B737-700 flight crew reported low engine oil quantity in cruise. Flight crew diverted and landed uneventfully.
Synopsis
B737-700 flight crew reported low engine oil quantity in cruise. Flight crew diverted and landed uneventfully.
Narrative
Enroute from ZZZ [to] ZZZ1 we noticed a low oil quantity indication while at cruise. The #1 Engine was showing around 14% oil quantity. On the previous leg we noticed Engine #1 was at 76% oil quantity while #2 was at 98%. We elected to have the oil serviced at ZZZ. Once we noticed #1 was once again low; we realized there was a potential oil leak. We were directly between ZZZ2 and ZZZ3 and decided to divert to ZZZ3. As the Pilot Flying; I disconnected the autothrottles and brought Engine #1 to idle in an attempt to preserve oil quantity. I requested the F/O (First Officer) run landing with one engine inoperative landing data for ZZZ3 Runway XXR. While in the descent we ran the landing with One Engine Inoperative Landing Checklist; but elected to keep engine running at idle as long as it had some quantity. We made an idle descent all the way to the final approach. The oil quantity continued to slowly decrease. Just inside of the FAF it was showing almost 0. We landed with Flaps 15 and Engine #1 at idle. There was no low oil pressure or other negative engine indications prior to landing. I shut down #1 Engine shortly after clearing the runway. We then ran the Brake Cooling Checklist. Cooling was not required.I was surprised there was no low oil quantity checklist in the QRH. I was also surprised to see very little guidance regarding low oil quantity in the manuals. The way it reads we could have initially continued to ZZZ1. I believe if we didn't make a timely landing; engine damage would have occurred.
Second reporter narrative
I was the First Officer and PM for this leg. This was my first trip after completing IOE and the second day of the trip. In cruise; I noticed a low oil quantity indication and alerted the Captain. He called for a low oil QRH and after a few minutes of both of us looking we determined there was no QRH procedure for our situation. We briefly considered a faulty gauge but decided that it didn't matter and that an immediate diversion was necessary as an oil leak could lead to catastrophic engine failure. We considered ZZZ2 to the NNW and ZZZ3 due west. I am very familiar with the airspace; airport; and general operations of the ZZZ3 area. For that reason and also because we both thought that there was more likely to be available gates; services; and potentially more readily available Maintenance we requested a turn and descent to ZZZ3 due to low oil quantity on the #1 engine. ATC turned us and gave a lower altitude. The Captain was PF (Pilot Flying) and remained so for the flight and so directed me to run standard and single engine landing performance. He also called for the one engine inop QRH. At the same time; we started working through the diversion tasks. Everything happened extraordinarily quickly; and the Captain kept the engine at idle all in an attempt to get the aircraft on the ground before anything developed further. He elected to keep the engine running and at idle in case we needed additional thrust at any point. Oil quantity dropped the entire way to ZZZ3 and even displayed 0 percent on short final. In the process of running; he QRH and rushing to get the other tasks done I began reading the No engine bleeds section which talks about using the APU for bleed air. This confused me and I asked the Captain if I should start the APU to which he replied yes before I realized that it was a section of the QRH that didn't apply to our situation. He said yes because there were other reasons to have it running and virtually no negative reasons to keep us from starting it; but technically I made a mistake in suggesting we should start it to comply with the QRH. We set the aircraft up for landing on [Runway] XXR and entered the single engine data; performed a quick briefing; completed the QRH; briefed Flight Attendants; and talked about a couple of contingency plans. The approach and landing were uneventful; and the Captain immediately shut down the engine with the leak on landing. We ran brake cooling and taxied in the to the gate.As far as the oil leak goes; I don't know. The leg prior to the leak we had noticed the oil had dropped a few percent from our departure to ZZZ flight and since we were in a base the Captain requested a top off for the oil. On my walkaround in ZZZ I did not notice any oil leaking or covering any part of the engine. On my walkaround in ZZZ3; I also did not notice any oil dripping or spilled on the engine; but Maintenance personnel informed us the bottom of the cowling had oil on it. I think I would have had to be on hands and knees to be able to see the oil that had come out of the engine but the lack of any leaking or dripping could have caused me to mistake it for a spill while filling or some oil that blew out in normal operations. For the incorrect QRH usage related to the APU I simply needed to slow down. We didn't really have the ability to create time but slowing down just a little bit likely would have trapped that error.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.