How to choose cruise altitude

[salty]

So, I'm planning my first long cross country and trying to weigh the options, go up as high as feasible, or stay down to shoot for highest speed or efficiency. For this 500 miles trip I'd only save maybe 4 or 5 gallons or 15 - 20 minutes tops, so my inclination is to just go up high for safety. 

Looking for philosophy you guys use and I'm sure the obvious things I'm missing. 

[peevee]

Might help to mention what you consider high and what you consider low.

 

my goal is to get as high as I can without getting killed by the head winds.

[salty]

So I'm a chicken in a C without oxygen, so probably 12k is as high as I'll go on this one, but I'll entertain advice on that even. 

Edited March 23, 2017 by salty

[1964-M20E]

OK you have a C model assuming naturally aspirated (NA) i.e. not turbocharged.  Also I'm assuming you are a flat lander i.e. or low mountains 6k feet or less.

Some will use O2 above 7k or 8k.

On a 500nm trip I generally go to 10k or 11k unless the winds are really not in my favor there.  This gives me cooler temperatures in the SE especially in the summer time, better efficiency, greater safety margin and I find fewer people are there.  Most SEL will fly 5k to 8k or below 3k.  Just my unscientific observations.

Favorable winds to me is the biggest factor in choosing altitude as well as ride quality I find above 4k to 5k AGL I generally get a pretty smooth ride and will stay higher with more head wind if it is smoother.

 

 

 

 

[Robert C.]

YMMV, but my decision making goes along the following lines:

1) Fltpln.com enter the route, check what ATC is likely to give me,check the winds aloft, time and fuel used calcs for the most efficient altitude.

now that I know the most efficient altitude:

2) Do I already know ATC won't give it to me? (try KSWF to KPNE at 8000ft...they'll laugh and give you 4000ft no matter how long you plead), then what altitude is the next best? Or if I might get it then what other factors play a role?

3) Safety over terrain: higher is better over rocks and water

4) Turbulence; if you know from experience that with a little wind there are always low level bumps I may go higher just to be more comfortable.

5) Aircraft performance: my Ovation is happiest sounding and feeling between 8000 and 11000ft.

Throw it all in the mix and something will tip the scales

[Bob_Belville]

Greg(?) if you're in Florida there is probably no terrain within 500 nm (GA/NC mountains excepted) to factor in to your decision. Your NA Mooney probably like 8-9000' best. W/O O2, I would not consider staying below 12000 as being "chicken" but everyone is different. If you get an oximeter (~$35) you can fly with more confidence that you're handling the altitude for 2 or 3 hours.

I have 2 oximeters and a SkyOx setup in my M20E which I use even at 8000  if it's for 3-4 hours. But I'm really old.  

  

[TTaylor]

I like to have 4 to 5 K of altitude agl for time to deal with problems.  I run the flight through one of the flight planners with several altitudes and pick the ones that gives me a good balance of safety and economy.  I will fly lower if there is a strong headwind and going higher will be a significant longer flight.  I don't like to fly much over 10,500 without oxygen for long periods because we find that everyone will get headaches after the flight. In the west I will do stair step flight planning to cross mountain ranges and drop back down again.  

[Jsavage3]

I live in south-central Ohio where ATC isn't much of a factor, i.e. I file for and almost always get direct.  I also like Fltplan.com and find it helpful in determining the altitude that causes the quickest enroute time.  Ride quality is usually more important to me (especially my wife!) though over winds, unless the winds are just crazy strong headwinds.  Since I always file IFR everywhere I go, you will typically find my 252 at 11,000 eastbound and 12,000 feet westbound.  Why there?  Because there I can plug in a single O2 mask and pass it around every 15 minutes or so, just to keep the headaches away, all the while keeping the pulse-Ox device connected to my finger.  If the winds are going to give me a 20+ knot push, then it's worth while, in my opinion, to strap on the rest of the O2 masks and go up.  I've been as high as 21,000 so far and the bird liked it...192 KTAS on 13 GPH.  I'm not real excited about wearing an O2 mask for hour after hour, so staying at 11/12K works for me in the 252.

When I had the J, we typically cruised at 8000 westbound & 9,000 eastbound...again, always IFR.  Did take her up to 13,000 once, but she was pretty tired up there with MP and KTAS dropping rather dramatically.  We had a portable O2 bottle in the J, but really only used it if we went above 9K.  I found her to be happiest in the 8-9K range.

