Fuel Burn at Different Altitudes

[greg1016]

We have fam flights all over the world, but we have no cockpit access yet. Stupid.

so what is the point then?

It should be clear that it's not strictly an able/unable decision. If it's annoying that pilots are trying to work out the best altitudes based on these factors, making sure that they have adequate reserves on arrival, I apologize. However, not everything up there is cut and dry, and nobody is screwing with you just for "funsies."

I'm sorry, but if a pilot tells me unable, receives a descent clearance, and then requests the altitude that s/he just told me they were unable, then that tells me they lied to me the first time. I would not be asking if I didn't need the altitude. Nothing wrong with saying able and requesting a different altitude at the same time, at least then I have the whole picture. The more information I have, the better.

Example 1: Aircraft underneath you is requesting your altitude. I ask you when you are able next highest. You respond able, I climb you, climb the other guy. You respond able, requesting to maintain altitude, then you get your request since you were there first, and the other guy has to wait.

Example 2: Adjacent center will not accept you at your present altitude (say you are at f360) the altitude options given are f390 or f310. I ask if you are able f390 by the boundary. you answer able, you get the climb, you answer able, request f360, you still get the climb, but I'll try to keep you at a lower altitude as long as traffic permits. you answer unable, then you get a clearance to f310 (and again I will keep you at your requested altitude as long as traffic permits). However, if you come back and change your mind about f390, it is only going to annoy me. Workload permitting, I will try to work out f390, but since you lied in the first place, getting you to f390 becomes a low priority. Also, in many instances, the second you respond with unable, I (or possibly the next sector/center) will already have someone else lined up for that altitude.

Bottom line is be honest the first time. Safety is the #1 priority, but when the aircraft in my sector are happy, so am I.

 

[greg1016]

That is terrible.

Hopefully it'll come back! It'd be good for you guys to see the boredom-PANIC-boredom-EMERGENCY nature of our jobs.


Sent from my TRS-80

sounds familiar

 

[dasleben]

so what is the point then?



I'm sorry, but if a pilot tells me unable, receives a descent clearance, and then requests the altitude that s/he just told me they were unable, then that tells me they lied to me the first time. I would not be asking if I didn't need the altitude. Nothing wrong with saying able and requesting a different altitude at the same time, at least then I have the whole picture. The more information I have, the better.

Example 1: Aircraft underneath you is requesting your altitude. I ask you when you are able next highest. You respond able, I climb you, climb the other guy. You respond able, requesting to maintain altitude, then you get your request since you were there first, and the other guy has to wait.

Example 2: Adjacent center will not accept you at your present altitude (say you are at f360) the altitude options given are f390 or f310. I ask if you are able f390 by the boundary. you answer able, you get the climb, you answer able, request f360, you still get the climb, but I'll try to keep you at a lower altitude as long as traffic permits. you answer unable, then you get a clearance to f310 (and again I will keep you at your requested altitude as long as traffic permits). However, if you come back and change your mind about f390, it is only going to annoy me. Workload permitting, I will try to work out f390, but since you lied in the first place, getting you to f390 becomes a low priority. Also, in many instances, the second you respond with unable, I (or possibly the next sector/center) will already have someone else lined up for that altitude.

Bottom line is be honest the first time. Safety is the #1 priority, but when the aircraft in my sector are happy, so am I.

I really must be doing a bad job at explaining this. Nobody is lying. Please, please come on out for a FAM when you can. I got to tour ZDC about 6 years ago, and it was a great experience.

 

[dasleben]

Before I go, I'll bring up a situation from my last trip:

We were cruising at FL360 approaching LEXAD, which is the first fix on L453 southbound from JFK in oceanic airspace. The oceanic controller had a number of European-bound aircraft transiting the airspace eastbound, so our requested level of FL360 was unavailable. He asked us if we were able to climb to FL380. Now, we have a display on our flight management computer called "optimum altitude," which looks at fuel burn based on weight. More importantly, climbing significantly above optimum altitude can put the aircraft into a situation where low-speed stall buffet and high-speed mach buffet are close together. This can be exacerbated by turbulence. As a general rule of thumb, we'll accept a climb to an altitude 2000 above optimum altitude. This provides adequate maneuvering margins for turbulence and turns. In our case, with an optimum altitude showing of FL355 and reports of convective weather ahead, we decided that descending would be the best course of action, knowing that once we burned more fuel, we could make FL380 safely. FL380 was probably fine at that point, but why push it? ATC cleared us down to FL310.

