Determining X/C Altitude

A

Apache123

Line Up and Wait
Joined
Aug 3, 2011
Messages
546
Location
Lake Forest, IL
Display Name
Display name:
Hey, Steve!
My "getting the book numbers" thread has made me think about flying higher in the Apache where 125kts indicated should get me close to that 170mph TAS at about 11.5k'. Most all of my cross countries are less than 100 miles though, and a good portion are typically under the Chicago veil.

Without regard for the wind, how do you guys generally choose how high to climb (in a normally aspirated)? If winds are a wash, at about 1200fpm it doesn't seem like it would do me much good to climb to 11.5 (from about 600'msl) on a 100nm trip.

Obviously on a 500nm trip with 100kts tailwind at 14.5k I'd be bringing the oxygen and doing my best to get up there asap. I'm just wondering at what point it would be worth it versus the slower true but less time in a climb/descent when it's less clear of a decision.
 
I usually go 7,500/6,500 on a 100 NM trip, and up to 9,500/8,500 on longer trips, winds notwithstanding in the Arrow II.
 
Also factor in fuel consumption, you'll need to calculate a few different scenarios and weigh the options of time en route, fuel burn, getting around Chicago veil (in your scenario,) etc.
 
Precisely -- so I wonder at what point it's worth the climb. Is it when at least 50% of the trip is at altitude? What rules of thumb do you guys tend to use?
 
I figure if I won't spend at least half the flight by time at the cruise altitude, a lower cruise altitude is better.
 
Being naturally aspirated, you might want to verify what the plane really does speed wise. My Aztec does pretty much the same TAS (155 ktas) and fuel burn (21 gph) the whole way from 5,000 ft to 11,000 ft and then drops off, depending on the load. I'd suspect your Apache to be similar. So, I pick based on winds and other weather (ice, etc).

For 100 nm, I won't bother going above 7,000 ft except in rare cases. As the trip gets longer, I'll increase altitude as appropriate.
 
Apache123 said:
Precisely -- so I wonder at what point it's worth the climb. Is it when at least 50% of the trip is at altitude? What rules of thumb do you guys tend to use?
Click to expand...


I would probably put the cut-off closer to 2/3 of the time and distance at altitude. For your scenario, I would also limit it to 7500' eastbound or 6500' westbound
FWIW
 
Apache123 said:
Precisely -- so I wonder at what point it's worth the climb. Is it when at least 50% of the trip is at altitude? What rules of thumb do you guys tend to use?
Click to expand...

The ROT I've read said to use time in thirds - 1/3 for climb, 1/3 for cruise, and 1/3 for descent. Of course for longer trips the service ceiling comes into play. The article also mentioned that a continuous cruise-climb was actually the most efficient but not particularly practical for most folks.

In practice I'll generally go in the low teens just 'cause: 1) I can, 2) less traffic, 3) better performance 4) sometimes have to for terrain clearance. Every now and then I'll go higher to chase favorable winds or avoid weather. Same for lower except try to stay at least 3000 AGL.
 
Last edited:
Apache123 said:
My "getting the book numbers" thread has made me think about flying higher in the Apache where 125kts indicated should get me close to that 170mph TAS at about 11.5k'. Most all of my cross countries are less than 100 miles though, and a good portion are typically under the Chicago veil.

Without regard for the wind, how do you guys generally choose how high to climb (in a normally aspirated)? If winds are a wash, at about 1200fpm it doesn't seem like it would do me much good to climb to 11.5 (from about 600'msl) on a 100nm trip.

Obviously on a 500nm trip with 100kts tailwind at 14.5k I'd be bringing the oxygen and doing my best to get up there asap. I'm just wondering at what point it would be worth it versus the slower true but less time in a climb/descent when it's less clear of a decision.
Click to expand...

What do you mean by 'winds are a wash'?

I think you'll find that climbing higher than 2-3K' AGL for a 100 mile trip isn't worth the trouble [unless you must].
 
MadseasoN said:
What do you mean by 'winds are a wash'?

I think you'll find that climbing higher than 2-3K' AGL for a 100 mile trip isn't worth the trouble [unless you must].
Click to expand...

Winds are a wash -- I was specifically referring to KTAS vs KIAS
 
This is an interesting topic. I fly a 220 mile leg each weekend. I recently cut my altitude from 8500 to 6500. It has improved my commute time and fuel burn. I may make adjustments for head winds from time to time but I found the extra time to get to altitude for this route is not worth it.
 
Apache123 said:
My "getting the book numbers" thread has made me think about flying higher in the Apache where 125kts indicated should get me close to that 170mph TAS at about 11.5k'. Most all of my cross countries are less than 100 miles though, and a good portion are typically under the Chicago veil.

Without regard for the wind, how do you guys generally choose how high to climb (in a normally aspirated)? If winds are a wash, at about 1200fpm it doesn't seem like it would do me much good to climb to 11.5 (from about 600'msl) on a 100nm trip.

Obviously on a 500nm trip with 100kts tailwind at 14.5k I'd be bringing the oxygen and doing my best to get up there asap. I'm just wondering at what point it would be worth it versus the slower true but less time in a climb/descent when it's less clear of a decision.
Click to expand...

Temperature, turbulence, winds aloft, cloud heights... All of it figures into my decission. My best zero wind altitude is the 7500-9500 range where I can maintain 180kts TAS at 21gph well LOP, by 10,500 I'm starting to lose TAS and by 11,500 I need a tailwind to make the numbers work or more fuel flow. 180kts is my traveling speed, 3 miles a minute is good enough for me in the speed/cost trade off, therefore I chose a plane that would do it while loafing along.
 
