Rolling takeoff with a TFE-731

[avbug]

Doesn't N1 also guarantee minimum rudder control if you have an engine failure? That, if you pop an engine with a power setting above N1 there is no guarantee that full rudder deflection will compensate for the yaw moment? Or was that just the EPR settings on the 20 series lears?

Rudder control at what speed? The only concerns you have with rudder control are establshed by the V1 speed, which must be above Vmcg, and V2, which must be above Vmca. Specifically, V1 must be at or above Vmcg, Vr must be above V1 and greater than or equal to 1.05 Vmca, with V2 being at or above 1.1 Vmca.

Vmc is a published number. Actual directional control under field conditions is whatever you get. The two are not the same.

EPR gives a true measure of engine thrust, whereas N1 does not.

1. N1 is the ONLY indication of thrust output in the TFE-731 engine.

While this is true, in that it's the only indication equatable to thrust, it doesn't actually describe thrust, but RPM. The thrust output of the engine decreases with age, such that what you're getting at 94% N1 today isn't what you'll be getting out of it five years from now.

Conversely, EPR tells you today what it tells you five years from now. If you were to compare an EPR indication (which isn't available for the TFE-731, you'd see the same EPR and the same thrust value being produced, but a higher N1 five years later as the engine must turn faster, work harder, and burn hotter to produce the same thrust with it's gradual deterioration.

2. N2 tells you the internal speed of the engine.

N2 tells you the percent speed for the high pressure spool only. Remember that the engine is two spool, and only part of it is turning that speed. Further, the N1 pickup is actually in the engine, at the back, in the core, whereas the N2 pickup is on the accessory drive.

3. ITT or EGT essentially tells you the health of the engine - a "fresh" engine will be able to achieve its rated thrust at a lower ITT or EGT than an engine with some time on it. As an engine wears, the internal tolerances open up and it takes more heat energy (fuel flow) to generate the same amount of thrust. More fuel into the engine results in higher temperatures. A new engine will have a greater margin than older engines.

ITT only speaks to you about the health of the engine when balanced against other parameters. The only time this is not true is when approaching a temp limit; in that case you don't care what the rest of the engine is doing, because it's time to limit it's function. Otherwise, ITT is only significant relative to the N1 or N2 indications...and really only to N2, even though N1 is used as the primary power setting device. N1 speed is a function not only of the core (N2) but of ram airflow and density...what N1 is doing doesn't say a lot about what ITT should be doing, whereas N2 certainly does.

4. The takeoff N1 Power Charts give you a specific minimum N1 required to achieved the charted takeoff performance. Garrett/Allied Signal/Honeywell looks upon that number as a "minimum" number.

It'a target number. As I said before, the only engine limit applicable here is the temperature, and no matter what the numbers are reading on the takeoff, the temps must be respected. If the book takeoff N1 is 94% but you get to 91" and find yourself temp limited, you stop pushing the power right there. Therein lies the value of making assumed temperature (reduced power) takeoffs, which do provide a margin as required and still allow your field and climb gaurantees...you always have the option of pushing up the power. The smart pilot will calculate a climb V2 based on current conditions an run the other numbers based on reduced power where it's possible...this allows the pilot better performance than what's predicted, and the option of increasing (restoring) to full takeoff power with certain knowledge of the safety parameters in the climb.

If the field is critical then holding brakes for the takeoff may be necessary, but if the takeoff distance is calculated out at five thousand feet and the runway is nine with no obstacles...a rolling takeoff is really the order of the day.

 

[Raskal]

Nice info avbug.

In the hawker 700's we fly, all but one have DEECs (one has EECs), and the standard is a rolling takeoff with levers to the stops unless the numbers are close. The XP is the same, with the cute little green lights for max cont. The 1000 is FADEC with Pratts so no issue. The manuals are all slightly different, but the gist seems to be that all max numbers are acheived with static takeoffs and that is the "preferred" method (from the crew vol. 2 book), but if distance available is good than rolling is fine making sure that max limiting (N1 or ITT) is acheived by 80kias.

