Determining Turbofan Cruise Altitudes

[C150J]

Hi guys,

We're pampered wayyyy too much at the airlines. So, what type of tables/software do you guys use to determine cruise altitudes?

Thanks!
J.

[minitour]

Well...........it depends. I'm a "wind hunter" so I only use the "10% thing" as a guideline.

10% Thing...
150 miles - 15,000'
200 miles - FL200

etc.

Once you get into long enough trips to end up between FL410-FL450 I look at winds. If I can cut 5-6 minutes off and not take a huge hit on fuel by flying at a different altitude, I will. Just depends on what the winds are doing. That's for my itty bitty light jet.

-mini

[SteveC]

Quick and dirty is 1,000 feet per 10 miles of trip distance.

150 miles, 15,000 feet;
300 miles, FL300;
450 miles, FL450.

When it comes time for filing I use fltplan.com and play with the altitude based on forecast winds. The website gives pretty accurate estimates of time enroute and corresponding fuel burns.

[SteveC]

Jinx.

[minitour]

Hah!

-mini

[C150J]

Originally Posted by SteveC

Quick and dirty is 1,000 feet per 10 miles of trip distance.

Thanks to both of you! Any rhyme or reason to the 10%/1,000' per 10NM? Just trying to gain a better technical understanding.

How different are your high altitude climb-thrust fuel flows from your cruise thrust settings? In the CRJ-200, I'm usually seeing 1,500-1,700PPH climbing in the teens/20s and cruise settings around 1,300-1,400PPH at those altitudes. Does that disparity (about 200-300PPH) exist in light jets?

Maybe I'm just thinking too much. There are so many variables (ISA, winds, TAS, traffic, etc.) that the "quick and dirty" formula seems to be a relief!


Thanks again!

[SteveC]

Originally Posted by minitour

Hah!

-mini

< arm slug >

[Orange Anchor]

I use fltplan.com. I took fuel burn readings at each altitude on the way to 35,000 which is below ceiling but we never get there. fltplan.com gives me alternate altitudes with burn and time enroute. We then choose best altitude on paper but modify due to turb or wx.

And you can run what-ifs. As we have refined the actual burns at altitude, our flt plans have become more accurate and we are usually within 100-200lbs. We are talking Citation here.

In Orlando this week, Air Routing Internationall unveiled their new flt planning tools. It does EVERYTHING from route planning as well as ALL costs including 3rd party charges when international.

sophisticated flt planning is becoming easily accessible to everyone and anyone just using the basics is WAY behind the curve.

[C150J]

Originally Posted by Orange Anchor

sophisticated flt planning is becoming easily accessible to everyone and anyone just using the basics is WAY behind the curve.

Cool! What are the fundamental metrics that this type of software uses? I'm assuming ISA, winds, and user-defined burns? Am I missing anything major?

Is there a quick-and-dirty FF calculation, like 2%/1,000 feet for TAS? I guess I could record what the burns are for a 250KIAS climb using the same thrust throughout my climbs tomorrow.

[Orange Anchor]

Originally Posted by C150J

Cool! What are the fundamental metrics that this type of software uses? I'm assuming ISA, winds, and user-defined burns? Am I missing anything major?

Is there a quick-and-dirty FF calculation, like 2%/1,000 feet for TAS? I guess I could record what the burns are for a 250KIAS climb using the same thrust throughout my climbs tomorrow.

The Citation uses the JT15D engine which is sort of like the PT6.. it's been around a LONG time. If you go to fltplan.com, you can put in your aircraft with a number of different options. You can load fuel burn/hr and for ours it is about 1200lbs first hour, 1100lbs second hour and about 950lbs for each hour after. We have about 4hrs fuel on board.

Another section allows you to enter at 1000ft increments, climb speed, rate of climb, cruise speed, cruise fuel burn and descent speed. So i just took the template and filled in the numbers using maybe 5-10 flights to average out the numbers.

Once the info is entered, you enter your flt.. for example ATL-ORL. The program spits out your chosen alt enroute plus 4 other altitudes on one page. It gives you about 8 altitudes on another page. The info includes winds, ISA, and headwinds/tailwinds. Oh, and the program allows you to enter your desired routing, gives you the preferred routing and shows you routing others have used. You choose.

In our example, the filed altitude is 350 with data for that along with 390, 330 and 310 as well as the others on another page. Fuel burn is 1570 using entered data and 1767 for hourly burn computations.

There are lots of other functions such as the ability to send faxes to the FBOs with any info you want to send; time of arrival, special catering needs, GPU, etc.

And for pilots here is the selling point... IT IS FREE! for the pilots. The FBOs and others pay to be on the page but for the pilot, it is FREE. I use it for the jet and for my Swift.

Also, you can download all the charts for a specific airport.

Skyvector.com is another site where you can get lots of info but not flight planning.

AOPA has its flt planning software on line also as does the FAA at www.faa.gov/pilots/flt_plan/

And for the flt departments there are lots of fee-based outfits. It is amazing that we used to crank out our flt plans for the KC-135 using the spider graphs and an E6B....

[minitour]

Originally Posted by C150J

Thanks to both of you! Any rhyme or reason to the 10%/1,000' per 10NM? Just trying to gain a better technical understanding.

Not really. It just kinda works. The T.L.A.R. method.

-mini

[Orange Anchor]

Originally Posted by SteveC

Quick and dirty is 1,000 feet per 10 miles of trip distance.

150 miles, 15,000 feet;
300 miles, FL300;
450 miles, FL450.

In the 20 series Lears, we did double the distance. 100 miles..FL200. Anything close to 200 miles was FL410. We often went from PDK to ACJ when we were flyin' Jimmah. about 120nm. We almost got to FL410 once before beginning descent. The -23 was a FUN machine. (okay.. so 120nm isn't close to 200nm but close enough when empty)

[TFaudree_ERAU]

Um...did anyone mention the weight to altitude charts?

[400A]

Originally Posted by TFaudree_ERAU

Um...did anyone mention the weight to altitude charts?

You mean use the POH/AFM! Surely you jest!

[C150J]

Originally Posted by 400A

You mean use the POH/AFM! Surely you jest!

Haha! I'm more interested in factors outside simple weight/ISA/altitude/mach capability charts, but the AFM is certainly a starting point that shouldn't be ignored.

[400A]

There are many factors that can influence your cruise altitude, besides optimum performance via the POH.

What kind of cabin altitude do you need to maintain?
Is time more important than burning more gas?
Turbulence?
Headwind/Tailwind?
WX (icing, CBs, etc)?

Most of those questions result in changes of en route time by minutes, but can vary fuel burned by hundreds of pounds and be the difference between non stop and a tech stop. Which is why I had my tounge in cheek comment, the POH/AFM will, in most cases, give you the fastest/most economical way to operate the aircraft, despite the "rules of thumb" people may have.

[ricecakecm]

I typically do the 1000 ft/10 NM rule +4000 or 5000 feet in the jets and King Air 350. So 100 NM trip = 15,000 ft cruise.