Q For Heavy Fliers (As In 747 et al)

[2000flyer]

A week ago I was sitting on a hillside off runway 6L in Anchorage watching the stream of cargo types departing for their wordly destinations, I've always been amazed watching 72's, 74's, etc., using nearly every foot of runway available.

I understand the concept and use of "FLEX" takeoffs. My question is how the information is derived. For example, in computing a FLEX takeoff on a 747, I assume before V1 you'll still abort for the obvious reasons. However, if you've figured a reduced power takeoff, thus a longer takeoff roll, how is V1 affected in relation to the loss of one or more engines? Is APR figured in? Does a 747 even have APR?

Not trying to get to technical here, just curious as I sat there watching heavies rotate with 1000' remaining. After more than 20 years of flying, I can still be awed!

Regards,
2000Flyer

 

[CLCAP]

When they "flex" they just go with an assumed outside air temperature that's higher than actual where they still meet BFL. That automatically lowers N1 (or EPR)

I don't think most companies flex when the runway is contaminated or when an ecessive climb gradient is required.

Correct me if I am wrong....

 

[chrisdahut24]

What is a "FLEX" takeoff?

 

[CLCAP]

Reduced power t/o

 

[Draginass]

Used to fly 747-200 out of Anchorage all the time. I'm sorry but I'm not sure what you mean by "APR." Do you mean "EPR" (exhaust pressure ratio)? Reduced thrust was used whenever feasible to save on engine wear/tubine creep. The FAA limits reduced thrust to 25% maximum. The reduced thrust EPR/N1 is usually dictated by the amount required to obtain a balanced field length. It's done by artificially raising the temperature value input (and thus lower the EPR/N1) until a balanced field length is obtained.

As far as V1 goes, by definition. it's based on losing an outboard engine. Usually with reduced thrust, V1 and Vr are very close together, as you would expect.

Usually reduced thrust is not allowed under very slippery runway conditions, performance related aircraft writeups, tailwinds, etc.

 

[fokkerjet]

First you need T/L tables for the airport and runway you plan on using. With this data, and using the procedures per the AFM:

1. Determine your actual takeoff weight from your load sheet (weight and balance).
2. Correct this weight for a high or low QNH off of standard. (a low QNH may cause a loss of payload due to lack of climb performance, high QNH the opposite).
3. The corrected weight must meet a minimum weight for the planned takeoff flap setting. (this is for Vmcg on our airplane).
4. Referencing that new weight, check both the climb limited weight column and the runway limited weight column and determine the maximum temperature for each off the T/L table.
5. From these two column temperatures, the lower of the two will be your maximum "assumed" temperature.
6. Referenceing another set of tables, determine the maximum approved reference temperature for your field elevation.
7. Reference another set of tables, determine the minimum approved assumed temperature.
8. Find your EPR setting for the assumed temperature.
9. Set your V-speeds per your actual takeoff weight

and off you go...........

After you do it a couple of times, you can figure out an assumed temperature for flex pretty quickly. The benifit is lower wear and tear on the engines while still guarantying that you meet all runway and climb requirements.

In the F2000, we can't use flex power, but in the 900EX, I believe we could. The Gulfstream IV and V have performance software in the FMS that offers you the option of a rated or flex takeoff with a key stroke.

 

[dsee8driver]

We use assumed temp of 49c for all our reduced take-offs. Obviously we have the usual limitations for EAI, tailwinds, etc.. How we get it is by looking at our performance chart and establishing what the max weight for that runway and desired flap setting at 49c and make sure we are well below it. Then look at the speed chart for that altitude and 49c and get the appropiate V speeds. Thats the simplified version.

 

[2000flyer]

Draginass,

I'm sorry but I'm not sure what you mean by "APR."

I'm referring to "Automatic Power Reserve". In short, you lose an engine, APR automatically increases thrust on the operating engine to compensate.

Fokkerjet,

In the F2000, we can't use flex power

You are correct, there is no "FLEX" on the F2000. Just push the power levers to the T/O detent and go.

To the others,

Thanks for the responses. As I said earlier, I understand the use and theory behind using a reduced power takeoff and the savings on the engines. I'm just trying to understand the relationship between the use of a lower power, longer takeoff roll and the assumed loss of an engine.

If you figure an 8000' BFL on a 9000' runway, obviously this figure will still guarantee a takeoff in the event of an engine failure post V1. If that is the case, is the last 1000' the "buffer" for the engine failure, or wouldn't your takeoff distance be somewhere less than 8000' with all engines operating? However, if as Draginass said, V1 and VR are very close, that may also answer my question.

Thanks to all for your responses.

Regards,
2000Flyer

 

[Draginass]

Since I never flew props, I'm unaware of the "APR." I've never seen a jet that automatically increases thrust on the good
engine(s) in the event of an engine failure. That would induce a further assymetric problem.

Like I said before, the power setting for full reduced thrust is based upon either:

1. Balanced field length - the point on the runway where the aircraft can lose an engine and either accelerate to takeoff speed (and clear the departure threshold by 35'), or abort the takeoff and stop in the remaining (and same) distance. In other words, where critical engine failure speed and and refusal speed is the same.

2. Reaching 25% thrust reduction, even if you have excess runway available.

Whichever is reached first.

Especially lightweight and with a long runway, you'll get the 25% reduction first.

