The Dash 8 vs. the Localizer.

[jmmoric]

I've had a number of occasions where Dash 8's have passed through the localizer while vectoring them in for an LLZ/DME approach.

We usually set them up with a 30 degree angle to the LLZ, giving the 1NM of straight flying before the LLZ, to intercept 2NM before the descent begins on the procedure.

That seems to work pretty well, and has never caused any problems in the past, and doesn't now with other types. But a few years back one of the carriers aquired new Dash 8-200, and since then, it's not unusual to have them fly straight through the LLZ, and us intervene to turn them back on.

Is there something in the operation of a Dash 8, that makes it tricky when intercepting a LLZ?

[GearDownThreeGreen]

It's just generally terrible at intercepting the LLZ. We do it all the time, and have just gotten used to/accepted how it works. We operate both classics and the Q400, and strangely enough the Q400 is generally the worst, but they all suffer from this interception problem.

However, the AP/FD does capture. It just does so at least 4-5 seconds to late. So please, give our aircraft a few seconds before you vector us back from the other side if the situation permits. The airplane is probably in the process of correcting its own :mad:

[RVF750]

Yes...they're crap. Beaten to it. The Q400 I had the pleasure of flying for 3,500hrs is pretty poor for LOC capture. It needs to be slowed down to have a chance and clean and above 180kts it will go through as a matter of course. (Pun intended).


Glad I don't have to manually nudge it on any more....

[pattern_is_full]

So is this just a case of Bombardier building a bigger, heavier, faster version of the Dash-8, and forgetting to reprogram the A/P intercept logic for a more, shall we say, ...leisurely... turning capability?

[GearDownThreeGreen]

So is this just a case of Bombardier building a bigger, heavier, faster version of the Dash-8, and forgetting to reprogram the A/P intercept logic for a more, shall we say, ...leisurely... turning capability?

I believe the avionics for the classics are delivered by Honeywell, whilst the Q400 is delivered by Thales. There should have been plenty of opportunities to create a good design from the start of the Q400.

[PeteB]

I believe it's the fact that when in LOC capture mode, the autopilot will only start to turn when the LOC needle starts to move, and yes, above 180knots the turn radius is too large and the aircraft will shoot through. Experienced pilots are aware of this, and use the following methods to avoid it:

Reduce speed to <185 kts.

Intercept angle of 30 degrees max. (Also, helping out the autopilot by easing it onto the localiser using HDG).

Better still, keep the autopilot in FMS mode, then the autopilot 'knows' when to start the turn to allow for the correct turn radius. Switch to ILS when the aircraft is established on the final approach track, or at least in the turn to intercept.

Having said all of that, the Q400 autopilot often just does a crap job regardless of how much you try to help it out....

[Sirijus]

I started not noticing it rather quickly, it's just one of the Dash's "funny" things..

Some of our pilots turn the aircraft to 15-20 degrees to localizer seconds before the intercept, seems to do the trick

[speedrestriction]

A generous helping of rudder also helps the automatics find the right path a little quicker.

[Alpine Flyer]

As posted, Dash 8-400 avionics have nothing to do with -100/-200/-300 avionics.

The low maximum intercept speed used for the new system by Sextant (now Thales) might have to do with Bombardier giving rather conservative design specs, after all they were trying very hard to make the -400 as close to the older A/C as possible to allow for minimum training converting to the new plane.

(Which IMHO led to a lot of potential for avionics improvement not being used.)

[jmmoric]

Sounds resonable, and much in line what I've seen. They're getting better though :)

Now a more technical question, why haven't they fixed it long ago (the autopilot)? Or is it one of the quirks modern aircrafts gets to live with? :confused:

[ACMS]

Speedrestriction-------"a generous helping of rudder....."

You must be joking surely?

A generous helping of rudder at 180 to 200kts........do you think that's a good thing for the Fin?? How's about Pax comfort??

I can't believe what I'm reading??????

Besides, the A/P would put in opposite Aileron to stop the sudden un commanded roll and thus negate what you tried to achieve wouldn't it??

I'd just love to hear what Bombardier or your Chief Pilot had to say....:D

[Piltdown Man]

Just out of interest, who is the manufacturer of the A/P software? Honeywell by any chance?

[RHS]

ACMS I don't think Speed restriction is proposing kicking the rudder round, but when it starts the intercept turn, keeping the feet in the loop and applying pressure to keep it coordinated certainly helps it around.

Certainly though, if you receive a late intercept heading, other types can deal with it. Our beloved dash, nope!

[despegue]

Can't you help her by reducing the intercept angle just before capture?

Of course you overshoot if you let capt. autopilot do all the work.

Proactiveness is vital on any type.

[RHS]

You can and a lot of people do just that, but seriously sometimes even then it can't cope that well.

I don't think there's many people who fly the dash who trust "captain autopilot" too much, the autopilot isn't great at all.

[dcoded]

haha.. We have the same problems some times on the CRJ as well!!
:8

[Tu.114]

If only it just overshot the localizer a bit...

Such an overshoot is usually followed by 2 or 3 oscillations about the localizer that may occasionally even put the aircraft beyond the one dot left or right of track, disallowing a descent on the glide. If "helping" the aircraft on the localizer by a twist on the heading bug did not cut it this time and such a situation is to be expected, using TCS or completely switching off the autopilot to manually establish the aircraft within the tolerances (disregarding the flight director for a moment) is the only way to avoid a go around.

The intercept algorithm is nothing Bombardier should be proud of.

[josephfeatherweight]

Similar problem on GLEX, does a terrible job at LOC capture. CL604 handled capture better but did a shocking job maintaining if there was a strong-ish crosswind. Maybe Bombardier needs some work in this department...

[Jwscud]

The Lear 40/45 identical. One wonders if Bombardier use the same logic on all types to save cash, and ignore the fact that the original program they're using is rubbish.