A couple of days ago I was approaching CYVR’s runway 08R in a Boeing 772, and was about to activate APPR mode to follow the ILS. The CRS1 was set to 83 degrees for the ILS.
I disengage VNAV when I reached 3000 ft. My GPS flight plan took me 3 nm south of the ILS ‘cone’ tip. I disengage LNAV shortly before I arrived at the point where I begin my base turn to intercept the LOC. I change source to NAV1 (ILS 08R freq). I wait to finally cross the “base turn point,” and then I manually adjust the HDG for a right turn to 50 degrees to intercept the LOC at 3000 ft. The ILS approach course for 08R was 83 deg. This gives me an intercept angle of 33 degrees (ik proper ‘max’ is 30 deg but close enough)
My plane is now stable at HDG 50 deg, 188 kts IAS, 3000ft. I tap “APPR” and suddenly my plane veers to the right (i dont mean adjusts a little bit to get an intercept of 30 deg–it was a sudden bank of 20,25 deg and never levelled). I have to disengage autopilot as the plane was turning away from the ILS and entering an uncontrolled trajectory. I then manually turn the plane 360 degrees to return to my base leg for a second attempt, this time smooth.
Hope I described it well…what just happened? Why did the plane go AWOL when all looked good the first time around. Stable SPD, HDG, ALT but yet when I engaged approach it turned unexpectedly. Remember I disengaged VNAV and LNAV before changing navigation source from GPS to NAV1 the first time. Then I hit APPR when I thought I was looking like a stable intercept.
Not sure if I had LNAV when activating APPR on the second time around tho.
if u have any questions just say so in replies; I’ll try to add any info necessary