B737NG F/D's on Initial Climb Out.
 

Automatic Flight Takeoff and Climb

There are certain times at LIFT-OFF during the initial climb out, the F/D pitch command at times indicates 20 degrees of pitch instead of 15. And latter reduces to 15? Is this something to do with the Rate of Rotation.

Also is it safe to pitch up to 20 degrees, following the F/D guidance ?

FD commands V2+20.. at least in our fleet, but I have myself always wondered if this is generic 737NG or a customer option.

If you are climbing into a strong headwind it's normal. You may want to restrict your pitch to max 20. Your company may have a saying in what they want you to do.

Most probably you under rotated or as said you got into strong headwinds.
Possibly also if you are relatively lower weight and high thrust(no ASS).
If the FD go back in a short period of time from 20 to 15 its probably due to an under rotation/speed too high,after corrected it goes back to a normal 15 deg(standard ASS and weight ratio).
Most airlines restrict the pitch by pilot action to 20 degrees,ie disregard FD).
Remember 25 degrees pitch up is considered an upset and needs to be dealt with as laid down in your QRH.

Your captains dont teach you anything?worried to ask them?

Also is it safe to pitch up to 20 degrees, following the F/D guidance ?


Never completely put your trust in flight directors. They are an aid to navigation - not the be all and end all as some pilots think. Know the correct pitch attitudes for the condition of flight and stick to that. Blindly following the FD needles has caused many pilots grief. Once you have learned to fly competently on raw data (FD off), then you will soon be aware if the FD is over-reacting to hasty pitch angle movements or leading you up the garden path. Even the manufacturer Boeing in the 737 FCTM recommends turning off the FD if the FD commands are not to be followed.

ONLY if it is unintentional.

Correct but its a fine line between unintentional and unaware of a potential threat.
Now the recovery of the 'upset' can be done in less drastic or different manner than the nose high actions as laid sown.For example if AP engaged,command increase of speed which will command the AP to reduce pitch.
If you approach a stall unintentionally,i hope you would recover as well:E

Of course, I'd consider unaware the same as unintentional in this case.

I'm just highlighting that a blanket - 25 degrees = upset is not correct.

Some of flying I do on the 737 I regularly see a lot more than 25 degrees, if I was to treat that as an upset then I'd never get the job done and that would be a shame because it's a lot of fun!

And if i had 25 deg every day doing my job,my QAR mailbox would explode:p
Taking my coat and out for some well deserved fresh air:ok:

20 degree ANU is also a "gate" for some FOQA programs where it starts looking at bank angle.

My company wouldn't care, but some Asian carriers would be talking to you if you bank over 15 Degrees with 20 ANU.

Back to the original question.

You will get an original pitch up indication of 15 degrees NU.

This should result in a V2 plus 20 speed.

If you underrotate, you are not following the FD, but the FCC does not know that. In fact this computer is made to believe that you as a competent pilot are following the FD command.

The computer only learns quick that a 15 degree NU command results in a higher than V2+20 speed.

Therefore the clever computer will give you now a 20 degree NU command, to try to satisfy the speed requirement.

When you now start pulling on the stick, speed will come back. The now happy computer will command the FD to come down, and meet you somewhere halfway..

If you rotate as you should, you will not see The FD showing you 20 degrees NU, unless you ferry the plane with TO trust.

The initial 15° initial NU is not a command by the way, if you use derate plus ATM (and probably improved climb speed schedule as well) an initial pitch of around 12° is likely enough, more would lead to a speed drop. There is a reason why there is a Note in the FCM which says that the FD pitch command is not to be used for rotation.

To be honest, pitch in excess of 20° seems to be rare in the NG, slightly more common in the 700 than the 800. However it was quite common in the classic during initial climbout, up to 25° was considered perfectly normal.

:D:D:D

Spot on Sheppey.
Keep your scan up and be prepared to fly through the flight director.

@ denti: if you use derate, plus max assumed, you FD's will still give you a correct pitch control. Ofcourse not at rotation, but in initial climb. Thanks, I have a FCTM.

You probably switch them off during TO, no?

And, while during an CatIII landing, make sure you look through them, just to be sure?

Isn't it quite easy really?

FD commands MCP (normally V2) + 20 kts and the pitch command will be whatever it will be. Light weight/high thrust the pitch command will naturally be higher than high weight/low thrust.

Is the question not answered now?

It's quite easy for someone who actually flies, not just claims to fly.

172 Driver and flyboymike, the question as I see it is why the bars drop back from 20° to 15°, not why they are sometimes 20° and other times 15°.

Who cares what they drop to (or don't drop to)? I don't even look at the F/D on take off, I look at my airspeed and the trend vector thereof.

We are cranky tonight...

The FD s drop as the FCC sees the V2 +20 requirement being achieved.
Read my pevious post.
Opposite, if one would climb extremely shallow after liftof, th FDs would indicate their max pitch.

Fly the airplane, not the flight director, then you won't have to worry about what it's dropping to.