Hi guys. Sorry about taking so long to respond to this thread. I’ve wanted to join in, but I had some oral surgery a couple of days ago (read: root canal from hell) and I haven’t felt like doing much typing.
HMR asked some very important questions. Guys who run around in FADEC and autothrottle airplanes don’t need to worry about this. Guys who never fly their airplanes out near the edge of the range envelope seldom really need to worry about it either – there’s nothing like the ability to carry a bunch of extra fuel to cover up inefficient flying techniques. It’s when you operate the way HMR was doing in his charter Astra that any inefficient operating techniques really make themselves known.
I was pretty fortunate when I typed in the Astra. Right after I received my type I had the opportunity to spend a week with one of the factory developmental and production test pilots. We flew coast to coast a couple of times and I had the opportunity to ask every possible question that I could think of – basically the “whys” and “how comes” of everything in the AFM. When we finished our trip I still had more questions so we went fishing. It was one of the most productive training experiences that I have ever had. I was the lead international captain at the place where I was working and I wasn’t very thrilled about flying an airplane that most folks said couldn’t fly as far or as fast as the factory promised. Since then, I’ve flown all 3 variants of the Astra well over 3,200 hours world-wide. It’s certainly not a G-V, but there’s not another mid-size jet that give you as much bang for the buck.
HMR knows my answers to his questions. I’ve had the opportunity to talk to him about this before. However, FWIW, here is the way that I do it when I’m flying an Astra. (Granted, you should always follow AFM procedures, but I would imagine that these would also be pretty close for other swept-wing Garrett powered jets as well.)
How do you set T/O power? The calculated N1 takeoff power setting is the minimum power required to achieve the required takeoff performance. In the Astra AFM it just states “Set Takeoff Position, Observe N1 and ITT”. This goes along with the letter that I received from Allied Signal that said you set the power levers to the stop and make sure no engine limits are exceeded. Note, that this is for the -3 as well as for the -40 SPX and G100. I am always amused when I fly with copilots who, while racing down the runway, are piddling around with the thrust levers trying to set the power to that MINIMUM setting – often to the point of missing the V1 and/or Rotate call outs. We’ve all seen guys do it. When I get a copilot who insists on messing with my throttles I simply mash them to the stops and call out “power set”, they get the point.
Climb power? Again, the AFM is quite clear when it says “Set Climb Power” and refers you to the “Thrust Settings – Two-Engine Climb” chart. The -3 powered Astra's AFM also allows for a climb power based on ITTs, but it is very conservative – you can normally pull significantly more power using the charts even if you’re flying a -40 powered SPX or G100.
This is where most guys start to veer off course when they’re trying to operate out near the edges of the range envelope. My test pilot buddy told me that if you want book performance you need to fly it by the book. In the case of the Astra, you have a couple of climb profiles to choose from. They have been determined (by computer modeling and flight testing) to give you the best combination of “down range” vs “fuel burn”. The range charts are predicated upon their use. A lot of guys have a tendency to fly fast in the climb. That’s OK, but realize that you’ll end up burning a couple hundred pounds more fuel for what amounts to very little time gained.
Cruise? Like others have mentioned, there are a million different techniques; but 999,999 of them are shakey when you take a close look at them. The AFM only says to “Set Cruise N1”. It also mentions Maximum ITT for cruise is 885 C. This is where most guys get it wrong. They take the temperature reference to mean that 885C is an OK power setting regardless of the N1 achieved. The power output of a TFE-731 is a function of N1. All performance and power charts are based on N1. N2 and ITT are only indicators of an engine’s health. A nice fresh tight engine will achieve the various target N1s with lower N2s and ITTs. As the engine gets time and cycles on it, it will require higher N2s and ITTs to achieve the same N1. What they are trying to tell you is that if it takes more than 885C to achieve the cruise N1 du jour it’s time to have the engine looked at.
As far as which power setting to use, the Astra flight manual give you three choices: long-range, constant mach, and maximum cruise. Long-range charts aren't very useful, they call for constant climbs and reductions in airspeed based upon in-flight weight - not something that you’re going to get away with when flying over water. The Constant Mach charts work best there. As Gulfstream 200 said, you don’t buy a jet to go slow. In the Astra, as well as most other jets, the cheapest way to fly the airplane is FAST. However, you gain very little by running faster than the N1 power settings given in the Max Cruise charts. Guys do this all of the time when they set power based on a particular ITT – most engines are better than minimum acceptable.
HMR said:
Let's take a look at a common scenario using my plane (Astra): The difference in true airspeed between balls-to-the-wall and “recommended max cruise” is (on avg.) 460 and 453kts, respectively. On a leg from KLAX-KTEB, using today’s temps/winds our “recommended max cruise” time would be
HMR said:
4:42 on 6700lbs of fuel. Using “Charter Power” the flight time would be 4:35 on approx. 7600lbs of fuel. The difference in time is 7 minutes or 0.1. The difference in fuel is 900lbs/135gals.
My charter buddies all fly with the extension tanks (600lbs fuel) installed. I'm able to carry 600lbs more pax/baggage and land with 300lbs MORE fuel. They beat us by seven minutes. I can cut that time in half with superior taxi skills.
HMR said it better than I could. Flying faster is better, but only up to a point. What sense does it make to spend an extra $400 or so worth of fuel to save $200 or so worth of airplane time?
‘Sled
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