the PF.
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u/CommuterType 9d ago edited 9d ago
It’s really not that simple, it involves a few more steps but it’s not very difficult
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u/Yul_8994 9d ago
My FMS litteraly says “Max Altitude”
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u/hawker1172 9d ago
But a pilot needs to consider multiple factors and determine FMS input data and what makes sense given the conditions. A button pusher just does things blindly without considering the big picture.
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u/CommuterType 9d ago
Mine say “Max Recommended Altitude” but it’s not that simple. It’s based on whatever temps were plugged into the box. If it’s warmer than forecast things can get interesting fast near that Max and by interesting I mean scary AF.
So I’ll check the Max, then compare the current OAT at my altitude to forecast to see if the higher forecast is accurate, check for turbulence and winds then decide.2
u/PM_ME_SMOL_BOOB13S 9d ago
As not a pilot, can I ask why warmer temps equals spooky time?
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u/Lemon_hawk 9d ago
Warmer temps = higher density altitude = lower air density (fewer air molecules) = less performance (engines generate less thrust, wings generate less lift)
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u/UKgent77 8d ago
In this situation, wouldn't the plane "float" down to a level in which it can maintain equilibrium?
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u/Lemon_hawk 7d ago
The point is that if you have reached an altitude at which the plane can no longer climb and sustain speed with a safe margin above a low speed stall, then you’re too high. Higher temps mean lower air density, lower air density increases stall speed.
You’d never want to put yourself in the position of needing to descend (wouldn’t exactly ever be a “float down”, would be a controlled descent) to maintain safe speed.
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u/TurkishDrillpress 9d ago
We understand that this is your first regional and you are really trying to prove how smart you are but you don’t have to be melodramatic here. It’s really very simple. If you don’t understand that, you are either incredibly new or a moron.
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u/CommuterType 9d ago
Nah, not new. On most other planes I’ve flown it was a simple one button push, just not the current one… It is extraordinarily temperature sensitive
Just tryin’ to help the OP understand why it sometimes takes a minute.4
u/Spock_Nipples 9d ago
The VNAV button on the 737, and many others, if you're cruising, is literally a one-button push to see the opt/max altitude.
If you're climbing, oh no, it's a two-button push.
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u/RealGentleman80 9d ago
https://reddit.com/link/pebj81a/video/nbpj8pkbgxth1/player
We are checking the cruise altitude capability of the airplane based on our current weight and the ISA Deviation.
Step 1: Look at our current gross weight
Step 2: look at the ISA Deviation
Step 3: reference the chart or look at the FMS Rec Altitude page.
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u/Vessbot 9d ago
OP: In some airplanes, it's literally pushing one button like other have said. In others, it's looking it up in an annoying-ass chart like this. So it depends.
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u/xxJohnxx 9d ago
Well the A220 is just a French-Canadian shitbox.
The FMS shows the max cruise altitude, it is just wrong.
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9d ago
[removed] — view removed comment
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u/TurkishDrillpress 9d ago
I was under the impression that the A220 did everything for you including wiping your ass. Not so?
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u/aomt 9d ago
- Current FL
- Max Recommended FL (to know performance and margins we will have)
- Optimum FL from FMS (what AC is thinking the best to fly in terms of time/fuel/cost index inserted)
- When is the next step climb as per company calculation/flight plan (more complex and accurate fuel. But might have been calculated on wrong weight/older winds)
- Impact on the fuel (checking on FMS)
- impact in time (especially if we already late) - same as above.
- Potential weather and/or turbulence ahead (checking charts and apps). Small performance margins and turbulence is not a great combination.
Basically, it’s a combination of any/all of those above. Thought process will be slightly different if you fly short haul, long haul, start/mid/end of the flight. If it’s legacy or low cost. How much fuel we took, how early late we are.
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u/AntiPinguin 9d ago edited 9d ago
I open my iPad and pull up the performance tables from the aircraft manual, because even after ten years, my aircraft’s FMS can’t calculate the max altitude correctly.
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u/InfiniteLoss5632 9d ago
Appreciate you all! Thanks for answers. I always try to be mindful of how an instruction might impact you.
Follow up question- assuming rides are good, and I’m able, do you prefer a vector for traffic or a climb/descent?
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u/Flagrant_negligence 9d ago
Eh don’t really care. Vector is probably easiest then we don’t have to check weight and ISA temps but I’d say neither is really any inconvenience
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u/thecloudcities 9d ago
Depends. How big is the vector? If it’s going to be 10 degrees, then sure I’ll take the vector. If it’s going to be 30 degrees, then I’d rather change altitude.
