Air carrier flight crew reported their aircraft; picked up from recent maintenance; failed to pressurize during the repositioning ferry flight.
Synopsis
Air carrier flight crew reported their aircraft; picked up from recent maintenance; failed to pressurize during the repositioning ferry flight.
Narrative
I picked up an out of base trip for an a/c coming out of Maintenance Control from ZZZ to ZZZ1 with no passengers and no assigned flight attendants. The aircraft was supposed to be complete and returned to service in the morning; however; it was not ready for departure until the mid-afternoon. The aircraft had executed an air return on the previous flight or a Bleed Trip off and pressurization issue which the maintenance actions had supposedly fixed. The aircraft was being dispatched with a 'One Pack Inoperative - APU Operates Normally' which required the aircraft to execute a Bleeds Off Takeoff and Landing and to operate no higher than FL250. The altitude restriction combined with a significant threat of convective buildups across the majority of the country caused a significant amount of coordination with dispatchers in order to arrive at an acceptable plan to accomplish the flight.Preflight was unremarkable. The brief covered all identified threats; to include ferry procedures for with no PAX or FAs; non-normal taxi location;); MEL required non-normal bleed configuration for takeoff; and enroute weather avoidance. Engine start and taxi from the Maintenance Operation Control was unremarkable. MEL procedures for the Bleeds Off takeoff were accomplished along with the usual checklists. Takeoff was unremarkable. MEL procedures for Bleed Configuration post takeoff were accomplished along with the normal after takeoff checklist.At the 10k' check the CA observed that the cabin was not pressurizing at the normal schedule and instead indicated approximately 8k' and climbing slowly. The CA told me to request a level off from ATC which was granted at 12k'. As we leveled off; the CABIN ALTITUDE Warning light illuminated with the associated altitude horn and a glance at the pressurization panel confirmed that the cabin altitude was also reading 12k'. We requested a descent back down to 10k' from ATC which was granted along with a vector off of the ZZZZZ departure course. We descended and leveled off at 10k' on the assigned heading and engaged the auto pilot. We elected not to execute the immediate action items of donning emergency oxygen masks and establishing crew communications because we had not experienced a rapid decompression; the cabin altitude had never climbed above 13k' where supplemental oxygen use is required by FAR; and in our opinion as CA/FO; the added confusion and barrier to communication would have been more detrimental to the situation than executing the descent expeditiously back to 10k'. (The fasten seatbelt sign was still on and therefore did not need to be turned on.)Once established in level flight at 10k' with the autopilot engaged; The CA passed PF duties to me along; with ATC communications and we received delay vectors from ATC while I began the non-normal checklist. Referencing the digital QRC in the FM on my EFB; with the exception of the Immediate Action discussed above; the Bleed Air switches were verified On; the Pack switches were verified as the right switch on and the left switch off per the dispatched MEL; the Pressurization Mode selector was moved to MAN; and the was held in Close and indicated fully closed. The cabin altitude remained at ambient altitude of 10k'. Since the cabin altitude was not controllable in MAN mode; the CA elected to deviate from the QRC; as we had no passengers on board chose to not deploy the passenger oxygen via the PASS OXY switch. Additionally; since we were already established at 10k' we did not perform an emergency descent; and ATC was already aware of our pressurization issue there was no further need to advise ATC or squawk emergency. Utilizing the hot link in the FM QRC; the CA continued the non-normal procedure and accomplished all of the steps up to the deferred items. At that point dispatch was notified via an ACARS 'Call Me' message followed be a DISP Message with 'Call me MX'. Dispatch and Maintenance Control were contacted via VHF voice and the consensuswas a return to ZZZ was the most prudent course of action. While still on delay vectors from ATC and with plenty of gas; the CA prepped the FMC for the expected approach to ZZZ; briefed the threats; the approach; and the expected taxi; and once ready the CA took the PF role from me; and we began the deferred items descent checklist. We informed ATC that we were complete with our checklists and ready for vectors back to ZZZ. When queried; we told ATC that we did not need to declare an emergency and did not require any assistance once on deck at ZZZ. Additionally; I began to coordinate with ZZZ Operations Control about our unexpected return and the need for either a gate upon arrival or to inform Maintenance Control that we would need to return for follow on maintenance.We received vectors for a visual approach to Runway XXL; backed up by the ILS; with pairing traffic to the parallel runway that once in sight the CA elected to select TA-Only on the TCAS in order to prevent the need for an RA induced Go-Around. The approach was unremarkable. We reconfigured the bleeds per the MEL procedure; and the landing checklist from the remaining non-normal checklist steps; however; the shuffle from one set of procedures to the other we both missed ensuring that the last step 'At pattern altitude: Outflow VALVE switch.... Move to OPEN until the outflow VALVE indication shows fully open to depressurize the aircraft' before accomplishing the landing checklist on the next page. Landing and taxi back to Maintenance Control were unremarkable. Upon shutdown; we discovered that the aircraft was still showing a cabin altitude of 8k' and the pressure differential was preventing us from opening the flight deck windows to speak with the MX personnel outside. After utilizing the MAN mode and the outflow valve to equalize the pressure; we had no further issues.Overall; a very busy and eventful short flight. We worked well as a team with good communication and automation management. My CA did a great job with crew coordination; in both the PM and PF role coordinating what we needed with ATC and backing me up on the book procedures. Where we deviated from the published procedures; in my opinion in was with good reason and helped to safely conclude this flight and we would do it the same way again under similar circumstances. Suggest a resolution post maintenance on a flight critical system such as cabin pressurization should require a better means of verification that the discrepancy has been resolved before releasing the aircraft for flight again especially if the aircraft is going to be utilized with revenue pax. Additionally; we would have benefited from a training scenario where a MEL modifies standard non-normal procedures as a refresher.
Second reporter narrative
[Report narrative contained no additional Information.]
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.