RVSM for C-90?

[wingnutt]

lol...actually the certified is 30,000, but theres an "all-engine" service ceiling of 28,900 feet, so RVSM definately wont be an issue

http://www.raytheonaircraft.com/beechcraft/king_air_c90b/KAC90B_Spec_Perf.pdf

 

[BoeingBoy]

Just a question about derated engines...aren't they normally bigger engines derated to lower power so they operate a lot cooler and have longer TBO's? I didn't think they gave you any added altitude capability...

 

[Lead Sled]

Just a question about derated engines...aren't they normally bigger engines derated to lower power so they operate a lot cooler and have longer TBO's? I didn't think they gave you any added altitude capability...

BoeingBoy...
There are two terms that come to mind anytime you start limiting aircraft engine power - "De-Rating" and "Flat-Rating" The differences between “Flat-Rating” and “De-Rating” engines can be confusing. De-rating an engine means arbitrarily assigning a lesser output to an engine than it was designed to have. There are a few ways to achieve the power output reduction on piston or turbine engines, for example - mechanically or “on paper” (charts and graphs which limit output).

Many turbine powered aircraft use derated engines. An engine derated to a particular thrust a SL will also be limited to that thrust at 40 000Ft. In other words, when they take away the power, you don’t get it back. There are many reasons to install de-rated engines. One of the major reasons is that they “loaf” – they’re operating at a certain reduced percentage of their design capability. This usually makes for increased engine life. Back about 30+ years ago, Parnelli Jones installed a P&W PT-6 in an Indy car and proceeded to eat everyone’s lunch. If it weren’t for some bad luck, he would have won the Indy 500 the first time out with it. The following year the race officials forced them to de-rate the engines – by adding restrictor plates to the air inlet – to the point that they lost all of their previous advantage. By the way, there is a company in Texas that has an STC to put the big Continental 0-520 (yes, they take an injected engine and refit it with a carburetor.) I think the engine ends up with a TBO of 2400 hours. Way back in the 60's, Piper decided that they needed an airplane to compete with Cessna's 150. Rather than design another aircraft, they simply removed the back seats from their Cherokee 150 and placed a restriction on the engine RPM - bingo... The New and Improved Cherokee 140. The first thing most folks did is get an STC to restore the RPM and get the 10 hp back. That’s de-rating.

Now, for flat-rating…
Generally speaking, it must be remembered that (in very simplistic terms) turbine engines are not supercharged, but rather normally aspirated - in other words, they lose power with altitude just like a Cessna 152. The percentage of N1 (on most turbofans) or EPR (many turbojets) required to obtain the engine's full rated thrust will vary significantly depending upon airport elevation and outside air temperature. For example, on one of the bizjets that I'm typed in, on a cool day at a sea level airport the engines will develop their maximum rated thrust with an N1 somewhere in the upper 80's say for example 88.7%. Go to a higher elevation airport on a warm day and the N1 will be higher, for example 93.4%. (As I type this, I'm looking at the Static Takeoff Thrust Setting Chart for our aircraft. Depending on the airport elevation and outside air temperature, the N1 settings vary from a low of 84.2% to a high of 96.1%.) These numbers will, of course, vary from engine to engine, but you get my point. On most older generation engines, the flight crew is required to come up with a takeoff power setting from a set of charts or tables. In later generation engines with DEECs (Digital Electronic Engine Controllers) or FADECs, the pilots only have to set the power levers into the takeoff detent and monitor things while the computer takes care of the rest.

Just to make things a bit more interesting, some aircraft have larger engines installed than they were designed for. These engines are "Flat-Rated" back down to what the airframe was designed to handle. In other words, say for example, an airplane was designed to use a pair of 40,000 LB thrust engines, the aircraft designers might specify a pair of 50,000 LB thrust engines and limit their thrust to 40,000 LBS. Why would they want to do this? Simple, remember that turbine engines are "normally aspirated" and start loosing power the moment they start to climb. By using a larger engine, the aircraft can operate at higher altitudes or temperatures before it runs out of power. The engine never produces more than the “airframe-rated” thrust (in this example 40,000 lbs), it’s just able to do it to a higher altitude.

Turboprop engines are similar, only instead of N1 or EPR, they usually measure their power output in Percent Torque. For those guys it's a bit simpler, they simply advance the power levers until the engines reach either their torque limit or their temperature limit. Typically, with flat-rated engines, they will "torque" out at lower altitudes, then as the aircraft climbs higher they "temp" out as the max operating temperatures become limiting.

Hope that helps you understand.

'Sled

 

[wingnutt]

holy cow...im glad he didnt ask why the sky is blue

 

[RTN137]

No RVSM for the 90's. Why would you invest the time and $$$ to gain 2000' of cruising altitudes when nobody actually flies at that altitude anyways. ATC doesn't like it when you fly higher in thousands of feet relative to your true airspeed. TAS= 230kts dont go above FL230, TAS=290 then FL290 and below. I've been told this is a rule of thumb and the center guys appreciate it.

 

[wingnutt]

interesting...hadnt heard that one before

 

[BoeingBoy]

Lead Sled,

Thanks for the through explanation.
Now if I could just find out why the sky is blue...

 

[Lead Sled]

Now if I could just find out why the sky is blue...

Heck, I don't know. However someone once told me that if God had meant women to fly He would have made the sky pink.

'Sled

 

[avbug]

The sky is pink. One just needs to fly high enough to get past the blue. Generally that takes a blonde...

The C90 is a teens kind of airplane. Not a really efficient one but a comfortable one. I've taken them on some long trips and could have sworn I was passed by seagulls.

A couple of folks mentioned taking the 200 to higher levels. I'm sure a lot of folks do, but I never thought the 200 was worth much above FL180. I wouldn't waste my time RVSMing that, either.