840 real world full fuel payloads

[JimC]

Eastbound altitudes are easy - go high, go fast, go far.

For westbound, do you guys generally find that fltplan and foreflight are accurate in their predictions?

There's no doubt I'll be doing a stop westbound 99% of the time, whether in an 840-5 or even a long range 980. I'm just trying to get some budget & time estimates put together.

[schrambow]

i find foreflight to be very, very accurate, there are different models and machines to select from i think,  But the stock 840 profile without even customizing or modifying it is very close. I use the max speed profile, but haven't tried the max endurance profile yet.   Almost to the point where i am still amazed of how close it is even after after about 2 years of using this specific profile.  Amazingly accurate compared to the old days for sure!

[donv]

For me, using fltplan.com, I find the time to be very, very accurate and the fuel burn less so. I generally burn less fuel that fltplan.com predicts, either on the advanced or hourly methods.

[Bruce Byerly]

Corey’s plane is on the money. A healthy -10 may get another 5-7% in efficiency in theory at equivalent speed. But usually everyone just uses “go fast” mode for the extra knots.  Either way, and particularly with a tailwind, there are some crazy long legs that you can do. At the end of the day, they are all not created equal at this stage in the life cycle so don’t just buy the first thing on controller that looks like it would look good in your hangar without employing careful diligence.

I would like to get my hands on Don’s plane. Where is the mystery missing fuel?! 

[donv]

Western Jet and Aero Air couldn't figure it out, and from the looks of some log entries, I think Eagle Creek took a shot as well. But you never know!

I would like to get my hands on Don’s plane. Where is the mystery missing fuel?! 

[donv]

I should add that I am pretty confident that Aero Air could figure it out, if I wanted to throw enough money at it. But I'm not convinced it's worth it.

[Adam Frisch]

Must be a kink or an interconnect tube that's messed up.

[JimC]

A healthy -10 may get another 5-7% in efficiency in theory at equivalent speed.

I checked the performance tables for an 840 vs a 980. The -10 is not more efficient, it just allows you to run hotter. At the same fuel flows, the speeds are less than 0.5% apart according to the book. The -10 gets its extra speed by pumping more fuel in. This will allow a greater range into a headwind, or course, so it can be slightly more efficient. It addition, the -10 gets to the efficient altitudes a few minutes earlier, so it gets a marginal range bonus there as well.

Both planes have the same published range profiles, so these slight variations weren't taken into account in the POH.

EDIT: I also went and checked Honeywell operating notes....
The -5 is listed with a SFC of .57 and the -10 is listed with .55...so it *should* be 4% more efficient, but the POH's don't show it.

[Bruce Byerly]

A healthy -10 may get another 5-7% in efficiency in theory at equivalent speed.

I checked the performance tables for an 840 vs a 980. The -10 is not more efficient, it just allows you to run hotter. At the same fuel flows, the speeds are less than 0.5% apart according to the book. The -10 gets its extra speed by pumping more fuel in. This will allow a greater range into a headwind, or course, so it can be slightly more efficient. It addition, the -10 gets to the efficient altitudes a few minutes earlier, so it gets a marginal range bonus there as well.

Both planes have the same published range profiles, so these slight variations weren't taken into account in the POH.

EDIT: I also went and checked Honeywell operating notes....
The -5 is listed with a SFC of .57 and the -10 is listed with .55...so it *should* be 4% more efficient, but the POH's don't show it.

It was Honeywell who published numbers showing the -10T 7% more efficient than a -5.  There was likely some theory supporting the claim - like total trip fuel or some other fun with numbers. BWTFDIK. 

Anyway, I think this is hair splitting. There’s much more to consider between planes these days than book figures and none of it can be quantified for flight planning purposes from this distance.

[JimC]

Anyway, I think this is hair splitting. There’s much more to consider between planes these days than book figures and none of it can be quantified for flight planning purposes from this distance.

We need a thumbs up emoji. Thumbs up!

[kent4142]

I don't know if this is the right place to post this or not - but here goes.

Monday I had the privilege of sitting in the "bosses'" seat (right rear), While my two adult sons (recently minted Commander Pilots), sat in the front!!!!  They did a great job!  I am a very proud Dad.

But I am also writing about some figures that we collected on this trip, and I wanted to see if they could be correct and if you all agree with them.  We flew from Deland, FL to St. Cloud MN on Monday.  It was a particularly brutal flight since we had 80 to 90 knot head winds almost all the way, aaaaaand we chose to deviate for some significant turbulence.

The flight lasted 5 hours and 47 minutes.  When we topped off with fuel for the trip home we put 374 gallons in our 81' 840 with dash 10 engines and optional extra tankage 475 gallons total.  This means we had 101 gallons remaining when we landed.  We flew the first half of the trip at about 286 knots true, burning 440 lbs per hour.  We contemplated stopping for fuel and decided that if we could land with 1 hour of fuel - we would continue to the destination.  If we calculated less than an hour reserve we stop for fuel.

We came up with the idea of pulling the power back a little bit, and see how that looked.  We pulled the power back to 275 knots and the fuel burn was 400 lbs / hr.  Sooooo, on this flight we had an ultimate of 7 hours and 28 minutes of fuel!!!!  This is absolutely unbelievable to me.

I know this is rough math but - 3,100 lbs of fuel (75 for taxi etc.) burning 400 lbs per hour @ 275 kts works out to 1,856 NM - leaving 1 hour of reserve fuel at the destination.  I know there is some climb fuel in there, but there is also some descent fuel savings.  But either way - that is just unbelievable!!!!

Now one more thing occurred to me on our return flight home.  We talked a lot about taking off over gross.  (The unspoken thoughts of the Commander community). I was thinking that if we left 70 gallons out (35 gallons from each wing) we would save 469 lbs.  Add to that the 75 lbs for taxi - plus my airplane has about 36 lbs of payload left with full fuel - That leaves 505 lbs for people and bags.  (Not too unacceptable in the general aviation world.)

So the moral of this long story is, by throttling back just 11 to 25 knots (depending on temperatures and day, I can be "legal" / safe (er) and still have 6 hours and 30 minutes endurance with 1 hour of reserve and still have 1,552 miles range (no winds of course).

I know in one sense this is obvious leave off fuel - if you have a big load, but it never occurred to me to pull the power back slightly to pick back up some range.  Another point is - not that many people want to sit still for that many hours.  But once in a while the situation might arise.

Or the counter point.  The airplane is so overbuilt just fill it up and go.  The over weight issue is fixed in the first hour - right?

Thoughts?

[donv]

My airplane is about 6830 empty, so with 2850 pounds of fuel I can carry almost 700 pounds of payload, and as you point out, go a long way. Personally, I don't go much more than 5 hours, although I generally do go fast, and I'm quite conservative on fuel reserves.

I don't like to see the low fuel light on in flight.

[Adam Frisch]

Nice, Ken! How old are the boys? I have a house rule for my boy and that's PPL before DL!

The 1000 model when it's up at FL350 will creep down to 390lbs/hr doing about 295-300kt. I bet at same speed as the one you pulled back, it would probably drop down to 350lbs/hr. That's why we love the Commanders! They have fuel economy like nothing else.

[kent4142]

Hey Adam,

The boys are 34 and 36.  I know that is plenty old enough - but, I remember carrying them both up to the top of a light house in Georgia just a few years ago!!!