Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Confusion
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Gate; General Flight Cancelled / Delayed; General Maintenance Action
Human Factors; Aircraft; Manuals; Procedure
I am writing this in regards to our low oil temperature we experienced on a Hawker HS-125-800XPC aircraft. What happened was that we were never able to get the #-1 Engine Oil Temperature to rise to at least 30 degrees Celsius which is the start of the green arc as required for operations. This day the outside temperature was -1 degrees Celsius which reflected temperature of the oil.  As per our procedures we started the #-2 Engine and then the #-1 Engine. Because the #-2 engine is started first; the oil temperature starts rising first and usually gets about a 5-7 degree Celsius head start on its way to rise up into the green which is at 30 degrees Celsius. This was the case today as both oil temperatures started at -1 degrees Celsius and were rising with the #-2 engine oil temperature ahead by about 7 degrees.  Now in my 12 years in the Hawkers; we have had different policies on when we could taxi depending on oil temperature. When I was first in the fleet; we could taxi with oil temperature not in the green but were not allowed to take off until oil temperature was in the green. Then at some point it was changed to not bringing the throttle off idle until the oil temperature was in the green. So in essence; unless you could taxi at idle thrust; you were forced to stay in the blocks until the engine oil temperature rose to 30 degrees Celsius which is the start of the green arc. It was like this for many years until the advent of the AOM. When the AOM first came out this was changed back to the old way where you could taxi out with oil temperature not in the green but couldn't takeoff until it was in the green. Then for whatever reason it was changed back to the previous way of having to wait until normal oil temperature until power could be advanced. However; as this affected doing the engine anti-ice check; this check had to be moved to the taxi check. I bring this up because back when the engine anti-ice check was in the after start flow; it mentioned you had to wait until 30 degrees Celsius until you could advance throttle. Then with the change to checking the anti-ice on taxi; most will taxi out when at least one engine oil pressure is in the green and then when the other engine oil pressure is in the green; do the anti-ice check. Therefore; what usually is the case is that pilots including myself will wait until one engine gets the oil temperature in the green at 30 degrees Celsius and taxi out that way by advancing the power on the one engine assuming the power application is minor. Usually this is possible because the number 2 engine oil temperature will get in the green around 2-3 minutes before engine #-1; as engine #-2 is started first. That was the case today. When engine #-2 oil pressure got in the green; we taxied out by advancing engine #-2 power lever just slightly. As engine #-1 was about 23 degrees Celsius and warming; I knew it would eventually get in the green and as I wasn't advancing the power lever on it; I figured I was operating properly. Then as we were taxiing the engine #-1 oil temperature stopped rising at its previous rate and slowed substantially. As we got to the runway; it had only risen to 29 degrees Celsius and was staying there. As engine #-2 oil temperature was at 51 degrees Celsius; and from what I know from past practice; the fact that engine #-1 oil temperature should have matched #-2 I called Maintenance to confer with them. I got Maintenance Controller X and we patched in ACP X and we discussed the situation. I was probably on the phone with them for 10-15 minutes and still during the whole time the engine #-1 oil temperature never rose above 29 degrees Celsius. We decided I should write it up and we went back to the ramp. I need to again emphasize that during this whole time the throttle on engine #-1 was always kept at idle as the engine oil temperature was never at 30 degrees Celsius or higher. We got to the FBO and I wrote it up. I got follow-up from ACP Y that the housing on the oil/fuel heater had cracked allowing cold fuel to mix with the oil.  One of the things that first caught maintenance's eye I guess; was that the oil tank was filled with jet fuel. I then talked to a Hawker Beech mechanic and I got the impression this is a very rare occurrence. One of my concerns however; is what if the oil temperature had just been enough to get into the green. Then; even though the oil temperature varied between engine #-1 and #-2; both being in the green; would have been acceptable for takeoff. I wonder what could have happened? I am also concerned if this oil; being diluted by fuel; could cause engine damage. From what I read in the maintenance notes the oil/fuel heater is being replaced. But I am curious if damage to the engine is looked at with the oil being contaminated? Now; besides filing a safety report over this event with the engine; there is another concern that is the real reason I am filing this. It has to do with the limitation on oil temperature in the AOM. I had explained to my SIC that we do have a limitation on the oil temperature and that it must be in the green to advance the power. As we had both engines running but only engine #-2 oil temperature in the green; we could taxi out as long as I didn't advance power on engine #-1. As we were light and only needed a small bump of the throttle from engine #-2 just to get us moving and then idle thrust was enough to taxi us; that this would work. This is the way I have always done it and have seen many other pilots do this. I felt that the limitations was; you could not advance power until you had oil temperature in the green. Well; my SIC decided to look up the limitation in the AOM when we were back at the ramp.  Imagine my surprise when he said that the limitation is for taxi and not throttle advancement. This must be our Operator's limitation; as this is not in the AFM. So therefore; the act of taxiing with the engine #-1 oil temp not yet in the green was incorrect; regardless if the throttle is kept at idle. I did not know this and would have never done this if I did. I guess I have been in this airplane for so long and with all the back and forth changes I was applying this limitation incorrectly as laid out in the AOM.
Reporter stated the Hawker 125-800XPC aircraft has Honeywell TFE-731 Engines; originally Garrett. Maintenance drained the #-1 Engine oil; changed the oil filter; ran the engine and returned the aircraft to service. This was the second aircraft in two months with a similar problem; but also had oil pressure fluctuations.Reporter stated the taxi-out limitations in the past was driven by the number of incidences of fuel control units blowing internal seals believed to be caused by ice crystals in the fuel; when the engine oil temp was not in the green and the throttles advanced. There were also numerous incidences of warnings in the cockpit for engine oil by-pass caused by the engine oil filter by-pass pin indicator popping and triggering the warnings. All of which was thought to be caused by engine oil temp not being in the 'Green' arc.
A Captain describes two situations involving their Hawker HS-125-800XPC aircraft with Honeywell TFE 731 engines. Pilot noticed the #-1 engine oil temperature would not come up to the minimum 'green' arc during taxi-out that was found to be an internally cracked fuel/oil heater dumping fuel into the oil system. He also explains the history and confusion for pilots between the company's Aircraft Operations Manual (AOM) versus the Manufacturer's Aircraft Flight Manual (AFM) regarding when to taxi with engine oil temperatures not yet in the 'green' arc.