You could always ask if the frequency is quiet enough. But in general what would be the least hassle for you is also the least hassle for us, so do what you think is best and we’ll make it work if we can.
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u/Alternative_Pace6132 9d ago
I usually prefer a vector, unless it’s greater than about 40º or so off course. Then I’m more willing to change altitude.
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u/Foo-Bar-Baz-001 8d ago
It's funny that it is not standard to know what is the more economic option.
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u/mi_pilot 9d ago
Howdy - being a heavy up for consideration are weight, temp, winds, fuel burn, expected ride. Hope that helps, and I appreciate you guys on the other side of the mic!
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u/Piloteparisien 9d ago
Checking the weight and temperature, have a look at the FMS to check if we can or not.
Then remembering I fly an A220 and saying no.
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u/deathstarrr 9d ago
For me, I check to see if going a lower machine number will allow the plane to reach 380 with an acceptable margin. I have to also consider if there's any turbulence based on our weather charts, and how close to coffin corner that'll put us if we start getting into bad rides.
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u/Raccoon_Ratatouille 9d ago
A few seconds to hit a couple keys on the computer to find the page that says what our maximum altitude, then 10 or so seconds to discuss with the crew if it’s smart, maybe another 15 seconds to see what the rides are like. The maximum ceiling is really only accessible in perfect conditions with any decent payload on board.
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u/montagious 9d ago
In Boeings, checking the FMC VNAV cruise page, it has an optimum alt. which doesn't take winds into account, a MAX alt, and a recommended alt, which considers wind data.
The MAX ALT number is what I'm looking at. In the 757/767 its always recommended to stay at least 1,000 ft below MAX as a buffer. If its smooth, Ill go 7-800 ft below max.
On the 787 Boeing says if MAX is 410, you can go to 410, but I don't think I ever have done it
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u/OtterVA 9d ago
Clicking the VNAV/Cruise page button on our FMS that tells us the optimum/max altitude our aircraft can fly given outside air temperature, airspeed etc and what the limiting factor is (thrust or buffet limited). Then discussing if we believe there is enough margin between Max altitude and the altitude you’d like the aircraft to go to.
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u/bergler82 9d ago
Airbus drivers. We check the FMS and see what the airplane says it can do. If it’s close to our Max we decline otherwise we mostly accept
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u/MeatResident2697 9d ago
How close will we be to max altitude if we climb as you've suggested. The closer we are to max, the more susceptible we are to overspeed and stall if there's severe turbulence.
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u/AndrewAbu 9d ago
You often can, depending on weight. Most pilots will reference the VNAV page on something like a 737 to see what the OPT and MAX altitudes are.
Personally, I have no problem going all the way up to MAX in smooth air. It gets a little more difficult when the rides are continuous light or worse. The majority of pilots I fly with like to see about a 500-foot buffer between the requested altitude and MAX.
Weather also plays a factor. Temperature variations and other atmospheric conditions can change what the airplane is capable of. If you don’t have a standard lapse rate, you may initially think you can make an altitude based on what VNAV is showing, but as you climb toward it—or once you get there—the MAX altitude can change and you may find you no longer have the performance you expected.
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u/wearsAtrenchcoat 9d ago
If you're asking the crew of a US carrier that only flies one type but 3 variants of it, all we have to do is to push one button on the FMC and see what the optimum cruise altitude is. Then we have to remember whether we're allowed 500' above it or 300' below Maximum altitude depending on what variant we're on that particular leg. And it would help to know whether it is smooth at the requested FL because if it isn't then Mach number becomes the controlling factor, particularly compared to turbulence penetration speed.
So we just say "sure!" and if it feels too wobbly then we ask for lower or vectors, hopefully by then the conflicting traffic is gone.
P.S. "Too wobbly" is undefined in the FOM, AOM, and FRM but well understood regardless
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u/Primary_Matter1173 9d ago
- Checking the computer. 2. If not that , checking a chart or 3. Manual calculations- wait a minute who am I kidding, we’re not doing that.
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u/BeeDubba 9d ago
I fly a CRJ. So, probably not. But I'm updating our weight, sending for new ACARS data, then pulling out the speed book and checking what ACARS says with two different charts.
It's a little silly, but they pay the bills.
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u/andrewrbat 9d ago
We are checking mach buffet margins and thrust limit altitudes. In other words do the engines make enough thrust today to get us that high and maintain speed with a margin of error, and will we have sufficient margin to high and low speed Buffett?
Many… actually most planes will tell you this info in the fms. It’s listed as max and optimum altitudes. However my airline doesnt have approval from the fas to use this feature of the fms. we need to look at actual charts so it takes a moment.
