EMB-175 flight crew reported deviating from an ATC clearance in an effort to avoid a wake turbulence encounter.
Synopsis
EMB-175 flight crew reported deviating from an ATC clearance in an effort to avoid a wake turbulence encounter.
Narrative
While descending on IRNMN2 arrival near CLIFY we were given a clearance to descend unrestricted to 2600. Initially we were descending about 1700 FPM. As our downwind was being extended farther and farther I began to decrease our rate of descent. My concern was joining final well below aircraft that were on final. I decreased our descent rate to 1000 FPM; then 500 FPM; then 300 FPM. On our base leg at 3200 our descent rate was 0 FPM. ATC told us to maintain 2600 ft. until established and cleared for ILS 24R. ATC queried us about why we seemed to be level. We told ATC it was for wake turbulence avoidance. After joining the localizer I descended about 500 FPM to 2600 ft. then intercepted the glideslope. I had a misunderstanding of the climb/descent rate requirement in the AIM. I thought ATC assumed at least a 500 FPM climb or descent rate but since we initiated a higher descent rate then I could temporarily reduce to less than 500 FPM since the average rate was still at least 500 FPM. I understand now after I reviewed the AIM that ATC expects a normal rate of descent until within 1;000 ft. of assigned altitude then a descent rate of 500 FPM to 1500 FPM. And if unable advise ATC. And that is where I failed as well. I should have requested a higher altitude to avoid wake turbulence. I should have verbalized my concern. We had just encountered wake turbulence earlier that day at FL320. So that wake turbulence encounter was fresh in my mind. [I suggest to] be assertive. Speak up and voice concerns to ATC. Also; ATC should not descend aircraft to 2600 on a 15 mile final which puts them 2000 ft. below other aircraft on final.
Second reporter narrative
On a right downwind at 6000 ft.; Socal Approach cleared us 'unrestricted descent' to an altitude (I believe 2200 ft.). We set 2200 and verified 2200 properly. I was Pilot Monitoring. Pilot Flying verbalized apprehension about descending to such a low altitude so far out from the airport. We are both familiar with LAX and the wake turbulence threat on the STAR and approaches; even behind something as small as an A320. We elected to shallow out our descent and stay higher on profile than descending immediately to 2200 since we were so far out from the field and WAY below GP. Around 3200 ft. we nearly halted out descent altogether (~0 FPM down) and Approach pimped us that they noticed we had been level at 3200 ft. for approx. 2 min. I responded we were performing wake turbulence avoidance. ATC said 'wake turbulence?' I said; 'in trail of any heavies'. ATC said 'you aren't behind a heavy'. ATC then said the normal: '170 to FAF; contact Tower' I could have been clearer that it wasn't necessarily only a *heavy* we were worried about (since I've felt rough turbulence behind an A320; and many have warned me the Airbus produces significant wake to be wary of as well). That could have justified our position better.I could have told Approach we'd like to level off to stay above preceding traffic GP for wake turbulence avoidance. Approach descended us low; very far out from the airport. Approach also probably isn't aware that ERJ pilots are wary of being behind even an Airbus. Also; the phrase 'unrestricted descent' sounded to me like Pilot Discretion descent to 2200. I believe even with a P.D. Descent; once the descent is commenced; intermediate level-offs aren't allowed? Therefore we should have kept up a 500 FPM descent at least once we began the descent to 2200 ft.; or asked for permission to level off.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.