Narrative:

My copilot and I leveled our aircraft off at FL330 for the first leg of our trip. About 15 mins into cruise, we noticed that the oil pressure on our #1 engine appeared to be lower than normal. We told one another that we'd keep an eye on it and if it became a problem that we would divert. About 5 mins passed and we then noticed that the oil pressure had fallen into the caution zone followed by the red zone. We received the master warning light that comes on when the oil pressure reaches 25 psi followed by the low oil and oil pressure warning lights on the annunciator panel. The copilot was the PF so I told him to continue flying the aircraft while I opened the checklist. I then proceeded to pull the #1 engine to idle so as not to put unnecessary strain on it and to possibly save some of the oil as it was not detrimental to our current flight conditions. I went through the checklist which stated to monitor the engine instruments (ie, the oil temperature and oil pressure gauges), but if the oil pressure went below 25 psi that we would have to shut the engine down. At that point, ZLA had given us a descent to FL290 which we so did. The first officer and I agreed that the best course of action at this time was to divert to palm springs which was about 30 NM to our southwest. I then proceeded to tell center that we needed to divert to palm springs for a precautionary landing. I did not declare an emergency at this time because we still had engine power available. At this point the oil pressure appeared to stabilize at 25 psi and was not going lower as with the oil temperature remaining in the green arc, so we opted to keep the engine running as long as it remained within parameters. We continued our descent and received a visual approach to runway 31L at palm springs. By the time we were lined up on final approach, I noticed the oil pressure to be going down to 20 psi, we opted to keep the engine running until landing, which we did. We performed a flaps 20 degree landing, rolled out, shut down the #1 engine and taxied to the ramp without further incident. After we parked the aircraft, we went outside for a visual inspection. We found oil all along the cowling and tail, the dipstick was dry, and there was oil pouring from the bottom of the cowling. I contacted my maintenance department, charter department, and spoke with the director of operations to advise them of the situation. I squawked the discrepancy, secured the aircraft, and refused to fly it until an inspection was performed. As it turns out, there was a bad o-ring on the oil dipstick cap for the #1 engine. Perhaps there should be a more frequent inspection required to check things like this on learjets.

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Original NASA ASRS Text

Title: A LEARJET 36 EXPERIENCED LOW OIL PRESSURE AT CRUISE ALT AND ELECTED TO MAKE A PRECAUTIONARY LNDG. DIPSTICK SEAL LEAKING.

Narrative: MY COPLT AND I LEVELED OUR ACFT OFF AT FL330 FOR THE FIRST LEG OF OUR TRIP. ABOUT 15 MINS INTO CRUISE, WE NOTICED THAT THE OIL PRESSURE ON OUR #1 ENG APPEARED TO BE LOWER THAN NORMAL. WE TOLD ONE ANOTHER THAT WE'D KEEP AN EYE ON IT AND IF IT BECAME A PROB THAT WE WOULD DIVERT. ABOUT 5 MINS PASSED AND WE THEN NOTICED THAT THE OIL PRESSURE HAD FALLEN INTO THE CAUTION ZONE FOLLOWED BY THE RED ZONE. WE RECEIVED THE MASTER WARNING LIGHT THAT COMES ON WHEN THE OIL PRESSURE REACHES 25 PSI FOLLOWED BY THE LOW OIL AND OIL PRESSURE WARNING LIGHTS ON THE ANNUNCIATOR PANEL. THE COPLT WAS THE PF SO I TOLD HIM TO CONTINUE FLYING THE ACFT WHILE I OPENED THE CHKLIST. I THEN PROCEEDED TO PULL THE #1 ENG TO IDLE SO AS NOT TO PUT UNNECESSARY STRAIN ON IT AND TO POSSIBLY SAVE SOME OF THE OIL AS IT WAS NOT DETRIMENTAL TO OUR CURRENT FLT CONDITIONS. I WENT THROUGH THE CHKLIST WHICH STATED TO MONITOR THE ENG INSTS (IE, THE OIL TEMP AND OIL PRESSURE GAUGES), BUT IF THE OIL PRESSURE WENT BELOW 25 PSI THAT WE WOULD HAVE TO SHUT THE ENG DOWN. AT THAT POINT, ZLA HAD GIVEN US A DSCNT TO FL290 WHICH WE SO DID. THE FO AND I AGREED THAT THE BEST COURSE OF ACTION AT THIS TIME WAS TO DIVERT TO PALM SPRINGS WHICH WAS ABOUT 30 NM TO OUR SW. I THEN PROCEEDED TO TELL CTR THAT WE NEEDED TO DIVERT TO PALM SPRINGS FOR A PRECAUTIONARY LNDG. I DID NOT DECLARE AN EMER AT THIS TIME BECAUSE WE STILL HAD ENG PWR AVAILABLE. AT THIS POINT THE OIL PRESSURE APPEARED TO STABILIZE AT 25 PSI AND WAS NOT GOING LOWER AS WITH THE OIL TEMP REMAINING IN THE GREEN ARC, SO WE OPTED TO KEEP THE ENG RUNNING AS LONG AS IT REMAINED WITHIN PARAMETERS. WE CONTINUED OUR DSCNT AND RECEIVED A VISUAL APCH TO RWY 31L AT PALM SPRINGS. BY THE TIME WE WERE LINED UP ON FINAL APCH, I NOTICED THE OIL PRESSURE TO BE GOING DOWN TO 20 PSI, WE OPTED TO KEEP THE ENG RUNNING UNTIL LNDG, WHICH WE DID. WE PERFORMED A FLAPS 20 DEG LNDG, ROLLED OUT, SHUT DOWN THE #1 ENG AND TAXIED TO THE RAMP WITHOUT FURTHER INCIDENT. AFTER WE PARKED THE ACFT, WE WENT OUTSIDE FOR A VISUAL INSPECTION. WE FOUND OIL ALL ALONG THE COWLING AND TAIL, THE DIPSTICK WAS DRY, AND THERE WAS OIL POURING FROM THE BOTTOM OF THE COWLING. I CONTACTED MY MAINT DEPT, CHARTER DEPT, AND SPOKE WITH THE DIRECTOR OF OPS TO ADVISE THEM OF THE SIT. I SQUAWKED THE DISCREPANCY, SECURED THE ACFT, AND REFUSED TO FLY IT UNTIL AN INSPECTION WAS PERFORMED. AS IT TURNS OUT, THERE WAS A BAD O-RING ON THE OIL DIPSTICK CAP FOR THE #1 ENG. PERHAPS THERE SHOULD BE A MORE FREQUENT INSPECTION REQUIRED TO CHK THINGS LIKE THIS ON LEARJETS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.