Knowing your autopilot

It is usual to be vectored from below the glide, but sometimes circumstances or traffic mean that you may well get vectored to join from above. This is usually no big deal, but in the quoted instance the crew were obviously not aware of just how far above the glide they were, and neither were they aware that they were making their task harder by agreeing to comply with the controller's requests to keep the speed up. By keeping the speed they were not able to get the rate of descent they needed to capture the correct profile, and neither could they configure the aircraft (flap settings and gear) in a timely manner for what was planned to be a Cat 3 approach. (When you need to be really well set up and prepared).

I'm sure that the crew knew that it was going to be a bit "tight" and further I am sure that they were reluctant to go around when the thought may have first crossed their minds. Having said this their first mistake, in my opinion was agreeing to the controller's request to keep the speed. Had they refused, and had they not been set around, there is a good chance that they would have recovered the situation, albeit in a rushed manner.

Personal opinion from someone who also makes mistakes from time to time!

Edited to add that the "gate" by which one should be "stabilised" is usually 500 in VMC, and 1,000 when in IMC. This crew was carrying out a Cat 3 approach, and by definition would have been flying in IMC.

…the DA42 into Frankfurt thread suggested that maybe it was time for a thread resurrection…

In the UK on an IR ride the AP might be switched on in HDG when S and L, and there doesn’t appear to be any formal learning objectives on AP use. FL or ALT changes are via VS, the more basic sets do not have a SPD mode, and HDG changes are via HDG bug. With VS set to an ambitious climb, speed will degrade, and then using HDG mode for a requested turn plus some turbulence might place the aircraft close to a stall.

The AP expects to receive the aircraft in correct trim for the requested action, and ATC expect a crisp response to instructions, however quite a few pilots do not appear aware of CWS (control wheel steering) mode. Am not sure how many pilots naturally go to CWS when there is a reasonably large change in flight profile, eg more than a 10 or 20 degree HDG change, but if it is not taught in the IR, learning about CWS would come from reading the manual?

Am sure there are other gotchas on AP use which might be worth mentioning.

I’ve always been a little confused as to the function of CWS. How does it differ from pitch/roll hold mode?

CWS lets you command the aircraft to climb, descend, or turn, using the control column, and then you can hand the aircraft back to the AP. If your bank angle is low, less than 6 degrees and you release CWS the AP should act as a wing leveller and roll wings level, otherwise the AP keeps the attitude as configured. CWS is not an AP disconnect, so there are restrictions (AGL minima, on LOC intercepts, coupled approaches). In turbulence it will hold an attitude and not try and maintain an altitude. All AP have roll hold, or wing leveller, but some the pitch hold may be driven by the baro.

Am not sure how many pilots naturally go to CWS when there is a reasonably large change in flight profile, eg more than a 10 or 20 degree HDG change

The problem with using CWS in that situation is you still have to follow up with the heading bug to prevent the aircraft resuming its original heading, so you’re doing more than simply leaving the AP engaged and turning the bug. However you can turn with any angle of bank you like in CWS so it can be useful to swing the aircraft round quickly in a hold or onto a LOC.

My KFC200 doesn’t have VS control so I use CWS to enter climbs and descents and then to adjust the pitch attitude as required. It works well and it’s much smoother than using the rocker switch.

More than half the After Start checks in the Navajo with Altimatic V are a/p related:

5 FD/AP Master Switch On
6 Autopilot Test, check RDY, On
7 Electric Trim On, Check
8 Pitch Command Adjust F/D up and down, align
9 AP Engage
10 Controls Overpower pitch and roll
11 Hdg Press
12 Pitch command Centre detent, Check Alt, Arm
13 Heading bug L & R check controls
14 Nav, Appr, Rev buttons Press sequentially
15 AP Release Press

We are shortly going to see an AAIB on a multiple fatality which appears to have arisen because the pilot did not know how his a/p worked.

And talking of which, would you expect the GA button on an Altimatic V to disengage the servos and only affect the FD, or would you expect to drive the aircraft to 10° nose up?

Timothy – I used to fly a Navajo and I cannot remember for the life of me what A/P was in it (I don’t remember the after start checks so presumably a different one) but I do remember that it would drive 10 degrees nose up.

Just to add to as well – I have seen a couple of references earlier in the thread to a lot of autopilots not reengaging if over powered and I think Peter mentioned at one point that he was led to believe that this was the case with most commercial jet stuff . Just to give a little bit of insight, in our 737s now if you apply force it will disengage the autopilot but it can be immediately re-engaged. Prior to a software update last year or the year before overpowering it would push it into CWS instead of disengaging.

Pirho wrote:

I do remember that it would drive 10 degrees nose up.

Yes, that is my experience. But a very experienced and highly competent and respected IRE recently told me that autopilots should be and are designed to disengage the servos on GA button press.