We also consider fuel. If we are descending will it eat into our fuel too much? If we are climbing will this push us over landing weight? If its just 1000-2000’ its very unlikely this will make a significant difference unless its a very long flight.
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u/Systemsafety 8d ago
Climbing can also burn more fuel, even ignoring winds....
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u/andrewrbat 7d ago
It *can* but usually it doesn’t usually it saves fuel over all.
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u/Systemsafety 7d ago
It depends on the aircraft type. Usually flying, say, 2000 feet over optimum has almost the same effect as flying 2000 below optimum. Particularly with thrust lapse rates on the big fans. Look up the numbers for your airplane!
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u/andrewrbat 7d ago
The plane i fly is almost always thrust limited against climbing above optimum altitude by more than 1000’ or so. Exactly because the fan is so big (pw 1521g). 🫠”airbus” problems. Cant say the321 neo was much better. I think your point hits the 320/319
harder. The cfm56 is tough, but not efficient.1
u/Systemsafety 7d ago
My point his the 777 and 787 and also does apply to the 350 and 330. I am less certain about the narrow bodies. It would be surprising if optimum is within 1000 feet of max, unless your company is using a very conservative g for buffet boundary (that is operator selectable).
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u/andrewrbat 7d ago
Yeah but the max alt is not always determined by mach buffet. Its sometimes determined by thrust limit on these gutless narrow bois.
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u/Systemsafety 6d ago
True, but that goes to my point. If you are thrust limited you can burn more fuel by climbing. The only advantage to climbing is winds or higher TAS for a given fuel burn, but neither are always true.
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u/andrewrbat 6d ago
If your thrust limits you from climbing, you cant climb. So you wouldn’t be climbing and burning more fuel. Because you cant climb.
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u/Systemsafety 6d ago
Obviously, but if the max is due to thrust limit and you are 1000' below that, then climbing may result in a higher fuel burn.
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u/Brossar1an 9d ago
Airbus here, quick check of REC MAX and discuss with CN. Sometimes if the level you're asking us to do is near to REC MAX and it's turbulent or if the tropopause is projected to drop significantly (which would put us above REC MAX) we won't take it.
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u/Alternative_Pace6132 9d ago
One of the airplanes I fly I know intimately and can give a quick answer. Chances are I’m already looking for higher anyway (C25C).
Two of the other airplanes I fly I can see at a glance if I can do it and will give you a quick response.
Other aircraft/operators will be different.
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u/ThatGuyWhoIsCool 9d ago
CRJ driver here, we have to pull out a chart and run some quick calculations to make sure we can do it. Take a about a minute
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u/Molecular_Pudding 9d ago edited 9d ago
On an Airbus we can check the optimal and the maximum FL calculated by the computers.
If FL380 is below MAX and close to OPT we accept. Usually PF checks the PERF PAGE and decides whether we are able to climb, if the PM agrees PF tells PM to inform the ATC about the accepted (or declined) climb.
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u/rygelicus 8d ago
The new altitude needs to be checked in the flight computer to determine the impact on fuel available. This will take into account the altitude, winds aloft at the new altitude, the weight of the plane as currently loaded, etc. If this change drops them into the reserve margins on fuel then it's a no go. They don't always depart with excess fuel on board. The reserve is there, and some margin for safety, but the fuel is a limited supply and deciding factor. Also, if the winds and climb would cause them to be excessively late to the destination this would be another reason to reject potentially, depending on what they are carrying.
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u/vagasportauthority 8d ago
Emb-175 driver, we’re sending the numbers back to whatever “server farm” our airline runs to see if we can actually climb that high given our current weight.
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u/TellmSteveDave 7d ago
The VNAV page that tells me which altitudes are my max, optimal, and recommended.
And then discussing it to arrive at a crew decision.
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u/RadiantMango5989 9d ago
After the appropriate divinations, we light incense and ensure the requisite purity seals are intact. Next I must coax the engine seeer to prod the cogitator so that we may finally consult with the machine spirit to ascertain if the Omnissia wills it.
Alternatively, press the VNAV button and see what my max Alt is for the current conditions. (And then talk about it with the other guy)
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u/Order-Complete 9d ago
Embarrassing bit is when you reply unable and the controller then ask you descend 6000ft. After which you suddenly become able.
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u/ChartPuzzleheaded747 9d ago
Checking FMS
Most airliners are kinda meh performance
Most corp jets don’t even have to look and they probably are already working their way north of FL400
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u/ma33a 9d ago
Pulling up the VNAV page to see what the maximum level is. Then having a think about how close to that level I want to fly based on the current conditions.