Does cruise altitude matter?

For a non-turbocharged piston airplane, does cruise altitude really
matter? I made a complicated spreadsheet which produces a fairly simple
answer: Unless you climb way above your critical altitude, trip time
and fuel burn don't vary much at all. Over a 500nm trip, it only takes
about 4% longer to fly at 3000MSL instead of 7000 (approximately critical
altitude) and 7% longer if you go clear up to 13000. For my plane that's
a difference of about 7-15 minutes out of 3+ hours.

How high does your critical altitude have to be (due to turbocharging
or jet/turbine) before it really starts to matter what your cruising
altitude is?

--
Ben Jackson

http://www.ben.com/

Ben Jackson wrote:

For a non-turbocharged piston airplane, does cruise altitude really
matter?

I have never seen a case in which a spreadsheet such as yours would have been useful.
My cruise altitude has invariably been selected by considering airspace restrictions,
winds aloft at different altitudes, or turbulence. Nothing else. If my wife is in the
plane, the factor that comes into play most often is turbulence; when she's not with
me, it's wind.

George Patterson
If you want to know God's opinion of money, just look at the people
he gives it to.

"Ben Jackson" wrote in message
news:fmLZc.105358$Fg5.1951@attbi_s53...
For a non-turbocharged piston airplane, does cruise altitude really
matter?

Well, according to the numbers you've provided, it makes between a 4% and 7%
difference. Whether that "matters" is up to each pilot, of course.

As George says, there's more to picking a cruise altitude than airspeed. At
the very least, groundspeed is an issue, and total trip time will be
affected by routing, which may be affected by cruise altitude.

Airspeed is not a very common priority for non-turbocharged airplanes with
respect to cruise altitude.

[...]
How high does your critical altitude have to be (due to turbocharging
or jet/turbine) before it really starts to matter what your cruising
altitude is?

The right question to ask is "how long does a trip need to be?" Assuming no
wind, you will always go faster climbing higher in a turbocharged airplane,
until reaching your critical altitude. The only question is how long you
need to fly at that faster speed to make up the time spent climbing. And
that depends on the airplane, and the loading for that day (factors that
affect the time to climb).

But again, there are other issues, and especially in a turbocharged
aircraft, it's unusual to find a no wind situation when you get to the
altitudes where the turbo is really helping.

In my turbocharged airplane, I fly as high as seems practical for the length
of the trip. Local flights, often only 30-45 minutes long, I rarely get
above 2000' AGL. For a long cross-country of 2-4 hours in length (I try to
avoid longer flights, as my bladder protests too much ), I will climb to
anywhere between 10,000' and 16,000', depending on the length of the trip,
winds aloft, whether I have to spend a portion of the trip underneath Class
B airspace (limiting my climb), and a number of other factors.

Pete

Ben Jackson wrote:

For a non-turbocharged piston airplane, does cruise altitude really
matter? I made a complicated spreadsheet which produces a fairly simple
answer: Unless you climb way above your critical altitude, trip time
and fuel burn don't vary much at all. Over a 500nm trip, it only takes
about 4% longer to fly at 3000MSL instead of 7000 (approximately critical
altitude) and 7% longer if you go clear up to 13000. For my plane that's
a difference of about 7-15 minutes out of 3+ hours.

Depends on what the wind is. If I can gain a better tailwind I climb.

For a non-turbocharged piston airplane, does cruise altitude really
matter? I made a c

For what it's worth, I've had to go (had to? Like that was a bad thing!)
Boston to Chicago a goodly number of times in a Mooney 201. Westbound almost
always it was at one of the lower available altitudes, and ALWAYS involved a
fuel stop in northcentral or northwesteern PA.

Eastbound, it was almost always at 11,000 feet, and nearly always it was
nonstop, with me landing with at least one tank half full (my personal
if-you-land-with-less-gas-than-that-you-screwed-up) minimum reserve.

It had everything to do with winds aloft..

In a no-wind situation... you are essentially trading time for fuel..
your max range versus power settings wont have a very significant
difference.. say.. 10% from the high value to the low value(this is
based on my experience in normally aspirated piston single engine 4 seat
aircraft)

Even turbocharged, the equation still applies.. time for fuel.. I can
make a big dash up high to say.. 25K feet.. and up there 200 INDICATED
gives me 300 TRUE on a standard day.. but you still have to burn pretty
much the same amount of fuel to maintain that "indicated" airspeed
(again.. 5-10% fudge factor).

So.. In the Velocity my friend is building, that we are gonna turbo.. we
can make a high power climb up high to that hypothetical 25,000 ft.. and
maintain that 200 mph indicated for 300 true.. but we will go through
our 60 gallons of fuel in a little under 4 hours.. and go about 950 SM..

We can fly lower/slower at a lower fuel burn and power setting.. and go
the same 900 or so miles, but take 5-6 hours to get there.

One benefit someone else mentioned is.. get up higher and get a
favorable tailwind.. and that will pay you sweet dividends on your time
and groundspeed.

