Let's Talk Jet Performance

[ksu_aviator]

Good work guys! Lots of mention of the AFM. I'm impressed. For clarity I should have narrowed it down to a non-DEEC TFE731.

This conversation started on some PT135 contract trips I did recently. Every pilot I flew with had the same peculiar method. While sitting around FBO's I started asking other TFE731 operators (Astra, Westwind) and every single pilot did it the same way:

1)T/O power is set to minimum N1 from T/O power chart.
2)Climb power is set using max allowable ITT on the "hot" engine and then matching N2's.
3)Cruise power is set by pulling the "cooler" engine back to "around" 845C and again matching N2's.

Just for clarification, I don't know about other aircraft, but the 731's on the Lear list the maximum power for takeoff due to an airframe limitation. At least that is what I was taught.

 

[enigma]

To answer the original question. My first jet was a TFE731 powered Lear35, and Flight safety taught that take off power was set per the AFM. Once I got on line, I learned that there were as many ways to set climb/cruise power as there were pilots. The most usual technique, and the one my boss used and required, was to set an ITT somewhat below max continuous and keep that temp setting. During his years of Lear piloting, he had observed that this temp was low enough to keep the other parameters from being exceeded. At first, I tried to keep the power charts out all of the time, but I eventually started using his technique. We probably sacrficed some performance, but he signed the paychecks.

It's been a while, but what I remember:


Your takeoff N1 setting is going to come from the book, based on pressure altitude and temperature. PF gets them close and PNF tweaks them to the N1 setting, within a few decimal points, (respecting the ITT limit, of course) you may have to bump them a little as you accelerate. Takeoff power restricted to manufacturer's limitation (usually three or five minutes).

Ty, I understand what you are saying, but I'd like to point out that when you use the book (properly) to look up take off N1, you will not exceed any limit. Especially ITT. The point is: if you set N1 according to ambient conditions and ITT exceeds the take off limit, you engine is NOT making rated power.

Man, I sure wish I could give those RR/BMW's a try. I'm getting tired of old, slow to spool JT8D's.

enigma

 

[Ty Webb]

Man, I sure wish I could give those RR/BMW's a try. I'm getting tired of old, slow to spool JT8D's.

enigma

I'm actually digging the CFM56-7's better than the RR/BMW's. At 22,000 per side, what's not to like?

 

[Lead Sled]

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.

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

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

 

[HMR]

Sled (wiping a tear from my eye), that was one of the most beautiful things ever written. You should get a root canal more often.

PS-Love the new avatar.

 

[NJA Capt]

What sense does it make to spend an extra $400 or so worth of fuel to save $200 or so worth of airplane time?

Its not the extra $200 for that flight that matters. Its the gas vs overhaul figures that matter. Flying slower gives you more time per flight. More time per flight gives you less flights per HS or OH. Gas is much cheaper than maint costs. IE....slower = fewer flights per overhaul (broad generalization).

Of course, I'm flying the most speed scrutinized aircraft in the history of civilian aviation, so we are max cruise (detent) all day long.

 

[flatspin7]

The following has turned into a friendly debate between our flight dept. and some others on the "proper" way to operate a turbojet. I'd like to hear thoughts from you FlightInfo'ers on how you do things and why.

To get us started:
How do you set T/O power?
Climb power?
Cruise?
For each segment do you primarily use ITT? N1? N2? Something else? Why?

Alrighty I'll throw the ERJ-145 into this frey
Before you push you set in the take off data telling the airplane weather you want a Full or Reduced thrust take off (Reduced is normal), Temp, and if you will have anti- ice for take off.
With this data the FADECs calculate a desired N1 for take off. All that is left for you to do is the place the power levers in the detent and have the Pilot Monitoring confirm the proper N1 has been achieved.

For Climb power you hit the button behind the throttle quadrant marked "CLB" and leave the throttles in the detent. This gives you max climb power.

For Cruise you hit the "Cruise" button and watch the airspeed. Depending on weight and altitude you may have to pull them back just a tad.

Its a pretty slick system considdering we dont have auto-throttles.
Outside of the thrust modes the primary reference on the engines in N1.

 

[Lead Sled]

Its not the extra $200 for that flight that matters. Its the gas vs overhaul figures that matter. Flying slower gives you more time per flight. More time per flight gives you less flights per HS or OH. Gas is much cheaper than maint costs. IE....slower = fewer flights per overhaul (broad generalization).

Of course, I'm flying the most speed scrutinized aircraft in the history of civilian aviation, so we are max cruise (detent) all day long.

I'm not disagreeing with what you are saying. Like I said, it's typically cheaper to fly faster and some of the reasons are the things that you mention. I also specifically said that FADEC airplanes weren't being discussed. You don't typically cruise those in the "takeoff" detent. I'm talking about operators who set cruise power in non-fadec aircraft using ITTs that result in N1s higher than charted figures. We're now in the era of $3 and $4 per gallon jet-a and there comes a point where the incremental difference doesn't justify it. Most corporate operators do not have the high cycle and high annual usage that the fractional or regional operators have. We'll never own an airplane long enough for it to be a factor in our operations.

This area is one of my "hot buttons". As pilots, we have direct control over how a lot of money is spent. Personally, I feel a responcibility to give my employer the most bang for his buck - in the long run it will keep his costs lower and I'll keep my job longer. It takes me a little longer to do my flight planning - I don't do it using "rules of thumb" like a lot of guys do. But isn't that the reason the boss pays me the big bucks? Bottom line is doing it correctly, by the book, doesn't cost you anything, you actually cut costs. The time factor? The 6 or 7 minutes are for a coast to coast flight. Most of our flights now days aren't coast to coast. The fuel savings remains, the time differential is even less. We can usually do on standard fuel what those guys need the extension tanks to do. Ask HMR he's seen it. I guess I just find it to be sloppy and lazy. Oh well, enough of this. I'll climb down from my soapbox.

'Sled