B737-800 flight crew reported an engine overheat light illuminated during taxi; prompting the crew to shut down the engine and return to the gate.
Synopsis
B737-800 flight crew reported an engine overheat light illuminated during taxi; prompting the crew to shut down the engine and return to the gate.
Narrative
While approaching number two for departure to runway XXL in ZZZ; we noted an amber OVHT/DET light illuminated on the Captain's recall panel which directed us to note the ENG 1 OVHT light illuminated on the fire detection panel. There were no abnormal engine indications; and no other indications on the panels; such as a fire bell; illuminated fire switch; etc. The Captain called for the QRC; and I began reading the checklist. I read the ENGINE OVERHEAT checklist; which had us disengage the auto throttles; close the number one thrust lever. The ENG 1 OVHT light remained illuminated; which directed us to the Engine Fire on the Ground checklist. Per the checklist; we advised the cabin; (had them check the number 1 engine from the window) put number 1 engine start lever to cutoff; and pulled the number 1 fire switch. At step X of the Engine Fire on the Ground checklist; the Captain advised me not to blow the bottle; that this was a scenario they had observed in training that said not to blow the bottle into an engine that was not on fire. The Captain verified with the cabin crew there was no indication of fire visually from the window; and called Maintenance Control for clarification. Maintenance Control said not to blow the bottle; that the checklist didn't say to blow the fire bottle on the ground; to make deferral item entry and return to the gate. During this exchange; the light extinguished and we returned to the gate.There seem to be discrepancies in a training scenario the Captain experienced recently; and direction from Maintenance Control. If it is the position of the maintenance team that we shouldn't discharge fire suppressant into the engine under this situation; it seems the checklist says otherwise; and this was confusing. I felt we were in a situation that could be imminently leading to an engine fire; and had no way of knowing whether there was a fire that wasn't being indicated in another way. The Captain was sure the training received recently trained not to blow the bottle; which appears to contradict the checklist. Luckily the light extinguished and there was no evidence of fire; but procedurally there seemed to be a discrepancy.
Second reporter narrative
After a long taxi of 45mins to departure runway XXL at [taxiway] 1 following at a distance an RJ; about #2 for takeoff; the caution panel illuminated resulting in discovering #1 Eng overheat light was illuminated. Highly unusual for just sitting there at idle.Following the QRC for engine overheat the throttle was confirmed closed; where it had been. CA then called the cabin to tell them we were going to be shutting down left engine and returning to gate but could they check it to see if there was anything unusual. Since the overheat light was staying on went to QRC Y as directed by QRC X. During the engine shutdown procedure the cabin called back to inform that nothing looked unusual and had looked at both ends of the engine. So when we got to step X; with the checklist directing rotate Engine Fire Swt and hold for 1 sec; CA said let's not blow fire retardant all over the engine when there is no indication of a fire from the handle not being illuminated and cabin crew verifying nothing is unusual. The typical timing for a light to go out is thirty seconds; which the FO mentioned and was the source of his discomfort with the decision to not blow the bottle. Had a discussion about which engine fire checklist we should have been addressing in the first place; X vs Z. Discussed how the sequencing in both has you rotating the fire switch with an overheat light staying illuminated without any reference to an acceptable wait time for cooling. Having just attended training; this scenario was presented with the instructor narrative being; 'don't blow the bottle if it isn't on fire when you are on the ground'. Since the FO wasn't comfortable with not following the checklist which keeps you on the QRC and plainly describes they want the fire handle rotated; I called Maintenance Control through Dispatcher and asked for them to confirm they did not want the fire bottle discharged on the ground for engine overheat light? Their response was it never should be discharged on the ground for that; or that the checklist does not direct you to blow the bottle. I was in the process of explaining the 'remaining on' concept when the light went out. Told Maintenance light went out; but it had been at least 3 mins since shutting down the engine. And then he said 'put it in the book; I hope you are returning to the gate'. We had already told ATC we were returning to the gate and were waiting for one to be assigned before the phone call. The taxi to the gate was uneventful. Maintenance came out to the aircraft; explained to them what had happened and one mechanic claimed it was a 'common thing for long taxis'. I hope not. But following up after the fact it appears there was no cause; nor fix done; just a reset. This allowed the aircraft to be released quickly as the bottle was not discharged.If the company does not want the fire bottle discharged on the ground for an engine overheat light there needs to be more guidance on the time allowed for letting it cool down. It took a good 3 to 5 minutes after engine shutdown for this light to go out. On the QRC for engine overheat there might be a decision tree for on the ground/inflight that doesn't lead to the Engine Fire procedure with the bottle discharge without fire verification?If they do want the bottle discharged; they should leave the checklist as is and not have a training scenario that is in opposition to the checklist.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.