Dave

(by the way.. if someone tries to replicate my fuel burn numbers.. this
was hypothetical.. I found an error in the power calcs we were making at
altitude.. and I think the fuel burn is actually low.. have to revisit
it.. I KNOW they have an error.. I used em to illustrate the concept)

Ben Jackson wrote:
For a non-turbocharged piston airplane, does cruise altitude really
matter? I made a complicated spreadsheet which produces a fairly simple
answer: Unless you climb way above your critical altitude, trip time
and fuel burn don't vary much at all. Over a 500nm trip, it only takes
about 4% longer to fly at 3000MSL instead of 7000 (approximately critical
altitude) and 7% longer if you go clear up to 13000. For my plane that's
a difference of about 7-15 minutes out of 3+ hours.

How high does your critical altitude have to be (due to turbocharging
or jet/turbine) before it really starts to matter what your cruising
altitude is?

(Ben Jackson) wrote in news:fmLZc.105358$Fg5.1951@attbi_s53:

For a non-turbocharged piston airplane, does cruise altitude really
matter? I made a complicated spreadsheet which produces a fairly simple
answer: Unless you climb way above your critical altitude, trip time
and fuel burn don't vary much at all. Over a 500nm trip, it only takes
about 4% longer to fly at 3000MSL instead of 7000 (approximately critical
altitude) and 7% longer if you go clear up to 13000. For my plane that's
a difference of about 7-15 minutes out of 3+ hours.

How high does your critical altitude have to be (due to turbocharging
or jet/turbine) before it really starts to matter what your cruising
altitude is?


How are you factoring wind into your spreadsheet? I have tried the same on
DUATS and came to the same conclusion as you. The difference is pretty
minimal in most cases, except when there is an inversion layer with a high
speed flow above it. In that case, it makes sense to climb up to catch that
flow (or stay below if you are going against it). I found that in most
cases the difference in enroute time is less than 5 minutes even for trips
as long as 3 hours.

Hi Ben

If you lived where I live, cruise altitude on almost any crosscountry
trip is based on altitude required to clear terrain.

That usually takes care of anything between 7500 and 11500ft.
After that, can we get a decent tailwind?
Can we dodge turbulence/weather?
Controlled airspace we usually fly over, or ask permission and fly
through.

So it all depends what you are calculating - time enroute , see tailwind.
Fuel burn? All I calc is that I have lots. I really don't see any great
monetary saving by selecting anything except tailwind, and I'm landing
in 2 - 3 hours anyway. That's as much as I want to handle in a 172
without a walk/washroom/coffee ion that order

HTH

Tony
--

Tony Roberts
PP-ASEL
VFR OTT
Night
Cessna 172H C-GICE

In article fmLZc.105358$Fg5.1951@attbi_s53, (Ben Jackson)
wrote:

For a non-turbocharged piston airplane, does cruise altitude really
matter? I made a complicated spreadsheet which produces a fairly simple
answer: Unless you climb way above your critical altitude, trip time
and fuel burn don't vary much at all. Over a 500nm trip, it only takes
about 4% longer to fly at 3000MSL instead of 7000 (approximately critical
altitude) and 7% longer if you go clear up to 13000. For my plane that's
a difference of about 7-15 minutes out of 3+ hours.

How high does your critical altitude have to be (due to turbocharging
or jet/turbine) before it really starts to matter what your cruising
altitude is?




--

Tony Roberts
PP-ASEL
VFR OTT
Night
Cessna 172H C-GICE

In article ,
Andrew Sarangan wrote:

(Ben Jackson) wrote in news:fmLZc.105358$Fg5.1951@attbi_s53:

For a non-turbocharged piston airplane, does cruise altitude really
matter? I made a complicated spreadsheet which produces a fairly simple
answer: Unless you climb way above your critical altitude, trip time
and fuel burn don't vary much at all. Over a 500nm trip, it only takes
about 4% longer to fly at 3000MSL instead of 7000 (approximately critical
altitude) and 7% longer if you go clear up to 13000. For my plane that's
a difference of about 7-15 minutes out of 3+ hours.

How high does your critical altitude have to be (due to turbocharging
or jet/turbine) before it really starts to matter what your cruising
altitude is?


How are you factoring wind into your spreadsheet? I have tried the same on
DUATS and came to the same conclusion as you. The difference is pretty
minimal in most cases, except when there is an inversion layer with a high
speed flow above it. In that case, it makes sense to climb up to catch that
flow (or stay below if you are going against it). I found that in most
cases the difference in enroute time is less than 5 minutes even for trips
as long as 3 hours.

If you fly east of the Rockies, low altitude can be pretty damned HOT!
If you climb above 5000 ft in the summertime, you can get some natural
"air conditioning" that you, your pax and the engine like.

tony roberts wrote in news:nospam-271FBE.19453002092004
@shawnews:

Hi Ben

If you lived where I live, cruise altitude on almost any crosscountry
trip is based on altitude required to clear terrain.

That usually takes care of anything between 7500 and 11500ft.
After that, can we get a decent tailwind?
Can we dodge turbulence/weather?
Controlled airspace we usually fly over, or ask permission and fly
through.

So it all depends what you are calculating - time enroute , see tailwind.
Fuel burn? All I calc is that I have lots. I really don't see any great
monetary saving by selecting anything except tailwind, and I'm landing
in 2 - 3 hours anyway. That's as much as I want to handle in a 172
without a walk/washroom/coffee ion that order

HTH

Tony


Around here (Ohio), pattern altitude will clear pretty much any obstacle
for many miles.