As for replacing SOP with technique, I don't think of the manufacturer's "recommendations" in the handling manuals as ironclad, they aren't limitations. Another example is the 700 book states a preferred climb of 230kias/.63. This is absurd in almost all conditions, the airplane is too much of a "dog meets brick" and climbs much better at 250/.68 or so, at .63 the energy dissipates too easily and it will flat out stop climbing. Recovering the speed at altitude is a looooooonnnnng process.

I certainly prefer rolling if possible, and I'm sure the pax do as well. I haven't really seen too many static t/o's in hawkers on normal runways, are there any operators out there that do that as SOP every time?

 

[soarby007]

Everytime I've gone to Simuflite in the Westwind, the takeoff power and numbers are based on static settings. That's what they say. I've seen the video at Simuflite and it's based on I think three airplanes, on specific 731's. I use rolling takeoffs as most runways have plenty of length. I talked with an instructor yesterday and he says the numbers are all based on static takeoff settings. I remember the video saying the same thing.

 

[avbug]

I'm looking at page 5-29 in the AFM for a LR35A with 731-2-2B powerplants, and it states:

"N1 s the value wher ethe temperature line intersects the pressure altitude line. N1's above this line may exceed the manufacturer's limit. N1's below this line may not meet the performance of this section."

As previously stated it also goes on to say "Operation at a specific N1 should always be within the turbine Temperature (ITT) imits. Turbine Temperature (ITT) limit must be observed."

Skipping ahead to the takeoff proceedure section, on page 5-37A of the same manual, I find the following:

The takeoff distances on the takeoff distance charts can be realized by following the engine failure during takeoff proceedure in section III and using the following technique from a standing start with brakes set:

1. Run engines at 750 C ITT for 15 seconds to stabilze fan speed (N1). Then set takeoff fan speed.

2. After brake release, continuously adjust fan speed (N1) up to 80 KIAS.

It then goes on to describe the rejeced takeoff proceedure.

As can be seen above, the values prescribed aren't minimum values, nor maximum values, as the only limiting fan speed is the maximum RPM limits, governing limits, and ultimate overspeed limits. The only engine limitation so far as takeoff will be temperature. As a practical matter, if you reach the prescribed N1 before temp, you should stop there, and if you reach temp before N1, you should stop there.

Again, N1 represents nothing more than fan speed, and does nto speak to thrust, despite being the only instrument available which comes close in this installation. N1 as a speed will produce different thrust at different altitudes, different temperatures, different levels of wear. A damaged or worn or dirty fan (or compressor, for that matter) will produce less thrust and be less efficient respectively, even though operating at a specified RPM. Efficiency goes downhill. Accordingly, while the performance charts for Part 25 airplanes are often hawked as being very accurate...banking on that performance with anything less than a new airplane in perfect condition is a fools errand.

You're taught in ground school and in manuals that the performance is "gauranteed." It's not.

It's been demonstrated, and in many cases, interpolated, but is no gaurantee. Much of the data in your manual is based on percentages of other speeds. Your V2 speeds, for example, are based on a book VMC speed...which has no factual relationship to the controllability of your specific airplane on a specific day at a specific point in time, temperature, and pressure altitude, to say nothing of engine condition.

The point is that if you run your flight to the ragged edge of the book numbers you may find that you run out of runway, or obstacle clearance, before you run out of numbers. At that stage numbers are meaningless. You'll be told margins are built in, but they're not enough; they're theoretical numbers applied for certification, in some cases to placate manufacturers or make life easier on their salesmen...take a look at the reduction in screen height on a wet runway as a good example of this.

To make the book numbers, you must do exactly as spelled out in the book. If that involves a braked takeoff, then you must do so...but if you're having to do that because your field is critical, then you're operating at the edge of the performance envelope, to which there is no gaurantee.

