Let's Talk Jet Performance

[Gulfstream 200]

Where are you guys getting that advice from? Just wondering.
'Sled

MY GUESS (emphahsis) is that when you look at the AFM its goin to give you a max temp you can run at. I would bet you will temp out before fanning out.

Why would you run less?

to save fuel?

I am a believer that time is money. I have heard guys tell thier boss they are running it a bit slower to save money on gas.....great way to limit yourself in corp aviation I believe.

The only differ I HAVE heard from this is from Garrett (or whoever that is now)....I have heard them come out with max rec. -20 degress (for example) to "prolong engine life" on their POS 731's. I say if it cant run at what the #'s say you have a problem - fix it. I dont know how this relates to the EEC engines.

Of course if thats your concern one call to them could get you the real scoop....

Best Bet: trade that farkin' dinosaur in on a fadec/Autothrottle equipped new ride!!!.....ah, in a perfect world...

.....


PS - having flown those EEC equipped 731's. I always thought there was little perf info on them. I was never taught much....just let it do its work....nobody seemed to have much support for these...or maybe I was too lazy to look for it. More likely the later...

 

[Jeff Helgeson]

Suck, squeeze, burn and blow....

....not much else you need to know!

 

[semperfido]

There may be some powerplants where that is OK, and FADEC and autothrottles are about the neatest thing since sliced bread but with regard to the EEC/DEEC equipped TFE-731s that a lot of us have, you won't find justification for it if you read the AFM. Fortunately the -731 is pretty tolerant. I've heard and seen a lot of different theories when it comes to -731 power management - for example, setting power according to some particular ITT, or N2, or N1, or my ever favorite: "fly it balls to the wall, the computers will keep you out of trouble." Actually the AFM is pretty specific when it comes to power management, the problem is most guys haven't really read the manual. [Note: Let the flames begin.]

'Sled

OK Sled I will agree with what you said.....but you didn't answer the original post.

 

[Ty Webb]

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).

At the top of final segment (usually 1500' AGL minimum and clear of obstacles) power is reduced to Climb Power, again, from the AFM, usually an ITT limit or a target N1 setting (some use N2). Usually, you take the hottest engine, set it at the target ITT limit, and then synch the other engine to the hotter ones N1 number.

For cruise, after accelerating to cruise speed, synch N1's to maintain selected speed, of course observing cruise ITT limit. Then you have a choice of Constant Speed Cruise (reducing N1 to maintain speed as aircraft gets lighter) or constant power (aircraft accelerates as fuel burns off).

For descent, usually an idle descent is most efficient, somewhere between 3:1 and 2:1 clean. Generally, a 3:1 descent rate will be (Mach * 3 = ft/min) or, if you are descending at .80 it will take about 2400 ft/min to maintain a 3:1 descent profile (Mach .80 * 3 = 2400).

On the approach, you will have calculated a go-around power setting (N1) to use in the event of a missed approach or go-around.

Hope this helps.

 

[slowto250]

Prior to Fadec, a variety of parameters was used. In most turbojets, EPR's were used for takeoff, and then a max temp (ITT) was used for climb or cruise. Remember though, that differing atmospheric conditions (ambient temp especially), could result in reaching a factory rcommended max operating value prior to reaching another. Soooo......for example, you cannot exceed max ITT even though a target N1 or N2 setting has not been achieved. Remember, ALL manufacturers operating limits must be heeded. Except when CFIT is an issue : )

 

[Guitar Guy]

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?

Setting power in N1/N2 vs. EPR vs. ITT is usually a question of the engine manufacturer and the particular model of engine. As other respondents have said, set it per the manufacturer's and/or operator's directions lest you make a pricey error.

 

[DrewBlows]

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?

1. Push the thrust levers into the T/O gates (Unless you are doing a reduced thrust takeoff, then push them to the Max. Climb gate).

2. Pull thrust levers to the Max Climb gate.

3. Airspeed.

4. How the heck do I know, I just do what they tell me to do.

But what do I know, I fly a Fischer Price jet.

 

[HMR]

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.

My thoughts:
1)There seems to be some confusion about this. I was blasting down the runway and at 90kts the PNF pulled the power back to the minimum setting. This happened with two different guys. The #'s in the book are minimum power settings. They are the lowest acceptable N1 settings that will allow charted T/O performance. If you use the minimum # it's to be set before break release, not by the PNF while on the roll.


2) & 3)What do N2's have to do with anything? Of the 10 or so pilots I asked, most said "you get a 'wah wah' when the N2's are out of sync, you know, it's like syncing the props on a twin". Huh?
I’m not making this up. I watched these guys fly around adjusting the rudder trim all day because the N1’s are split and the plane is flying sideways. Every pilot I’ve flown with on contract trips does it like this and they all have 3,000+ jet PIC. What gives? Where in the AFM performance section does it ever mention N2?
These examples are all engine related. I won’t even get into some of the other “theories” I learned.

As for me… I’m too dumb to make up my own procedures. I just follow the AFM as closely as possible. The test pilots/engineers went through a lot of trouble to make those T/O, Climb and Cruise charts. Makes sense to use them, no?

As for all you lucky dogs flying the big iron with your FADEC’s and Auto-Throttles... Don’t you have a suitcase to pack?

 

[HMR]

MY GUESS (emphahsis) is that when you look at the AFM its goin to give you a max temp you can run at. I would bet you will temp out before fanning out.

Why would you run less?

to save fuel?

I am a believer that time is money. I have heard guys tell thier boss they are running it a bit slower to save money on gas.....great way to limit yourself in corp aviation I believe.

Understood. 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 boss couldn't care less about the few hundred bucks we save on fuel vs. engine time. What he likes is us not having to use our extension tanks and losing half the baggage compartment.

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.

 

[Ty Webb]

I was taught not to reduce power on the takeoff roll, unless you are going to exceed a limit. In other words, push 'em up, but don;t pull 'em back.

I was also taught to synch N1 down low (ie climb) and N2 in cruise because down low the fan is doing more of the thrust, at altitude, the core is doing more. ALso, for trend purposes, I believe Honeywell calls for synching N2's). The 2 FADEC aircraft I have flown, however, seem to synch N1 at altitude.

While we're at it, if you program the winds before takeoff, the FMS on the 717 will give you a shallower climb (higher indicated airspeed) into a headwind, and a steeper, lower airspeed climb into a tailwind (now, that's schmarrt!).