Edited April 10, 2017 by Jsavage3
Spelling

[salty]

 

22 minutes ago, Bob_Belville said:

Greg(?) if you're in Florida there is probably no terrain within 500 nm (GA/NC mountains excepted) to factor in to your decision. Your NA Mooney probably like 8-9000' best. W/O O2, I would not consider staying below 12000 as being "chicken" but everyone is different. If you get an oximeter (~$35) you can fly with more confidence that you're handling the altitude for 2 or 3 hours.

I have 2 oximeters and a SkyOx setup in my M20E which I use even at 8000  if it's for 3-4 hours. But I'm really old.  

  

Yes, It's Greg.

Flying into low mountain area in NC, 1300 ft highest point, destination airport is 700 feet, with terrain rising around it. I'm nervous about that also as I haven't flown outside Florida yet, but I'm very aware of the terrain issue. 

I have an oximeter. 

[Shadrach]

I fly where the winds are most favorable. I know my engine well, monitor it carefully and feel comfortable cruising as low as 2000agl during the day unless the terrain is consistently hostile in the event of an off field landing. Winds tend to be lighter at night. I typically like to be 4000agl or higher after dark.  I am comfortable cruising 12-13,000msl but I do check my O2 sats in route. Though I am currently a flat lander, years of intense backpacking in the 10-13k range has indicated that I'm not prone to hypoxia at those altitudes. However, I don't count on that always being the case so I monitor to be sure. I run Max power LOP down low into the wind and peak EGT up high with favorable winds. My lowly stock F has enjoyed ground speeds well in excess of 220kts on very modest fuel burns because both pilot and aircraft are happy to climb above 8000.

[SantosDumont]

I usually cruise at 11.5/12.5. 

[Godfather]

With a NA engine I'll drop down to 3-4K fighting a headwind but may come back at 14k to catch a tailwind. Very versatile to have a plane that can do both imo. 

[Jeff_S]

Any normally aspirated airplane will do best in the 8K to 11K range simply because that's where your engine will still develop a good amount of HP for cruise power, while also benefitting from reduced drag of thinner air. You can go faster down low but burn more gas, or speed will taper off as you climb higher but you'll burn less gas and get overall better mileage (assuming you lean correctly and no wind factor).

So really, as has been noted by those above, a lot of your decision will come down to winds, and where in that range you'll get the overall best bang for your gasoline buck. (There is a pun not so cleverly hidden there!) My default altitude in ForeFlight is 8K feet, and then I'll use the wind planner to see if I want to modify that based on prevailing winds (and direction of course, even vs odd thousands).

[Bob_Belville]

8 hours ago, salty said:

 

Yes, It's Greg.

Flying into low mountain area in NC, 1300 ft highest point, destination airport is 700 feet, with terrain rising around it. I'm nervous about that also as I haven't flown outside Florida yet, but I'm very aware of the terrain issue. 

I have an oximeter. 

Greg, my home drome, KMRN, is 1270' with higher terrain close by, under the approaches to both 3 and 21. Morganton, Hickory, etc. are the foothills. If we go 25 miles to the west we're dealing with 6000'+ terrain, Mt. Mitchell at 6684' is the highest peak east of the Rockies and there are several other peaks nearly as tall nearby.

You will have no trouble flying into the airports in this area, if you get close to MRN please drop by, we love to see MSers.  

[salty]

I'm flying into hbi, but this trip is going to be a turn and burn, if weather permits, I'm hoping to get back home same day. 

[Mooneymite]

Altitude for IFR flight.

Avoid:  Terrain, Ice, turbulence, head winds in that order.

Optimize:  groundspeed, fuel burn, passenger comfort.

Consider:  sightseeing and gliding distance in case of engine failure.

For VFR, cloud avoidance becomes an over-riding consideration.

Edited March 23, 2017 by Mooneymite

[N201MKTurbo]

Sometimes it's nice to fly along at 1000 AGL just to see the world.

[Bob_Belville]

28 minutes ago, salty said:

I'm flying into hbi, but this trip is going to be a turn and burn, if weather permits, I'm hoping to get back home same day. 

Asheboro is a pretty easy airport. Low hills to the east and south of 3-21.

[peevee]

24 minutes ago, N201MKTurbo said:

Sometimes it's nice to fly along at 1000 AGL just to see the world.

it's kind of interesting reading the thread I notice how much I have changed changed how I fly with the 2 mooneys I've had.

