B-737 First Officer reported a hydraulic leak in one hydraulic system during cruise. The flight diverted to an alternate airport; landed; and received a tow from the runway to the gate.
Synopsis
B-737 First Officer reported a hydraulic leak in one hydraulic system during cruise. The flight diverted to an alternate airport; landed; and received a tow from the runway to the gate.
Narrative
While in the cruise phase of flight at FL370; the Master Caution (MC) flickered very quickly illuminating and immediately self extinguishing. Out of normal SOP and training when the MC illuminated; I called out 'My Aircraft' focusing solely on the control of the aircraft as I was the PF. This happens frequently when flying with center tank fuel. The MC will flicker as its approaching empty center fuel tanks triggering the PM to switch the center fuel tank pumps to 'off.' This time however; we still had plenty of fuel in the center tanks making us question why the MC just flickered. My Captain stated to me that there is center fuel and that no abnormal lights are illuminated anywhere in the flight deck on the lower or overhead panels. After paying attention to and determining the flight path of the aircraft was in a desired state/mode under control of the A/P and that the airspeed was exactly on the speed I had programmed for cruise in the VNAV cruise page under control of the A/T; I also began to also investigate what caused the MC to flicker while still giving plenty of attention to the flight path and airspeed of the aircraft. I thought I may have seen something rapidly illuminate and extinguish at the same time the MC did on my recall panel. I pressed on my recall and realized that I believed it was the 'HYD' that quickly flashed in the middle on the left side of my recall panel. After letting go; all recall items and both MC extinguished. This thought of maybe seeing the HYD light flash instigated me to open the 'SYS' page on the lower DU of the MFD where hydraulics information is shown. We both noticed how low the System A quantity was as it was showing 40 and dropping rapidly. We checked and all engine and electric pumps and they were all in the 'ON' position. Knowing the quantity was going to be at 0 within the next minute; I called for the 'Loss of System A QRH checklist.' The Captain began reading out loud the Loss of System A QRH checklist. The quantity was at 0 on System A when beginning the QRH. After step X and placing the A HYD PUMP switches both to 'OFF;' we noted that the quantity then rose to 13. The Captain continued the QRH procedure up until the 'deferred items' which we reviewed to understand what systems are INOP; noted that we have to stop straight ahead on the runway and have gear pins installed and then saved for later when ready to lower the landing gear. While flying the aircraft and managing the flight path and speed; I send a quick and simple message the dispatch; through ACARS; saying what QRH procedure we were working through. By the time we completed the QRH to the deferred items; dispatch was aware and had maintenance ready for the Captain to talk to. I made mention that we have ZZZ and ZZZ1 directly at our 12 o'clock. the Captain and I agreed that we will [request priority handling] but to hold off for just a moment while he called dispatch. I took control of radios while continuing to control the aircraft and the Captain called Dispatch on VHF 2 through AIRINC where he spoke with dispatch and maintenance discussing the situation and discussing a diversion. I did remind the Captain during this the ZZZ2 tower would be closed by the time we would get there making it a non suitable destination to continue to with a [priority handling] aircraft when we have ZZZ1 close by. While the Captain was on VFH 2; I made the required report to ATC and reported that we lost our System A hydraulics; we are not [requesting priority handling] just yet; they can plan on us diverting to ZZZ1; but that I would like to remain at our current altitude and route to ZZZ2 (ATC cleared up direct to ZZZ2 airport awhile back) for the time being but would like to slow to M0.73 to start buying us some time for planning. ATC kindly noted everything and approved for us to slow down. Captain came back on radio and intercom with me and we agreed that ZZZ1 is the most suitable diversion airport since they have ATC; [priority handling] equipment; the weather was good and VFR in ZZZ1; maintenance base for Company X; managers and supervisors to help get our PAX re booked; as well as a 737 base for Company X and could potentially get another plane and pilots to get our PAX continuing into ZZZ2 relatively quickly. We also agreed we will [request priority handling] since we could be spraying hydraulic fluid all over the gear bay and that we will have to stop on the runway after landing and shut engines down since we need maintenance to install gear pins before we move again. I continued to control the aircraft and radios while the Captain began to brief the Flight Attendants and then give a PAX PA. While he was completed these duties; I officially [requested priority handling] with ZZZ Center and informed them that we would like to divert to ZZZ1. I was given direct to ZZZ1 airport and to report souls & FOB. I then requested that ZZZ Center reach out to ZZZ1 App and Tower to see what runway we could expect. I explained that XXC was the only runway showing open in the D-ATIS and NOTAMS and would be shown as closed as well if we landed on that runway since we will be stopping and shutting engines down on the runway. I advised that if ZZZ1 is able to open any other runways for us; we would be happy to accept Runway YYC; YYR; XXL; or XXC. We began a descent and we alerted by the MC and recall panel 'HYD' again. This time looking to the overhead panel; Low Pressure was illuminated on the engine pump for #2. We the completed the QRH for hydraulic pump low pressure which calls for that pump to be placed off. The quantity was rising and stopped at 106. Soon after; it settled at about 94. At this point; the only pump we have 'ON' is the #2 electric pump. While on our descent into ZZZ1; we received a hand off to ZZZ1 App where they informed us that they are opening Runway YYR for us and that we can expect the ILS YYR ZZZ1. I still had us slowed back in speed buying us plenty of time as well as buying our ground ops and maintenance plenty of time so they could have tow equipment ready for us. The captain completed his duties and briefs and took control of the radios again while I continued to be PF. I set up for an briefed the ILS YYR at ZZZ1. We briefed that we would bring the aircraft to a full stop on the runway; stow the boards and