Remember, Part 25 doesn't offer gaurantees, it offers requirements applicable to certification, just like Part 23. It just offers more requirements to achieve certification, and increased performance requirements for certification. Guess what? Certification is done. Class is over. Time to go to work...what you find in the field won't necesarily be what the manufacturer's test and certification crew found at their home field.

 

[SoundBarViolatr]

Well, not quite. If you made every takeoff according to the book you would sit at the end of the runway, set takeoff power and wait, for what? 45 seconds for the numbers to stabilize before releasing the brakes. Show me anyone who does that.

Like I said, I'm not advocating that you ignore book procedures. I'm the one who constantly harps that if you want to achieve book performance you have to fly the airplane by the book. What I am questioning is the wisdom of some pilots to dink around adjusting the power levers so as to set precisely a power setting that is guaranteed to provide nothing more than the minimum required takeoff performance - all while the airplane is accelerating like a raped ape and while all this is going on the guy in the right seat is missing the speed callouts. Tell me you've never had that happen. Set the power and don't mess around pulling it back to a precise N1 if you happen to go over a little bit. The engine folks aren't going to care; as far as they're concerned you can shove the power levers to the stops and leave them there for up to 5 minutes without exceeding any limitations - provided your computers are set up correctly.

LS

I may vey well be missing something here, but where I come from we line up, set the power, then release the brakes. PNF calls power set and brakes are released by PF. From a static takeoff you have all the time in the world to set takeoff power because you are technically not supposed to release the brakes until the PNF calls power set and the throttles are left alone. Rolling takeoffs were a little more challenging but attainable nontheless. And we had to use steam tapes!!

The 5 minute limitation on most of the engines I have operated are limiting you to the plotted t/o power. That is why you are required to pull back to climb power after 5 minutes. As far as I can remember, unless you have clickers, your engine output is only governed by throttle position thus the need to plot numbers. And yes, from what I've seen the engine folks are going to care because anytime a hard limitation is exceeded you are SUPPOSED to pull the engines apart. Call me crazy, but it's just the FE in me who's actually seen this.

I should have specified that my comment was not specifically for the westwind but something in general instead. At least thats how we operated in the AF.

 

[Lead Sled]

I may vey well be missing something here, but where I come from we line up, set the power, then release the brakes. PNF calls power set and brakes are released by PF. From a static takeoff you have all the time in the world to set takeoff power because you are technically not supposed to release the brakes until the PNF calls power set and the throttles are left alone. Rolling takeoffs were a little more challenging but attainable nontheless. And we had to use steam tapes!!

The 5 minute limitation on most of the engines I have operated are limiting you to the plotted t/o power. That is why you are required to pull back to climb power after 5 minutes. As far as I can remember, unless you have clickers, your engine output is only governed by throttle position thus the need to plot numbers. And yes, from what I've seen the engine folks are going to care because anytime a hard limitation is exceeded you are SUPPOSED to pull the engines apart. Call me crazy, but it's just the FE in me who's actually seen this.

I should have specified that my comment was not specifically for the westwind but something in general instead. At least thats how we operated in the AF.

It's apples and oranges. We're not talking general techniques here; we're talking engine specific techniques as per documentation received from the engine manufacturer. Engine operation is generally not a "one size fits all".

Regarding Avbug's comments on EPR indications... although technically correct, they really don't apply since the TFE-731 series engines aren't provided with EPR gauges. N1 is all we have, the performance charts are referenced to N1 settings, so N1 is all we have to use.

LS

 

[avbug]

Regarding Avbug's comments on EPR indications... although technically correct, they really don't apply since the TFE-731 series engines aren't provided with EPR gauges. N1 is all we have, the performance charts are referenced to N1 settings, so N1 is all we have to use.

I believe that's really my point. Or part of it. The N1 is all you have, but let's remember that the N1 doesn't actually tell you about thrust, and when it does, it's not correct, esepecially with age. For the crowd that tells us a rolling takeoff throws all the data out the window...it doesn't, though it does delay it, and it does unbalance the field. The problem is that the data is NOT gauranteed, nor is it necessarily correct.