Low for me is 11 or 12k and that's only if it's to duck the wind. normal is 15,5 and 16,5 but that's primarily because of the terrain west of me, High is in the flight levels, where I can only justify it if I'm on a 2 or 3 hour leg.

[N201MKTurbo]

Just now, peevee said:

it's kind of interesting reading the thread I notice how much I have changed changed how I fly with the 2 mooneys I've had.

Low for me is 11 or 12k and that's only if it's to duck the wind. normal is 15,5 and 16,5 but that's primarily because of the terrain west of me, High is in the flight levels, where I can only justify it if I'm on a 2 or 3 hour leg.

The curse of the turbo! You never get to enjoy the scenery any more...

[peevee]

4 minutes ago, N201MKTurbo said:

The curse of the turbo! You never get to enjoy the scenery any more...

over the terrain I fly, I like the view from 170 and higher JUST FINE!

I also like to know that if the big fan quits I can glide about 2 miles per 1,000 feet once it feathers.

 

Of course I'll probably curse that turbo some day if a V band clamp lets go and she flame cuts through my cowl and starts a fire.

[jlunseth]

I have a turbo.  Depending on winds I will fly anywhere from 2,500 to 22,000 with the rare excursion to my ceiling, which is 24k.  I use Foreflight to make the decision.  It is not ideal but it works ok.  Once you have entered a flight plan, go into Maps and make sure the flight plan is displayed in the flight plan screen at the top.  Tap Altitude and it will give you the winds aloft at all the altitudes that are practical for that flight.  I can then compare the winds aloft.  I will then pick the altitude that looks best.  Foreflight does not have an automatic means of accounting for the speed increase by going to a given altitude, so once I have picked the altitude it is necessary to go in and enter my estimate of what the TAS will be at that speed.  There are many days when the headwinds increase by about the same (or more) than the increase in TAS with altitude.  On those days I will pick a low altitude, preferably high enough to get out of the low level turbulence generated by diffential warming.  That means something over 2500 to 3500.  Even if the TAS I wind up with is 145 Kts., with a 20 kt. headwind, the GS of 125 will be better than going to say 18k, getting a TAS of 165 but a headwind of 50.  It is not worth the climb and I will only lose GS.  There are also days when the tailwind does nothing but go up with altitude, and on those days, depending on weather, I will go as high as the trip length allows.  Lots of days in the middle too.  I know you are an NA aircraft, but the same method applies to NA.

You can also sign up for an account with www.fltplan.com and put in a custom profile for you aircraft.  The custom profile allows fltplan to allow for the increase in TAS with increases in altitude, and also to allow for the time to climb at climb speed.  However, it only displays a few alternative altitudes at a time, you put in an intended altitude, say 10,000 and it will give you a few altitudes to either side of that (higher or lower).  If you want to see a higher slot you have to manually change your flight plan information to insert a different altitude.  I prefer the Foreflight method.  Weather aside, I would not worry about flying at anything under 12k without O2. 

If things work out that the GS is about the same at many altitudes, then I usually choose the highest altitude that I can fly, under 18k.   At 18k I have to put on a mask as opposed to a cannula, which is a pain with my glasses and I can't conveniently eat or drink.

[Piloto]

On my trips to KAGC from KFXE I check on my Garmin Pilot for ETE based on existing winds conditions and altitude. For today it shows that 4,000 feet would be the shortest time. The ETE computation takes into consideration time to climb to altitude. I found it pretty accurate once you program your plane performance accurately. Of course there are other factors in altitude selection like weather, turbulence, icing and obstacle clearance

José 

Edited March 23, 2017 by Piloto

[salty]

I have Garmin pilot all set up with my planes performance stats, but as I mentioned, it's only a few gallons or minutes difference anyway, so I was thinking the altitude would be better than saving 15 minutes on a 3.5 hr trip. 

[Mooneymite]

6 minutes ago, salty said:

I have Garmin pilot all set up with my planes performance stats, but as I mentioned, it's only a few gallons or minutes difference anyway, so I was thinking the altitude would be better than saving 15 minutes on a 3.5 hr trip. 

Heresy!

To a true Mooniac saving time/fuel/money is sacrosanct.  There's no such thing as a "few" gallons, or minutes.  

Battered brains, rough ride.....yeah, but save 30 seconds!  

Edited March 23, 2017 by Mooneymite