-8 nose pitching up on the takeoff roll

[Nighthwk 566]

Don’t know if this is a bug in the flight model, or simply a consequence of the situation, but I thought I’d bring it up anyway.

It seems that whenever I depart in the -8 at extremely light weights, as in 40k pounds of fuel and no payload, the nose will pitch up to the point where the tail strikes the ground on the takeoff roll.

Since I like the almost comical climb rates, I almost always go TOGA on departure, but is the increased thrust the cause of this nose up tendency?

The first time I experienced this was a takeoff using TOGA and flaps 20, so I decided to use flaps 10 on the next departure in the interest of reducing lift and surface area for the thrust to impact. 

Sadly, this did nothing and it took full forward action on the yoke to keep the nose down. My CG seemed to be within limits, and the stab trim was near 2, as calculated by the takeoff page. I’ve tried a derated takeoff, but I can’t recall if the nose pitched up or not.

The issue for me also happens upon landing, with full forward stick also being required to hold the nose down. 

Any ideas on what this might be? I’ll probably end up a bit of cargo in the forward hold to keep the nose down from now on, but it’d be nice to know if it’s something I’m doing wrong or an issue with the model itself.

Thanks in advance (and sorry if it’s my own fault..)!

[scandinavian13 5,192]

9 minutes ago, Milton Waddams said:

Any ideas on what this might be?

Trim, and technique.

Trim may be calculated on the FMC a bit on the high side (better to rotate than not, and roll longer). There's a trim switch on the real aircraft yoke for a reason. Retrim, and don't yank back on the yoke so hard.

There are people up front for a reason. Do what you need to do to operate the plane safely. The FMC is a TOOL, not an answer/absolute.

[Nighthwk 566]

10 minutes ago, scandinavian13 said:

Trim, and technique.

Trim may be calculated on the FMC a bit on the high side (better to rotate than not, and roll longer). There's a trim switch on the real aircraft yoke for a reason. Retrim, and don't yank back on the yoke so hard.

That’s the thing, I’m not tugging on the yoke at all when it wants to pitch up. If anything, I have a bit of nose down pressure applied. I’ve experienced this IRL flying an Archer on soft field or standing takeoffs where you need nose down pressure to compensate for the increased airflow over the elevator. 

I’ll try trimming to the nose down limit, and apply forward pressure, on my next departure and see if that eliminates the issue. 

[downscc 1,327]

You cannot be serious... you are using full TO power when you are only 40 klbs above empty weight?  John, you've got enough power to lift a million pounds!  Dial back to DTO2 50C and see how it goes.

[scandinavian13 5,192]

Yeah, I'm with Dan.

If you want the absurd climb rates (no judgment - the first thing I did with the 777-200LR/F when I got it was set it to empty payload, a tiny bit of fuel, no derate, and just launched it), keep in mind that you can set the derate for takeoff (which will then auto-set climb), but then override any automatic climb setting to get rated climb thrust.

[Nighthwk 566]

I knew it was something I was doing wrong..I’ll drop it back to DTO2 which will probably eliminate the issue.

Sorry for being so dumb, and great job on the -8! 

[icaruss 18]

6 hours ago, Milton Waddams said:

I knew it was something I was doing wrong..I’ll drop it back to DTO2 which will probably eliminate the issue.

Sorry for being so dumb, and great job on the -8! 

Have you check in the manuals.  Is your power, takeoff weight, and CG within the normal envelope?  If so, then for a given correct trim value the aircraft shouldn’t rotate this way. If not, then there is your answer.

[Nighthwk 566]

14 hours ago, icaruss said:

Have you check in the manuals.  Is your power, takeoff weight, and CG within the normal envelope?  If so, then for a given correct trim value the aircraft shouldn’t rotate this way. If not, then there is your answer.

I have, yeah. Power is just straight TOGA, CG is within limits, if not a bit towards the rear with only fuel as payload, and trim is near the nose down limit, but still within the envelope.

I’m planning more test flights today, checking it with a dereated thrust setting, less flaps in, and some cargo in the forward compartment. Should fix the issue, though from Kyle is saying, it’s all about technique. 

[VHOJT 160]

I have noticed this on the 744 on my last two flights since the update.

I loaded fuel first, then payload.  And tried the other way around.  But it keeps filling the stab tanks even though it is not called for with the fuel load (~141 tonnes).

TOCG seems to be normal - about 22% according to the readout.

But using appropriate thrust (D-TO 40 @ 390 tonnes, ZFW 240.8), and the correct stab trim as suggested by the FMC, the aircraft starts to pitch up before VR as calculated by TOPCAT.  This never happened to me once before the update.

Once the stab tanks are burnt during climb, TOCG is now down at 18% and highlighted as out of range.  If I reload payload now, it goes back to 22% and the highlight disappears.

Any advice?  Am I encountering any sort of bug?

 

[Jet Tech 31]

I can only reference the 747-400 on this but when we operated short reposition flights on an empty aircraft the Captain would typically remind the F/O to keep moderate nose down pressure on the control column during take off to avoid a tail strike at light weight even with Derated Thrust and then allow it to slowly rotate at Vr.   It was also a handful to land since it just never seemed to want to get back on the ground and would float forever.

