Narrative:

At cruise, 2 1/2 hours into the flight at FL390, we got a low pressure intermittent light on the #1 engine driven hydraulic pump. After about 1-2 mins, it changed to a continuous low pressure light. No quantity or pressure anomalies. Turned pump off according to the QRH. While referring to the information in the QRH, the #2 engine driven pump low pressure light came on steady again, no quantity or pressure problems, and we turned #2 off also. I called ZZZ operations for a patch to dispatch. When dispatch came on, I informed him that we had an unusual hydraulic problem and asked him to get maintenance control on the line. When maintenance got on the line, I described the sequence of events. Maintenance control immediately said 'yeah, that's a common problem on the 700 after a long time at high altitude.' he said that moisture gets into the 'regulators' (?) and eventually freezes up, causing low pressure lights. He said that the pumps should operate normally once we descend to a lower altitude. We requested FL280 and after cruising to FL280 for less than 5 mins we got both pumps back and proceeded to ZZZ1 at the lower altitude. On arrival in ZZZ1, maintenance met the plane. The maintenance guy has a procedure that he had received from maintenance control for draining the moisture from the regulators. He had never heard of this 'common problem' either. I've discussed this at every plane swap and debriefing and have found one other instance of this occurring. I discussed with the equipment training instructor's (new guy) on monday and he said he had heard of it and he understood that the 'B' pumps could be lost for the same reason. Callback conversation with reporter revealed the following information: the reporter stated with both low pressure warning lights illuminated the pressure and quantity was normal. The reporter said they were not certain that hydraulic pressure was available from the engine pumps as the electric motor pumps may have switched on automatically. The reporter stated after descent to FL280 temperature -7 degrees C, the lights went out. The reporter said this B737-700 fleet has other reports of this engine driven hydraulic pump low pressure warning lights on at high altitude with low temperatures. The reporter said the problem is being investigated by boeing aircraft. The reporter stated what corrective maintenance action was made at ZZZ after landing is unknown.

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Original NASA ASRS Text

Title: A B737-700 IN CRUISE AT FL390 OUTSIDE AIR TEMP -39 DEGS C HAD BOTH L AND R ENG DRIVEN HYD LOW PRESSURE WARNING LIGHTS ON CONTINUOUSLY. PRESSURE AND QUANTITY INDICATION NORMAL.

Narrative: AT CRUISE, 2 1/2 HRS INTO THE FLT AT FL390, WE GOT A LOW PRESSURE INTERMITTENT LIGHT ON THE #1 ENG DRIVEN HYD PUMP. AFTER ABOUT 1-2 MINS, IT CHANGED TO A CONTINUOUS LOW PRESSURE LIGHT. NO QUANTITY OR PRESSURE ANOMALIES. TURNED PUMP OFF ACCORDING TO THE QRH. WHILE REFERRING TO THE INFO IN THE QRH, THE #2 ENG DRIVEN PUMP LOW PRESSURE LIGHT CAME ON STEADY AGAIN, NO QUANTITY OR PRESSURE PROBS, AND WE TURNED #2 OFF ALSO. I CALLED ZZZ OPS FOR A PATCH TO DISPATCH. WHEN DISPATCH CAME ON, I INFORMED HIM THAT WE HAD AN UNUSUAL HYD PROB AND ASKED HIM TO GET MAINT CTL ON THE LINE. WHEN MAINT GOT ON THE LINE, I DESCRIBED THE SEQUENCE OF EVENTS. MAINT CTL IMMEDIATELY SAID 'YEAH, THAT'S A COMMON PROB ON THE 700 AFTER A LONG TIME AT HIGH ALT.' HE SAID THAT MOISTURE GETS INTO THE 'REGULATORS' (?) AND EVENTUALLY FREEZES UP, CAUSING LOW PRESSURE LIGHTS. HE SAID THAT THE PUMPS SHOULD OPERATE NORMALLY ONCE WE DSND TO A LOWER ALT. WE REQUESTED FL280 AND AFTER CRUISING TO FL280 FOR LESS THAN 5 MINS WE GOT BOTH PUMPS BACK AND PROCEEDED TO ZZZ1 AT THE LOWER ALT. ON ARR IN ZZZ1, MAINT MET THE PLANE. THE MAINT GUY HAS A PROC THAT HE HAD RECEIVED FROM MAINT CTL FOR DRAINING THE MOISTURE FROM THE REGULATORS. HE HAD NEVER HEARD OF THIS 'COMMON PROB' EITHER. I'VE DISCUSSED THIS AT EVERY PLANE SWAP AND DEBRIEFING AND HAVE FOUND ONE OTHER INSTANCE OF THIS OCCURRING. I DISCUSSED WITH THE EQUIP TRAINING INSTRUCTOR'S (NEW GUY) ON MONDAY AND HE SAID HE HAD HEARD OF IT AND HE UNDERSTOOD THAT THE 'B' PUMPS COULD BE LOST FOR THE SAME REASON. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED WITH BOTH LOW PRESSURE WARNING LIGHTS ILLUMINATED THE PRESSURE AND QUANTITY WAS NORMAL. THE RPTR SAID THEY WERE NOT CERTAIN THAT HYD PRESSURE WAS AVAILABLE FROM THE ENG PUMPS AS THE ELECTRIC MOTOR PUMPS MAY HAVE SWITCHED ON AUTOMATICALLY. THE RPTR STATED AFTER DSCNT TO FL280 TEMP -7 DEGS C, THE LIGHTS WENT OUT. THE RPTR SAID THIS B737-700 FLEET HAS OTHER RPTS OF THIS ENG DRIVEN HYD PUMP LOW PRESSURE WARNING LIGHTS ON AT HIGH ALT WITH LOW TEMPS. THE RPTR SAID THE PROB IS BEING INVESTIGATED BY BOEING ACFT. THE RPTR STATED WHAT CORRECTIVE MAINT ACTION WAS MADE AT ZZZ AFTER LNDG IS UNKNOWN.

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.