Virtual Pilot Takeoff

[andmiz]

Hi Hardy

I did a search of the forums to see if there has been discussion about this in the past but have not been able to find anything.

I have noted in the initial thrust setting prior to TOGA application by the virtual pilot, that 1.25 EPR is the target, which seems excessively high.  It's almost high enough to be a takeoff thrust setting.

Can you or anyone else care to comment?  The purpose of 'standing up' the thrust levers to a nominal value prior to takeoff thrust application is to ensure that all the engines accelerate symmetrically, and any lagging engine doesn't cause the aircraft to potentially depart the runway (especially on a contaminated runway).  Along with this, through the application of this initial thrust increase, it can be seen that the HPSOV's close, through the increase in EGT, and subsequent decrease once the thrust stabilises. 
We use 1.10 EPR as our target, not 1.25.  Can anyone else from a 747 operator confirm what they use?  Once 1.10 is set, I expect to see the EGT rise, settle and then decrease slightly as the HPSOV's close, prior to the selection of TOGA.

[Hardy Heinlin]

Hi,

the purpose of the initial "standing up" is clear.

The virtual pilot's target is not an EPR value but circa 50% of the mechanical thrust lever range, i.e. 25° TLA (thrust lever angle). There is no special intention behind the fact that 25° results in 1.25 EPR. The result is different on all three engine models.

If you can tell me more about the PW's ratio of TLA versus EPR I might be able to fine-tune the translation in the EEC for the fuel valve control.

The PW engine's EPR model is the most complex of all three engine types as it uses the "core EPR" method to indicate EPR values -- unlike the RR model which uses the "integrated EPR" method. The PW EPR varies very much with the Mach number. As you know, in an idle descent it can go as low as 0.75 EPR, where the RR wouldn't indicate less than 0.98 EPR.

It could also be that I should fine-tune the TLA/EEC ratio on all three engine models towards a more curved, less linear translation? Comments are welcome.

Should we have more fine-control in the upper thrust range or more in the lower thrust range?

For example, I could put the PW's 1.25 EPR target along a curved, non-linear translation on, say, 35° instead of 25°. That would give more fine-control for lower thrust settings (0°-35°), and less fine-control for settings above 1.25 EPR (35°-50°).

Do you see what I mean?


Regards,

|-|ardy

Once 1.10 is set, I expect to see the EGT rise, settle and then decrease slightly ...

This effect is modelled in PSX as well. I just tested it with the Tel Aviv takeoff situation. I set engine #1 thrust to 1.10 EPR, the EGT rises to 350°, and then decreases to 333°.

[andmiz]

Thanks Hardy

I'm not sure where I'll be able to find data to corroborate that thrust lever angle is non-linear to EPR, but I'd argue that is the case.  I'll see what I can do and email it to you.

What I can say, is that sitting the base of the levers at the "1" mark on the thrust quadrant is typically where the thrust lever angle matches approximately 1.10 EPR (or 45% N1 on our -8's).

[Hardy Heinlin]

The "1" mark is 10° TLA, right?

I thought in all airlines and on all engine models you would set the throttles approximately vertical, and that this position would be circa 25° TLA.

If my current translation is OK, I just need to tell my virtual pilot to set 10° instead of 25°. Would that fix it?


Regards,

|-|ardy

[skelsey]

Hi Hardy,

I believe it is more an N1/EPR value which is being targeted than a specific TLA. The FCOM (RR) suggests 1.20 EPR, though the FCTM implies there may be engine-related differences:

When the airplane is aligned with the runway
centerline ensure the nose wheel steering tiller is released and apply takeoff thrust by advancing the thrust levers to approximately 1.1 EPR (PW or RR) or 70% N1 (GE). For RR engines, initial EPR settings up to 1.2 are considered acceptable to improve engine operation. Allow the engines to stabilize momentarily then promptly advance the thrust levers to takeoff thrust (autothrottle TO/GA)

(My emphasis).

Of course, the most important thing is that the thrust is set /the engines spool up symmetrically rather than the precise value.

[Mariano]

I agree. Our manuals also call for ~70% N1 for GEs and ~1.10 EPR for PWs. There is no mention of physical thrust lever position.

Best regards,

Mariano

[Hardy Heinlin]

Thanks, guys. I guess my brain wasn't in the takeoff position but in the standard touch-and-go training pattern where you set the throttles "vertical" after touchdown -- at least in Lufthansa training (GE engines).

In the next PSX update (10.82) I'll make the virtual pilot aim at the above thrust settings instead of 25° TLA.


Regards,

|-|ardy


Just for comparison: Do other airlines use in their touch-and-go training descriptions (Boeing text in most cases) not the wording "throttles vertical"? I think in that moment you want to control the throttles in a tactile way rather than visually on the EICAS.

[Hardy Heinlin]

Our manuals also call for ~70% N1 for GEs and ~1.10 EPR for PWs.

I also have this 70% value in mind and it's TLA-wise in the mid range. I think I used it as a reference for all engine models.

70% N1 is certainly not at 10° TLA, is it? Why is the initial "stand-up" thrust setting so high on the GE, and so low on the PW?


|-|ardy

[skelsey]

Just for comparison: Do other airlines use in their touch-and-go training descriptions (Boeing text in most cases) not the wording "throttles vertical"? I think in that moment you want to control the throttles in a tactile way rather than visually on the EICAS.

The airline FCTM I have is Boeing standard on this:

The trainee should do a normal final approach and landing. After touchdown, the instructor selects flaps 20, sets stabilizer trim, ensures speedbrakes are down and at the appropriate time instructs the trainee to move the thrust levers to approximately the vertical position (so engines stabilize before applying go-around thrust). When the engines are stabilized, the instructor instructs the trainee to set thrust.

Note: Flaps 20 is recommended after touchdown to minimize the possibility of a tail strike during the takeoff.

747-8
Note: The speedbrake aural will sound on landing until the thrust levers are advanced.

After reverse thrust is initiated, a full stop landing must be made.

At VREF, the instructor calls "ROTATE" and the trainee rotates smoothly to approximately 15° pitch and climb at VREF + 10 knots. The takeoff configuration warning siren may sound momentarily if the flaps have not retracted to flaps 20 and the thrust levers are advanced to approximately the vertical position.

[Hardy Heinlin]

Thank you for the confirmation, Simon.


|-|ardy

[Hardy Heinlin]

Initial thrust settings are now engine model specific in PSX 10.82:

http://aerowinx.com/board/index.php?topic=4191.0


Regards,

|-|ardy

[andmiz]

Thanks Hardy