140 KIAS is an important airspeed in the PA-46 Piston’s, but it is not an airspeed you will see listed in the POH for any particular reason. Why 140 KIAS? It’s all about the cooling.
In either a Continental or Lycoming powered Malibu, the highest temperature I like to see for any cylinder is 375F. Yes, the POH lists 425F as the upper limit for CHT, but ask just about any mechanic worth his salt and he’ll tell you that heat is enemy #1 for any piston engine and the cooler you run your CHT’s, the more life can be expected from your engine. I don’t know any PA-46 pilot who will let the CHT’s get anywhere near 425F. In talking with seasoned, trusted PA-46 pilots, I’ve heard 375F discussed as the arguable “upper limit” for most who care about the life of their engine (meaning, they are the ones paying the bills!). The challenge is keeping the CHT’s below 375 on warm days, especially during the climb.
The POH lists 125 KIAS as the cruise-climb speed. If you fly this airspeed at max gross weight on a fairly warm day, you will get about 600-700 ft/min in the climb for a Mirage and 700-800 ft/min in the straight Malibu. Interestingly, if you fly at 140 KIAS, you get almost exactly the same rate of climb. Most pilots I fly with during recurrent training use 120-125 KIAS and invariably will operate near 400F on their CHT’s while lumbering up to the selected cruise levels. I’ve found if you put your autopilot on 500 ft/min rate of climb, and let the airspeed build up to 140 KIAS, you can then reset the autopilot to a higher rate (depending upon your specific airframe/engine/prop combination) and achieve nearly the same rate of climb as you find at 125 KIAS. I then fly 140 KIAS all the way up to my cruise altitude by simply adjust my autopilot rate of climb so as to maintain 140 KIAS or greater. If flown this way, you’ll have CHT’s less than 375 on all 6 cylinders, and you won’t give up rate of climb. Additionally, you’ll be cruise-climbing 15 knots faster! Since it will take you 45+ minutes to get up to the flight levels from near sea level, the extra 15 knots does mean you’ll get to your destination faster.
Did I make you think about your CHT and how high CHT’s can negatively affect your engine? I hope so…give 140 KIAS a try…I think you’ll soon begin operating above 140 KIAS for climb. I’ve been doing so for a long time now…