Apache123 said:
Winds are a wash -- I was specifically referring to KTAS vs KIAS
Click to expand...

Maybe I'm not understanding. Do you mean GS vs KTAS? The main difference between KIAS and KTAS is due to temperature not winds.
 
MadseasoN said:
Maybe I'm not understanding. Do you mean GS vs KTAS? The main difference between KIAS and KTAS is due to temperature not winds.
Click to expand...

He means the difference in winds at different altitudes being discounted from the question, ie, 0 wind through max altitude.
 
MadseasoN said:
What do you mean by 'winds are a wash'?

I think you'll find that climbing higher than 2-3K' AGL for a 100 mile trip isn't worth the trouble [unless you must].
Click to expand...

Depends on the plane. Many planes, especially N/A, get into their efficient range starting around 5k. Depending on your climb rate and the specifics of the plane, you can definitely see a benefit.
 
On shorter trips, 75-100 miles, 35-4500 feet, 100+ it depends on winds, but usually 6-7000 in the 182. In the 421 I usually go 6-7000 on the under 100 mile trips and 9-11000 on 100-200 mile trips, 200+ is usually mid teens, real long trips 400+ upper teens to low 20's.:D
 
ROT on a standard temp day: climb for 10% of your total flight time.
 
3000' AGL is about where I like to be if winds are a wash and it's not 100F outside. Heh.

Around here that means 9000 MSL. So that's usually where I'm at unless there's stuff to climb over.

Habit mostly. Everything is generally downhill from here so 9000 means I'm really high at the other end.

Climbing higher usually takes longer than it'll buy. Usually.
 
How about using one of the programs like FltPlan.com, Flitesoft, etc. to help you determine the best routing, including DPs and STARs, and the times and fuel burns (taking into account the winds aloft)? Then it's a simple matter to select the best altitude. It takes all of the guesswork out of altitude selection. Rules of thumb are handy ways to double check your flightplanning, but they are no substitute for running the honest-to-goodness calculations. Relying on them will get you into trouble sooner or later. There are several programs available ranging in price from free (FltPlan.com) to what you'd pay for one hour of flight time or less (Flitesoft). Once you get your data dialed in, they're bang on and can save you quite a bit of money.
 
Last edited:
My favorite is weathermeister. It will compute the winds and come up with the ideal altitude for either TIME or ECONOMY. Often there slammed all the way to the minimum or the maximum if there's substantial winds,

Of course, as CRon points out, sometimes it's not worth the effort to climb up there even for a few minutes of optimal cruise.
 
There is no hard and fast rules with this. It all depends on the airplane, weather at the time, and how efficient you want to be. I like going as high as I can as it gives me more options, but I am not going to fight a heavy headwind to do so. Look through your charts and see if they match what you are getting. If so then it should be easy to determine on a certain day how the performance with change.

Cheers
 
Forgot to mention, Foreflight will do the comparisons for winds and altitudes now also...
 
As a geek, I'll suggest this:

Do some flight testing... And then make a spreadsheet.

First, climb as high as your airplane can practically do so. Start a timer when you take off. Every 500 (or 1000, you can interpolate) feet, take down the time, fuel flow, IAS, indicated climb rate, and ideally OAT and such as well (for calculating DA).

Next, at your top practical altitude and every 1000 feet on the way back down, take down IAS and fly a full minute on N, S, E, and W headings and take down GPS groundspeed once it stabilizes.

Halfway down each of those 1000-foot steps, take down speed, descent rate, etc.

Then, take all of the above data and create a second spreadsheet where you can put in a trip distance and have it calculate what the time and fuel burn would be at each altitude. You can put in some test cases and develop rules of thumb to make it easier in the future.

I did this once minus the flight testing, and came up with the fastest trip being roughly to climb 1000 feet AGL for every 18nm of length, and the most efficient being 1000 feet per 20nm. That doesn't apply to every type, though.

If you want someone to help with this, I'd love to - I don't think I'm too far from you. And I've been wanting to do it in the Mooney, so maybe we could trade off data-collection and flying duties. :)
 
flyingcheesehead said:
As a geek, I'll suggest this:

Do some flight testing... And then make a spreadsheet.

First, climb as high as your airplane can practically do so. Start a timer when you take off. Every 500 (or 1000, you can interpolate) feet, take down the time, fuel flow, IAS, indicated climb rate, and ideally OAT and such as well (for calculating DA).

Next, at your top practical altitude and every 1000 feet on the way back down, take down IAS and fly a full minute on N, S, E, and W headings and take down GPS groundspeed once it stabilizes.

Halfway down each of those 1000-foot steps, take down speed, descent rate, etc.

Then, take all of the above data and create a second spreadsheet where you can put in a trip distance and have it calculate what the time and fuel burn would be at each altitude. You can put in some test cases and develop rules of thumb to make it easier in the future.

I did this once minus the flight testing, and came up with the fastest trip being roughly to climb 1000 feet AGL for every 18nm of length, and the most efficient being 1000 feet per 20nm. That doesn't apply to every type, though.

If you want someone to help with this, I'd love to - I don't think I'm too far from you. And I've been wanting to do it in the Mooney, so maybe we could trade off data-collection and flying duties. :)
Click to expand...

You don't need to fly a four course ASI calibration test at each altitude, you can just (perhaps on a separate flight) determine the ASI error at two to four indicated speeds in the appropriate range and extrapolate those measurements across the cruise speed range from low to high altitude.

Also it's important to let the speed stabilize for several minutes when collecting performance data at various altitudes, and it's best to let the autopilot fly the plane when doing so to minimize the altitude variations.
 
You must log in or register to reply here.
Back
Top