There is no "BUFFER." The takeoff data itself is BASED upon losing an engine (outboard engine on a 4E airplane) at V1. If it were not, there would be no need to compute anything except a Rotate speed.

As far as your analysis of a 8000'BFL vs. a 9000' RL. If this were the case and 8000' was the BFL that the data came up with, THEN, you would be at the maximum 25% thrust reduction. In order to get a 9000' BFL, you'd have to reduce thrust more than the 25% maximum. So in that case, the last 1000' of runway would be a freebee, but not theoretically needed, even in the event of engine failure.

Now, in the event that you DON'T lose an engine (with reduced thrust), your takeoff roll won't be significantly shorter even with the loss of an engine. Keep in mind that (depending on conditions), you're V1 and Vrot are going to be fairly close. Assuming that you lost the engine right at V1 and continued the takeoff, then the only advantage that the other engine will give you on the runway is that short time between V1 and Vrot. Climbout performance is entirely another matter.

It's been a really long time since I've tried to explain these concepts, so I may not be doing such a hot job.

As an aside, when I was flying 747s, we had CF-6 engines and actually had 3 ways to "reduce" thrust. One was the traditional "reduced" thrust and the other two were fixed thrust reductions of 2% and, as I remember, 10%. Since most of the creep on the engine is in that last 2% of power, even this modest reduction produced good results, but certainly not the fuel savings that a reduced thrust takeoff provides.

BTW, some airlines use the term "Standard" instead of "Reduced." They say it sounds better in the event that a layman pax hears the term.

 

[2000flyer]

There is no "BUFFER." The takeoff data itself is BASED upon losing an engine (outboard engine on a 4E airplane) at V1. If it were not, there would be no need to compute anything except a Rotate speed.

As far as your analysis of a 8000'BFL vs. a 9000' RL. If this were the case and 8000' was the BFL that the data came up with, THEN, you would be at the maximum 25% thrust reduction. In order to get a 9000' BFL, you'd have to reduce thrust more than the 25% maximum. So in that case, the last 1000' of runway would be a freebee, but not theoretically needed, even in the event of engine failure.

Draginass,

Thats the answer I was looking for. Thanks!

APR has nothing to do with props. On the Falcon 2000's CFE738 APR (Dassault calls it "Automatic Performance Reserve") works like this:

If takeoff power is set and one engine N1 drops 10% below the other engine:
- Max ITT limit increases from TO limit of 864°C to APR limit of 890°C.
- Crew and Pax airconditioning valves close to minimize bleed extraction.


If CLIMB power is set there after:
- Climb N1 schedule increases from Max Climb to Max. Continuous.
- Crew and pax airconditioning valves remail closed.

There are other limitations such as OAT and time limits, but basically thats how the system works.

Again, thanks for the great explanation.

2000Flyer

 

[Draginass]

Interesting. I've never heard of it, but so many different manufacturers use different concepts and assumptions in designing their aircraft. The design philosophy that Rockwell used in the B-1 is pretty unique and certainly not much like Boeing.

A lot of laymen think pilots can just jump into another jet and be able to fly it. Given the significant differences between the manufacturers, we know that's an impossibility.

 

[fokkerjet]

One thing about our T/L tables, maybe it's all T/L tables, but we can't see how much runway we are going to require. Our tables list the maximum weight allowable for climb and runway based on temperature. As long as I'm equal to or under the charted weight for the temperature, all I know is that I'm going to be no worst than balanced when I release the brakes.

Performance computers on the executive jets that I've flown will tell you the required runway distance based on the conditions that it senses, or you have entered, and all you need to do is look at the airport diagram to see how much extra runway is available.

 

[cl-65link]

The CL-65 has APR for Take-off(GE CF-34 engine).

 

[2000flyer]

A lot of laymen think pilots can just jump into another jet and be able to fly it. Given the significant differences between the manufacturers, we know that's an impossibility.

Amen!

2000Flyer

 

[Dieterly]

...also some newer jets are certified to reduce the thrust up to 40% of the full t/o thrust.

 

[Broke in CVG]

I am going through upgrade class now in the CL65 and we just went over this in detail yesterday....

Normal thrust settings will guarantee 121 climb performance (accellerate from V1 to V2 and positive slope to 35', 2.4% climb to 1000 or OCA , and then positive slope during acceleration, flap retraction and 1.2% climb to 1500 at Vfs.)

Reduced or FLEX thrust can be used if the actual weight is less than the max performance weight. This reduced thrust number ends up being equivalent to normal thrust without APR at the higher "assumed" temp.

V1 is the combination of sufficient kinetic energy to lose half of available thrust, and still achieve acceleration and climb requirements, balanced with the ability to dissipate that energy with the brakes before running out of load-bearing pavement. When less thrust is used, applying thrust over a greater period of time (bumping V1 higher) coupled with less mass (lighter TO weight) produces similar kinetic energy state.

Thus since we are at a lower weight, this reduced thrust and higher V1 will give us the same 121 climb profile guarantee even without the APR working (2% bump) or increasing to max thrust.

 

[metrodriver]

according to the charts you are OK, but I've been on some flights where at V1 there is so little runway left that I wondered if the remaining runway would be enough in case of an abort.
But we all know it's not lift that makes a big plane fly, it's the paperwork