B737 flight crew reported an altitude deviation on arrival into ORD; citing fatigue; late runway change; and a wake turbulence encounter as contributing.
Synopsis
B737 flight crew reported an altitude deviation on arrival into ORD; citing fatigue; late runway change; and a wake turbulence encounter as contributing.
Narrative
On the arrival into ORD after flying the redeye in; we were flying on the FYTTE 7 arrival and had briefed the approach initially for runway 09L. Below 18;000 ft; we began to hit significant wake turbulence from a proceeding 747 ahead in which we inquired about. Upon checking in with ORD approach we were assigned the RNAV 10C. PM loaded and confirmed the approach. We briefed the new approach and continued on vectors where we still hit significant wake turbulence from the 747 ahead. Approached cleared us to cross KYLAA at 7;000 cleared for the RNAV 10C. Autopilot was on; approached was armed and the aircraft intercepted the course normally and touchdown zone elevation was set. As the aircraft began to descent in VNAV Path; I looked on the chart to verify what the DME was for KYLAA at that point realizing there was no way to find DME since we were on an RNAV arrival and the full approach was not loaded in the FMC so KYLAA was not depicted on the approach. The Approach Controller then inquired about what altitude we were descending to; told us to turn level off at 6;000 ft. Upon leveling off we were then cleared for the visual to 10C where a normal landing occurred. I believe some of the contributing factors were fatigue from the redeye flight; dealing with the wake turbulence; expecting a different runway/different approach than normally used for the assigned runway as well as not realizing the initial fix we were cleared to cross was in fact not loaded in the FMC.
Second reporter narrative
Coming in on a redeye on the FYTTE 7 STAR. ORD Advertising ILS to 9L as well as RNAV 10C. We opted to set up for the ILS 9L. Chicago Center had us in trail of a B747. The numerous incidents of being affected by the B747's wake became a distraction that continued throughout the arrival and approach. Chicago Approach; on initial contact; assigned up the RNAV 10C. As PM; I set up the FMC but did not select any type of transition. We were rushed/interrupted due to multiple calls from approach with various instructions as well as distracted by the continuous encounters with the B747 wake. We heard approach query a flight that was two or three ahead of us as to their altitude being low for where they were on the approach. The FO and I quickly discussed that and said we'd use DME (as we normally would do on an ILS) to ensure we crossed the various fixes at the charted altitudes. FO asked me to extend SHARN. Approach assigned us a heading to intercept and to maintain 7;000 until KYLAA. We intercepted the LNAV course and went straight to rote procedural steps of setting TDZE and selecting VNAV without ensuring the other fixes (KYLAA and PEPAW) were in the legs page and became a bit perplexed (most likely due to fatigue from the redeye) that we didn't have DME to reference the fixes mentioned. By the time we figured out we had started down too early; approach control admonished us for descending too early; assigned us 6;000 ft and subsequently assigned us the visual to 10C when we reported the airport in sight. Rest of approach; landing; etc.; uneventful. We wrongfully assumed that we'd have DME as a backup to identify the fixes on the approach to ensure altitude compliance. Ultimately it is our responsibility to correctly fly the approach however; the numerous radio calls from approach (after assigning an approach we didn't plan;) the high number of 'bites' from the preceding B747; fatigue; and an assumption that we would have some sort of DME backup were factors in this occasion.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.