Now I do the calculations. I fly the numbers in the book. But I also do something else very important. When the book says it's a 5,000' takeoff, I don't depart a 5001' runway. I can afford to do the rolling takeoff because I plan ahead with ample extra runway. Some aircraft will toss things around the cabin if you run them up and then release the brakes.

While yes, the performance charts are referenced to N1, we need to know and understand that those charts are referenced to the N1 on the engines of the aircraft that flew out the certification runs, and that much of the data is interpolated...it's not actual data. We need to know that all our instruments lie to us, from airspeed to altitude to precession gyro errors...and yes, the N1 lies to us, too. As it ages it gets forgetful, and as ambient conditions change, it's ability to predict and report thrust decreases in accuracy. That alone should convince us not to play around near the performance margins when considering takeoff distance, abort distance, obstacle climb criteria, engine out criteria, etc. Be conservative.

For the crowd crying that the numbers go out the window with a rolling takeoff...the numbers are already out the window. Many pilots either don't know this, or choose to ignore it. Give yourself ample room, and a lot of extra, either by choosing the runway or by choosing the load. Take less fuel. Make more stops. Go on a diet. Do something.

A rolling takeoff is not a crime.

 

[fu69atacafyeah]

A rolling takeoff is not a crime.

I agree with everything you say except this...LOL...If the book requires you to do a Static takeoff and you dont, well then you're exceding/violating a limitation...BUSTED says Mr. FAA

I agree though, be on the conservative side and we all could shed a few lbs...

 

[Lead Sled]

I believe that's really my point. Or part of it. The N1 is all you have, but let's remember that the N1 doesn't actually tell you about thrust, and when it does, it's not correct, especially with age. For the crowd that tells us a rolling takeoff throws all the data out the window...it doesn't, though it does delay it, and it does unbalance the field. The problem is that the data is NOT guaranteed, nor is it necessarily correct.

Now I do the calculations. I fly the numbers in the book. But I also do something else very important. When the book says it's a 5,000' takeoff, I don't depart a 5001' runway. I can afford to do the rolling takeoff because I plan ahead with ample extra runway. Some aircraft will toss things around the cabin if you run them up and then release the brakes.

While yes, the performance charts are referenced to N1, we need to know and understand that those charts are referenced to the N1 on the engines of the aircraft that flew out the certification runs, and that much of the data is interpolated...it's not actual data. We need to know that all our instruments lie to us, from airspeed to altitude to precession gyro errors...and yes, the N1 lies to us, too. As it ages it gets forgetful, and as ambient conditions change, it's ability to predict and report thrust decreases in accuracy. That alone should convince us not to play around near the performance margins when considering takeoff distance, abort distance, obstacle climb criteria, engine out criteria, etc. Be conservative.

For the crowd crying that the numbers go out the window with a rolling takeoff...the numbers are already out the window. Many pilots either don't know this, or choose to ignore it. Give yourself ample room, and a lot of extra, either by choosing the runway or by choosing the load. Take less fuel. Make more stops. Go on a diet. Do something.

A rolling takeoff is not a crime.

I agree with all of the above. I believe that this is the reason that I was told what I posted early on in this thread...

The only thing I ever remember hearing from Honeywell, ne Allied Signal, ne Garrett was that takeoff distances began from whatever point you were at when the takeoff N1 was achieved - regardless of your ground speed...

Perhaps I should have been more clear, they were referring directly to the question of a rolling takeoff - they are allowed, but the distance required BEGINS at the point that the specified N1 is achieved, regardless of the speed of the aircraft or the distance that it has traveled prior to achieving the required N1. In other words, if you need 5000' and you take 1000' getting the power set, then you would need 6000'. It's probably not the most precise way of calculating the required distance, but it's pretty conservative and will keep you out of trouble.

LS

 

[avbug]

I agree with everything you say except this...LOL...If the book requires you to do a Static takeoff and you dont, well then you're exceding/violating a limitation...BUSTED says Mr. FAA

No, not busted says Mr. FAA. A proceedure is not a limitation. Try again.