Intro

New E175 pilots will at some point experience a small lurch and either a light feeling or getting pushed into their seats somewhere outside the final approach fix when on an autopilot-coupled approach. I’ve heard pilots nickname this behavior the “hobby horse,” drunken sailor,” “pirate ship,” and probably a few more that I can’t remember. It’s a somewhat annoying quirk of the E175 that many find uncomfortable. Fortunately, there’s a good explanation for the behavior and some easy ways to mitigate it.

Why does it do this?

In most arrival-to-approach scenarios, the airplane is descending in PATH mode, where the lateral track is defined by the FMS and the vertical track is defined by the baro-VNAV system. When the armed vertical mode is either an ILS glideslope (GS) or an LPV, that next descent segment is vertically defined by a reference to the ground (either a glideslope antenna, or a WAAS-refined coordinate path on the LPV). If the temperature is substantially warmer or cooler, or the baro pressure is high or low, then the FMS baro path and the ground-based GS or GP will be some vertical distance away from one another. When the airplane detects sufficient proximity to the ground-derived GS or GP (sometimes up to a dot deflection), it will grab onto it, leading to a vertical lurch as it re-establishes itself on that new vertical guidance. If the PATH was below GS/GP, you’ll get a pitch up, followed by a pitch down to intercept. Conversely, a high path will give you a down/up ride (see illustrations below).

Capture behaviors when your path is on, under, or above the GS.

What you see on your PFD right before the capture. Look for the blue preview before you arm APP.

Why is it so aggressive? My guess is that it’s easier to write software that aggressively joins and gets stable further out, versus a more sedate join behavior that either won’t capture with as much vertical deflection or would take longer to do so smoothly and potentially result in a few missed approaches. The folks writing the software were probably trying to account for situations like blustery breeze and Cat II/III coupled approaches and figured that comfort is secondary to safety and consistency.

How can we address it?

The good news is that this behavior is easy to mitigate. Before arming APP mode, check the preview needles. If you see the blue preview GS (or white GP diamond) way off vertically, that’s your clue. There are a couple things you can do:

1. Disconnect the autopilot and hand fly.
2. Hover over the TCS button when you arm the approach, then if you see the GS jump the fence, hit TCS and nudge the yoke to center the GS gradually, then release the TCS and verify that GS and LOC are the active modes. Hit APP again if not.
3. Hit FPA and shallow or steepen the descent about a quarter degree until the preview is centered, then hit APP (see sketches below).
4. Do nothing. If it’s a busy day or you’re hard IMC doing a Cat II/III and you don’t have the mental bandwidth to mess around with it, just let it’s do it’s thing. Sure, it’s a little uncomfortable, but it’ll work out.
   

Using FPV will smoothly get the airplane on top of the preview before you arm APP.

Using FPV will shallow out the join to make it more comfortable.

Conclusion

The aggressive glideslope capture in the E175 is a somewhat annoying, though not unsafe behavior that probably has a good reason for being the way it is. With practice, you’ll get in the habit of anticipating it and mitigating the little lurch.

Does your team have a different nickname for the behavior? PM me your answer and I can add it.