For instance if POH says 140kt at 65 % 8000’ (Best Economy Mixture). Is it only true when they come from the factory or do you see similar figures on older aircraft? How can it it be realized? New paint, engine etc?
Seems that many older aircraft perform less than 95% of the specs.
The truth is that cruise performance doesn’t degrade much over time. At least not just because of mere age, or wear.
Of course, if you start adding lots of antennas, remove wheel pants (spats), ding the aircraft badly or mess up the rigging, then it is a different story.
Even many new aircraft don’t quite make the quoted cruise speeds. But we are talking a very few knots at most. The reason why many people think that aircraft don’t meet POH speeds is that they fail to even broadly replicate the conditions listed for those speeds. In some cases, cruise speeds are published only for weights well below MTOW. The air must be 100% still. Aircraft perfectly clean. Ball perfectly centered, etc.
Also, what is often forgotten is that over time, airspeed indicators may lose accuracy, too.
I found the performance figures for the DA40s, DA42s and SR22s I fly or flew, very accurate.
Mine marginally exceeds book.
Mine seems to go faster than book, but as Bosco says I am probably not replicating the POH conditions exactly, because they are not settings at which anyone would fly, being slightly ROP (“best economy”) and well ROP (“best power”). Mine certainly does about 10-20% more range than the book, but then the book doesn’t say how much should be left in the tank at the end.
The other Q to ask is whether the ASI is calibrated. There is a procedure using three GPS ground speeds – see previous threads.
Jonas wrote:
For instance if POH says 140kt at 65 % 8000’ (Best Economy Mixture). Is it only true when they come from the factory or do you see similar figures on older aircraft? How can it it be realized? New paint, engine etc?
I actually see book figures — or very close — in our club aircraft (which you are also familiar with Jonas). Certainly with our 2007 Cessna 172S. But also when I test flew our 1979 PA-28-181 last week after engine overhaul and with a new prop. (Or, to be very precise, I got 2 kt less, but since the a/c didn’t have a nose wheel fairing that was to be expected — the POH says 8 kt less without all three wheel fairings.)
boscomantico wrote:
Also, what is often forgotten is that over time, airspeed indicators may lose accuracy, too.
It’s not that common, though. We test our glider ones annually and it’s not often they will disagree with the test gear.
The mechanical one in my aircraft is probably 70 years old but it agrees exactly with the ASI on the Garmin G5 (as does the vintage altimeter, one of the old white and green numbered ones).
My book, unusually, does recommend LoP, and it is those figures which are marginally exceeded.
What I do find comforting is that the LoP fuel burns, which must have been calculated long before JPI was even thought of, are exactly what happens when you lean according to APS techniques (within 1 ph).
Interested to know what discrepancies people find between speeds published in the POH vs. actual speeds in flight. I wish I could find the extra 15 or so knots in the cruise that my Arrow once recorded in perfect test conditions 40 years ago 
It has a new engine, so no problems there, but engineering tells me the age of the paintwork is the biggest culprit – even on a well-looked after aircraft – and numerous antennae etc that get added over time. Can we ever replicate these speeds after time?
You can try replicating few points of the PoH table for ROC/IAS/FF for various MP/RPM/DA/OAT settings and then analyze if it is engine power, aerodynamics or instrument errors?
- Power issues (e.g. engine max MP*RPM? max HP?) will be more pronounced in performance deterioration at slow speeds (e.g. ROC at Vy?)
- Airframe aerodynamic issues (e.g. paint, antennas) is mainly performance at high speed (e.g. cruise TAS) and are highly non-linear in FF
- Instrument error is easy to spot
My bet it is mainly aerodynamics but you can’t be sure unless you put the new engine on a dyno (do you get max MP? max HP?) and you are able to replicate FF with mixture close to PoH figures, if you have done all this and still not able to get around it then you may have to get a new shiny propeller 
PS: I don’t think it is wise to check aerodynamics using the ROD with engine off ;)