[mfrantz 45]

3 hours ago, VHOJT said:

But using appropriate thrust (D-TO 40 @ 390 tonnes, ZFW 240.8), and the correct stab trim as suggested by the FMC, the aircraft starts to pitch up before VR as calculated by TOPCAT.  This never happened to me once before the update.

Once the stab tanks are burnt during climb, TOCG is now down at 18% and highlighted as out of range.  If I reload payload now, it goes back to 22% and the highlight disappears.

VR calculated by TOPCAT is completely irrelevant. You can't use the stab trim suggested by the FMC and then reference the TOPCAT VR and not the FMC calculated VR. The FMC calculations are performed specifically for the aircraft you are flying and this should be your reference.

A CG within limit at takeoff does not guarantee that the CG will remain within the envelope during climb/cruise or landing. Correct load distribution is crucial for the 747. One of the new features with this release is that setting a payload % from the FS ACTIONS>PAYLOAD page will perform a distribution of cargo load in a way that CG is not only within limits at takeoff but is also set so that it is less likely to get out of the envelope during flight and landing. This is why CG was corrected when you reloaded payload during flight.

If you are worried about CG the EFB OPT WT AND BALANCE application can be useful. Use the READ FROM AIRCRAFT feature to get your sim payload and fuel data into the EFB, enter estimated trip fuel and watch the CG values on the graph. You can fine tune by moving around cargo and passengers on the EFB and/or adjusting fuel and see the effects on the CG values in the graph. The EFB application incorporates the fuel burning schedule to calculate the landing CG. When satisfied use the SEND TO AIRCRAFT button to send the desired configuration to the sim.

[VHOJT 160]

4 hours ago, mfrantz said:

VR calculated by TOPCAT is completely irrelevant. You can't use the stab trim suggested by the FMC and then reference the TOPCAT VR and not the FMC calculated VR. The FMC calculations are performed specifically for the aircraft you are flying and this should be your reference.

A CG within limit at takeoff does not guarantee that the CG will remain within the envelope during climb/cruise or landing. Correct load distribution is crucial for the 747. One of the new features with this release is that setting a payload % from the FS ACTIONS>PAYLOAD page will perform a distribution of cargo load in a way that CG is not only within limits at takeoff but is also set so that it is less likely to get out of the envelope during flight and landing. This is why CG was corrected when you reloaded payload during flight.

If you are worried about CG the EFB OPT WT AND BALANCE application can be useful. Use the READ FROM AIRCRAFT feature to get your sim payload and fuel data into the EFB, enter estimated trip fuel and watch the CG values on the graph. You can fine tune by moving around cargo and passengers on the EFB and/or adjusting fuel and see the effects on the CG values in the graph. The EFB application incorporates the fuel burning schedule to calculate the landing CG. When satisfied use the SEND TO AIRCRAFT button to send the desired configuration to the sim.

Hi Michael,

This is with the 747-400.

On these occasions, TOPCAT VR was within 1 or 2 knots of the FMC calculation.  Basically, since the update, it seems to want to pitch up before VR?  This has never happened to me once before the update (~100 flights).

Which way around is it best to load the 747-400?  Payload first in the FMC, or fuel first?  I always let the FMC load the aircraft as it sees fit.

Cheers,

Edited October 5, 2018 by VHOJT

[scandinavian13 5,192]

1 hour ago, VHOJT said:

This has never happened to me once before the update (~100 flights).

Using the same exact weights and CGs?

1 hour ago, VHOJT said:

Payload first in the FMC, or fuel first?

Doesn't matter.

[VHOJT 160]

1 hour ago, scandinavian13 said:

Using the same exact weights and CGs?

Doesn't matter.

This is what I’m saying. I’ll trial a takeoff from an old flight plan from before the update to make sure - but I have used very similar weights in the past. CG I couldn’t tell you - I let your FMC work it’s stuff and make sure nothing is highlighted when inputting load. 

I am going by the FMC basically - if it loads the weight and fuel I ask it to, and the CG isn’t highlighted, and I use the FMC stab trim and VR  and thrust - it shouldn’t pitch before VR at heavy weights with an assumed temp, right?

I’ve also checked the STAT page and my flight controls show centred. 

I notice the plane now loads fuel in the stab tank at lower fuel weights than it used to, has this changed?

Edited October 5, 2018 by VHOJT

[Sekkha 15]

On 10/5/2018 at 3:24 AM, VHOJT said:

  Basically, since the update, it seems to want to pitch up before VR?  This has never happened to me once before the update (~100 flights).

 

I experienced the same behaviour with the 744, never saw that before the recent major update..

TOW was around 856000 (payload set by RANDOM function) / Flaps 20 /  TO 1 derate / CG 22,6 / Trim settings 6,7 and Vspeeds as suggested by FMC (and except for V1 identical to those suggested by topcat) :  Vr = 170

Aircraft startet rotating on its own some knots before reaching Vr.....