set the flaps to 40; set brakes; switch electrics to the APU and shut down both engines; and that we would just let the tow team tow us to the gate from there instead of starting engines again and taxiing to mitigate any further threats. We agreed on a plan to ask Approach control for direct to ZZZZZ and down to 4;000 ft so that we could get to 4;000 ft and get fully configured with the manual gear extension and QRH checklist complete and well as the before landing checklist completed before crossing ZZZZZ inbound. ATC cleared us to 4;000 ft and cleared up direct ZZZZZ. We executed exactly how we planned and had the QRH and Before Landing Checklist complete before ZZZZZ. Upon bring the aircraft to a stop; we did exactly what we briefed and I took control of the radios to talk to ARFF while the Captain made a PA to the cabin. ARFF completed a 360 degree sweep of the aircraft not seeing anything out of the usual and stated that they would hang out there with us while maintenance and tow team completed their duties and that they would then follow us all the way to the gate. Maintenance notified us that there was fluid spraying from the bottom of the #1 engine but that it would be okay to tow to the gate. We were towed to the gate where we were met with supervisors; managers; and maintenance to help with pax deplaning and for reports; AML entries.Cause: There were storms that passed through ZZZ3 before our departure which had us about an hour into the DOT delay clock after pushing back to when we were cleared for takeoff. All fluid levels were normal during the preflight inspections and before start checklist. The new procedure in the FO'sAfter Start Flow is to 'Clear Secondary Engine Indications' in order for them to auto display when a secondary engine parameter exceeds normal operating range. Since this was completed in the after start and we didn't take off for at least another hour; I wonder if it would help to put this step in the 'Below The Line' section of the Before Takeoff Checklist. If there had been a leak on the ground while sitting on the taxiway for an hour with the engines running; there is a possibility we would have noticed this before taking off. When the Captain calls 'Below The Line' on the Before Takeoff Checklist; we could possibly mitigate this by checking the SYS page; checking the ENG page and then selecting the ENG page one more time to blank out the secondaries for takeoff as a flow. This is just a thought I have had since this incident.
Second reporter narrative
While in cruise en route from ZZZ3 to ZZZ2 we noticed the master caution light flickering briefly. The FO guessed that it was related to the hydraulic system and checked the hydraulic quantity as a precaution. We discovered system A quantity was low and dropping rapidly. We were unsure what checklist to run at first but as we discussed it the quantity continued to drop and as it approached 0 we decided that the Loss of System A checklist was appropriate. We accomplished the checklist except for deferred items and I then called dispatch via AIRINC to discuss the possibility of a diversion. After briefly considering continuing to ZZZ2 we decided diverting to ZZZ1 would be the best course of action as the checklist required maintenance to meet the plane on the runway to install gear pins and the tower at ZZZ2 was due to be closed by the time we arrived. During the call Maintenance Control was patched in as well. We also [requested priority handling] with ATC. About 10 minutes after running the checklist; after signing off with dispatch and Maintenance Control and shortly after starting our descent to ZZZ1; the low pressure light for Engine Driven Pump 2 (for hydraulic system B) came on. We found the quantity for system B high and rising; the quantity indication peaking at 106%. We ran the Hydraulic Pump Low Pressure check list which called for shutting off EDP 2. After running the checklist the quantity indication for system B soon settled at 94% and stabilized. The electric pump for system B functioned normally throughout the flight. Also worth noting is that while system A quantity dropped to 0 at one point we found it at 13% shortly after running the Loss of System A checklist and it eventually came up to 20% after landing. As we approached ZZZ1 only runway XXL was open but we were soon informed that Runway YYR would be opened for us and that after landing we could remain on the runway as long as necessary. On approach we lowered the landing gear via the manual gear extension handles as called for by the Loss of System A check list but otherwise the approach and landing were normal. After landing we discussed what needed to be done with ARFF; maintenance; and station personnel who came to meet the aircraft on the runway. I also briefed the passengers and flight attendants about the situation. We started the APU and shut down both engines so maintenance could safely install gear pins. Although it would have been possible to restart the engines and taxi to the gate; operations met the aircraft with a tug so we left the engines off and we were towed to gate X without incident where we deplaned and I wrote up the malfunctions in the AML. At the gate a mechanic mentioned that a substantial amount of hydraulic fluid was leaking from the drain mast on Engine 1.Cause: It was unclear and remains somewhat unclear if running the Loss of System A checklist when we did was the appropriate course of action. We didn't get the indications on the overhead panel consistent with what the checklist shows as an indication of Loss of System A (Low pressure on EDP 1; EMDP2; and Flight control A) but there is no checklist for low hydraulic system quantity or a known hydraulic system leak. This led to some confusion as to what checklist to run but as the hydraulic quantity was rapidly approaching 0 we considered system A to be lost and decided that was the appropriate checklist. I'm still unsure if this was the best course of action as the manual does mention the system A quantity shouldn't drop below 20% due to a leak in EDP 1 or its associated lines because of a standpipe in the System A reservoir. The manual also mentions that fluctuations in the hydraulic quantity indications in flight are normal due to a variety of reasons. While the quantity indication for system A did drop to 0 it rose again after running the Loss of System A checklist and eventually stabilized at 20% after landing. I'm still unsure if system A was truly 'lost' and if that checklist was appropriate. Some clarification on how to handle an obvious hydraulic leak; either via QRC changes or training would be helpful.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.