View Full Version : 737 Autopilot, when engaging..

14th Jan 2004, 20:31
Hi folks

As I teach it for JAR, I'm trying to get the correct answer to this question;

What happens when you engage an autopilot in terms of stability enhancement, and or AF modes that engage automatically. Please assume that the FD is off and no modes are armed or selected at the time you engage the AP.

Possible answers are;

1. The aircraft will level the wings and provide lateral stability.

2. Pitch attitude will be maintained (Longitudinal stability).

3. Vertical speed will be maintained (V/S is engaged automatically)

...and so on. So, could you Pilots who are actually flying, in particular, the 737 please let me know what would happen. All the literature I've read goes in to detail on the modes, as does this site, but does not specify what you get on initial selection of an AP.

Many thanks in advance.

14th Jan 2004, 22:38
If I understand the question correctly, the answer is ultimately: the aircraft will try to maintain attitude (including bank!).

The roots break down like this:

The AP as referred to on a 737 calssics is no more than a hydraulic system, that flips the controls but itself have no clue which direction or why.

Then you have the flight director system, which is the smart-ass that can compute the acceleration vector required. But still, it doesn't know where to and why either.

The FMC is the knowlegable one that is capable of assembling the flight trajectory.

(Yes, the pilot is the one who thinks he knows WHY)

Now, in full automation mode, the whole AFDS works like this>

The FMC has the uploaded track, profile and speeds, the FD constatnly recalculates how the airplane should behave to achieve so, and the AP works the ailerons and elevator+stab.trim.

Now this union at work may be broken at any point, i.e. you may disengage the FMC (LNAV &VNAV modes] from the FD and decide to tell the airplane where to go manually by using HDG SEL, ALT HLD, LVL CHG or V/S by setting the dials on MCP (glareshield panel). OR, you may want to disengage the autopilot and hand-fly the aeroplane on the FD'd bars - but it makes no real sense.

In conclusion, what the questions aims at is the CWS regime when the autopilot is engaged but knows not where to go. At that point it only tries to maintain given attitude. If you hand-fly to another attitude, it keeps it. Hence the name Control-Wheel-Steering. I was told this used to be the primary AP regime on -200s.

With no FD mode activated, the AP has no clue and engages in CWS, that's what happens.


One more thing, I've been through the ATPL theory only 13 months ago, but what I recall is that the VERY EXACT wording used to be the enigma behind many questions. It seems to me that the answers do not match your question very well - or be it the other way around. Incidentally, I remember this too. :*

Cheers, FD.

15th Jan 2004, 02:03
When the AP engaged and proper mode selection is not made AP will switch CWS ROLL,CWS PITCH mode and flashing amber CWS PITCH,CWS ROLL announciaitons on FMA will come on and ask you to give proper roll and pitch modes.As you select your roll and pitch modes it will engage and FD will display your selection(ie: press HDG SEL and FD will show guidance to selected heading on MCP)

15th Jan 2004, 03:42
Well, of course. But I feel the question says the FD would not be engaged at all.


15th Jan 2004, 06:18
Engaging the autopilot will not change the aircraft’s inherent stability; and even if the aircraft has stability augmentation e.g. Mach Trim, that system is more than likely to be engaged in manual flight as well as auto flight.

Selecting an autopilot ‘on’ will normally engage the existing flight guidance system (FD) modes or, if none selected, the autopilot basic pitch and roll modes. The basic modes are type dependant, usually attitude hold (Pitch and Roll), or a combination of Roll, Hdg Hold/Select with Vertical Speed.

Thus What happens when you engage an autopilot in terms of stability enhancement? … nothing.

16th Jan 2004, 01:51
OK correct me if I get the question wrong.

The airplane is in the air and no AP and FD selected.To engage the AP(the criteria is no force applied on the control wheel and the stab trim AP switch is normal) you have to press either one of AP A or AP B buttons.Lets say you pressed AP B.Since each side has its own FCC, right side FD master switch(MA) will illuminate.(With one or both AP's engaged the master FCC is determined by which AP is engaged first regardless of which FD is turned on first.)So now we have AP B engaged and FD system ready to display commanded modes.But there isn't any since no mode selected and AP is in CWS mode.As you select your Vertical and Lateral modes AP will start following them,FD's will display it with vertical and horizantal bars on attitude indicator and FMA will announciate the selected modes.

16th Jan 2004, 23:18
On the 737-3/4/5, if the FD is on with no lateral or pitch modes pre-slected, and you engage either autopilot, it will default to MCP SPD and HDG SEL.

As part of Special procedures which vary from our outfit's normal SOPs - departing certain CAT C airfields such as CBY and INN, in order to afford increased situational awareness, Autopilot use is maximised. We can therefore select an AP at 500'R after take-off, and all you have to do is select the autopilot and adjust the speed in the MCP Speed window to Top-bug - And there you are......barrelling on up initially at up to 6000fpm! :cool:

So...just to recap, in the above situation you get, after AP selection, FMA annunciations of TO/GA, MCP SPD, HDG SEL & CMD.

If No FD selected on then as others have mentioned you end up with CWS in both pitch and roll.

The only downside (on the question of Stability) is IMHO the 737-3/4/5 autopilot is quite frankly crap in pitch, especially in FMC/MCP SPD, and lags quite badly in correcting to the commanded speed - (it seems worse on the 300 and 500 than the 400). In the aforementioned situation, I've found it necessary to adjust the speed in the MCP window a good few kts above or below what i really want in order to coax the autopilot into adjusting pitch in an, *ahem* ...more timely manner than it would like to.

Flight Detent
18th Jan 2004, 11:16
I agree with Bumblebee! (for the most part)

2nd Feb 2004, 22:35
If No FD selected on then as others have mentioned you end up with CWS in both pitch and roll.

Indeed, and that is THE question: What are the muscles of CWS.

But to answer this at last, one need to decipher the original wording at first. In order to do so, one needs to know the regimes of AP system on 737, their limitations (AP with no FD trips to / engages only in CWS) and functions. Only then you can asses the outcome on stability as the question suggests.

For a student, it is quite uneasy.

Edited for typos