The International Cessna^® 170 Association

I know this is a dumb question, but I will ask it anyway. Stock '56 170B with 0300.
On a recent trip from So. Cal to Texas, I leveled off at 9,500 and attempted to throttle back to 2200 per the book with normal leaning technique. I was solo and fairly light in weight, probably around 1900 pounds. Howsomever, I could not maintain 9500 at this throttle setting. The 170 would slowly lose altitude. IF one cannot hold altitude at this throttle setting, Why is this data provided in the Performance charts??? I was going for range. Can anyone suggest an efficient throttle setting for max range as I have a coast to coast ferry flight in the near future. Appreciate any suggestions from one and all.
Chuck

The only dumb questions are the ones not asked.
I have to say that while I've found the cruise performance data in the Owner's Manual to be accurate at 2450 (and wide open throttle at higher altitudes), ...I've never actually checked the reduced power settings up there. I will never forget my own experience flying out to Las Vegas, NV to the convention and being at 12,500', heavily loaded over Winslow, AZ carrying full power and watching the vertical airspeed indicate alternately between 100 fpm climb and 1000 fpm descent in thermals. It was pretty disgusting until I calculated my density altitude and found it to be almost 16,000 feet! (Eat your heart out Joe!)

Chuck, I'd first ask you if you are abosolutely certain that your tachometer is accurate. Have you actually had it checked with a digital tach checker?
The next thing I'd ask is if you checked your actual density altitude on the flight in question. How do you know your actual density altitude wasn't somewhere near the service ceiling?
Lastly, did you check your TAS? In other words, were you attempting to hold max available airspeed and altitude?....or did you allow airspeed to reduce to the 84 mph IAS the cruise data shows? (on a std day 97 mph TAS will only indicate about 84.)
Anyway, I hope my response helps lead you to the solution of your problem. Indication errors, and density/pressure altitude errors can be misleading. As for long-range cruise, my personal preference it to carefully check the winds aloft and select the most favorable altitude based upon that info, and then my secondary decision is to select the highest altitude my aircraft will still achieve max TAS with full throttle. (This almost always works out to about 7500' +/- 1K.) This still provides about 500 sm range with reserves.
NOTE: Don't forget that the cruise performance charts do not consider increased climb fuel consumption and do not provide for any reserve fuel. When the factory computed that data they flew it until flame out and then things got very silent up there at 12,500'! (They also did not consider alternative fuels which provide fewer BTU's and/or which may have reduced performance due to alcohol or MTBE's. According to the data FAA provides range reductions up to 15% are possible for those causes. In conjunction with the 10% error disclaimer in the cruise performance charts that leaves an error possibility of up to 25% less range. Also the latest TCDS revisions specifically prohibit fuels with alcohol in our airplanes.)

Manny thanks George. Those are all very good ideas. I, too, always check the winds aloft because that's usually the largest effect one can expect. Headwinds being obviously bad effects and tailwinds good. I also try to shift the cg aft (within limits) a bit when possible. You actually had a loaded 170 at 16000 density ??? That's very impressive for a stock 170.
Appreciate the comments.
Chuck

N170CT wrote:Manny thanks George. Those are all very good ideas. I, too, always check the winds aloft because that's usually the largest effect one can expect. Headwinds being obviously bad effects and tailwinds good. I also try to shift the cg aft (within limits) a bit when possible. You actually had a loaded 170 at 16000 density ??? That's very impressive for a stock 170.
Appreciate the comments.
Chuck

Unfortunately it also opens me up to Joe's accusations that it proves I'm light-headed.
On the most recent journey to the Tehachapi convention, Jamie, all her clothes, snacks, gifts, and I flew up first to SLC to visit friends, and passed over Jim Bridger's pass at 14,500 indicated. I was pretty impressed myself considering that I have a cruise prop. Of course that was near the end of the flight and we were probably down to gross weight by then.

I have flown 04:45 hrs @ 8500', 2100 RPM used 30 gal.

my last XC in the 170, RT Anchorage-Skagway in Sept.:

76.6 gal.
14.0 hrs
5.5 gph

±1150 nm
82.4 kts avg ground speed
94.7 mph " " "

7500' down, 8500' back
2400 rpm
8042 prop