Well, about 30 minutes later, fat dumb and happy in non-radar airspace, we were satisfied that we could safely make FL380. We called back to see if FL380 was still available, which it was not.

From ATC's point of view, were we lying the first time?

 

[derg]

I use the old "mad dog" rule. Optimum is "max". "max" is out of the envelope. If the requested is above the opt, it's not worth it.


Sent from my TRS-80

 

[greg1016]

Before I go, I'll bring up a situation from my last trip:

We were cruising at FL360 approaching LEXAD, which is the first fix on L453 southbound from JFK in oceanic airspace. The oceanic controller had a number of European-bound aircraft transiting the airspace eastbound, so our requested level of FL360 was unavailable. He asked us if we were able to climb to FL380. Now, we have a display on our flight management computer called "optimum altitude," which looks at fuel burn based on weight. More importantly, climbing significantly above optimum altitude can put the aircraft into a situation where low-speed stall buffet and high-speed mach buffet are close together. This can be exacerbated by turbulence. As a general rule of thumb, we'll accept a climb to an altitude 2000 above optimum altitude. This provides adequate maneuvering margins for turbulence and turns. In our case, with an optimum altitude showing of FL355 and reports of convective weather ahead, we decided that descending would be the best course of action, knowing that once we burned more fuel, we could make FL380 safely. FL380 was probably fine at that point, but why push it? ATC cleared us down to FL310.

Well, about 30 minutes later, fat dumb and happy in non-radar airspace, we were satisfied that we could safely make FL380. We called back to see if FL380 was still available, which it was not.

From ATC's point of view, were we lying the first time?

I completely understand 30 minutes later, you burned off some fuel and can go higher, you could also respond able f3XX in XX minutes/hours. With that kind of information, I can usually make everyone happy. It is the immediate unable, followed by able after being cleared to descend (where the aircraft obviously doesn't want to descend) that really bothers me. Maybe we weren't on the same page there.

Also FYI, LEXAD is a radar fix, L453 is in radar coverage all the way to PAEPR southbound. That section of airway is a little different than most radar areas because it is bounded by W-72 to the west and non-radar to the east, which makes vectoring for better altitudes less of an option. If you were on L454 you would be in non-radar at OKONU (more likely ROLLE since OKONU is used for northbound traffic). Your traffic was probably on M204 to SOORY, but could have been on one of the other M routes on the way to canadian_atc land.
For planning purposes, the WATRS oceanic area generally only accepts odd number flight levels southbound and even flight levels northbound on the L routes. Back ashwards right? Well those airways actually have northwest to southeast courses (something like 179-359).

 

[greg1016]

and for the record, I don't recall ever having a DAL, UAL (COA), AWE, or AAL ever complain to me about being stuck at an altitude that they aren't requesting. Utmost professionals.

 

[TripSix]

Before I go, I'll bring up a situation from my last trip:

We were cruising at FL360 approaching LEXAD, which is the first fix on L453 southbound from JFK in oceanic airspace. The oceanic controller had a number of European-bound aircraft transiting the airspace eastbound, so our requested level of FL360 was unavailable. He asked us if we were able to climb to FL380. Now, we have a display on our flight management computer called "optimum altitude," which looks at fuel burn based on weight. More importantly, climbing significantly above optimum altitude can put the aircraft into a situation where low-speed stall buffet and high-speed mach buffet are close together. This can be exacerbated by turbulence. As a general rule of thumb, we'll accept a climb to an altitude 2000 above optimum altitude. This provides adequate maneuvering margins for turbulence and turns. In our case, with an optimum altitude showing of FL355 and reports of convective weather ahead, we decided that descending would be the best course of action, knowing that once we burned more fuel, we could make FL380 safely. FL380 was probably fine at that point, but why push it? ATC cleared us down to FL310.

Well, about 30 minutes later, fat dumb and happy in non-radar airspace, we were satisfied that we could safely make FL380. We called back to see if FL380 was still available, which it was not.

From ATC's point of view, were we lying the first time?

From this ATC's point of view, no - you (the crew) were not. Nor was any crew in any example I've seen in this thread.

I can't count the number of times I've made a call - be it who was going to be high/low in a crossing, who was getting which runway, how I was going to split a tie, etc etc etc, and as it played out, for reasons that are too numerous to list, it changed. With that, comes a new set of clearances for the adjustments. Was the first one a lie?

When I assign you a runway, and 45 miles from the airport you get a "change the runway, expect yada yada yada," did I lie to you the first time? I would really hope you (or any flight crew) would not view it as such.

Like you, there are numerous factors taken into consideration when I make a decision. Those variables are ever changing - for both of us. When they change, I view it as just that. Adjust accordingly and move on. That's pretty much my job description in a nutshell...

 

[dasleben]

I use the old "mad dog" rule. Optimum is "max". "max" is out of the envelope. If the requested is above the opt, it's not worth it.


Sent from my TRS-80

Yeah, after doing some more reading during this thread about optimum altitude, I can see why. What concerns me are the guys who don't give any thought to buffet margin, and think the best way around weather is over it.

 

[derg]

Yeah, after doing some more reading during this thread about optimum altitude, I can see why. What concerns me are the guys who don't give any thought to buffet margin, and think the best way around weather is over it.

Ever after that CRJ law darted into that neighborhood in Missouri (??) I'm dismayed more people don't pay attention to it.


Sent from my TRS-80

 

[Polar742]

So the OP was working a heavy, possibly a B744.

In that jet, we like to fly optimum, as it gives you ~1.5g buffet margin, plus the best fuel burn based on weight and winds based on the cruise speed. Changing the speed will affect the Optimum altitude causing it to go up or down.

If we climb to our "max" altitude, we are down to 1.2 or 1.3g buffet depending on the operator's specification. So now, we are over burning plus lose maneuvering margin. Not a good place to be if there is a chance of a rough ride.

According to Boeing, each 1000' deviation from optimum is approximately 1% fuel burn penalty. Not such a big deal when a jet is fat on fuel. However, if you're trapped 4000' low, (we do our fuel math in kgs) I'm on a flight burning 100kgs with steps 6 and 9 hours downline, now I'm 8 tonnes short. When I have reserve gas of 9.5 tonnes, things get busy in contingency mode.

That being said, the FMC will generate it's time estimation of when we can step, so we can maybe work something up for :30 to :60 mins downline.

So myself, and most guys take all these factors in account when we accept an altitude or not. There are times where the airplane thinks it's capable but due to other factors frm the knowledge, experience and judgment of the pilots, we don't want the climb. Also, that same knowledge, experience and judgment may take the initially declined climb if the requested descent will have a way more adverse impact on the aircraft and flight. The FMC looks up, but we have to trick it into giving us contingency numbers at lower altitudes.


$5 of my $.02

 

[Screaming_Emu]

I use the old "mad dog" rule. Optimum is "max". "max" is out of the envelope. If the requested is above the opt, it's not worth it.


Sent from my TRS-80

That's a good idea. Most aircraft performance data is assuming a brand new airplane and everything working the way it should. Not ones where you need significant left aileron trim and right rudder trim because somebody bent it.

 

[FM_Weasel]

From ATC's point of view, were we lying the first time?

I see what you're saying but you're talking about 30 minutes worth of flying time before the scenario changed. I think it's understandable that situations can change in half an hour, but saying "unable" doesn't communicate that. You weren't really "unable" at the start, you were unwilling. Unwilling for justified reasons, sure.

What's wrong with saying "FL380 is bad for us, can we take lower for now and advise when we can accept FL380?

I'm out of my element here, I don't do any high altitude work, it just seems like an area where communication could be more clear, perhaps on both sides. And I'm aware of how frequency congestion can preclude lengthy explanations. I just think we could find something better than "unable."

 

[dasleben]

I'm out of my element here, I don't do any high altitude work, it just seems like an area where communication could be more clear, perhaps on both sides. And I'm aware of how frequency congestion can preclude lengthy explanations. I just think we could find something better than "unable."

I agree, it seems like both sides aren't 100% sure what the other wants or expects. I generally say something like "Unable, we're still a little heavy," which is meant to communicate that we'd be looking for a step climb later on down the road. It's not correct phraseology of course, and not something that may be communicated properly where the controller isn't a native English speaker.

 

[Autothrust Blue]

Ever after that CRJ law darted into that neighborhood in Missouri (??) I'm dismayed more people don't pay attention to it.


Sent from my TRS-80

When I was a shiny jet jock, I would glance at the buffet boundary charts...I mostly got odd looks from the other seat, but at least I looked. (The ERJ has a pretty wide operating envelope even up towards the top.)

Not dying is near the top of my list of things to do at work.

 

[Roger Roger]

Not dying is near the top of my list of things to do at work.

Quoted for truth.