A Great Mission Comes to an End
We just spent the last week at the European Glider Organization (EGO) meeting in La Spezia, Italy. It was early Monday morning, the day after we arrived, that RU17 woke us up in the middle of the night with a call to our blackberries. Dockserver had just sent us the warning "Glider: ru17 Event: ABORT".
A glider abort doesn't necessarilly mean the sky is falling. What it does mean is that the glider figured out that something is not right, and rather than trying to deal with the problem alone, the glider decides to come to the surface so we can work on it together. But the message Dockserver was relaying to us from RU17 wasn't good. In glider-speak, RU17 sent the following line with the abort:
ABORT HISTORY: last abort cause: MS_ABORT_LEAK
One of RU17's internal sensors had detected a leak, and it was aborting the mission to return to the surface until we told it otherwise.
Now the leak detect sensors are very sensitive, and with good reason. Any small amount of seawater can cause electrical systems to short and fail. We have seen leaks before, and have continued to fly the glider, adjusting the flight characteristics to shallow up the dives and climbs to keep whatever water drops that were rolling around inside away from the electronics. In the past we have been able to keep a glider flying towards its recovery point by this method. It all depends on your glider's location when you find the leak and how much risk you are willing to accept. If it is a local deployment, we just keep the glider at the surface and send a boat out the next day - minizing risk. If it is far away, you sometimes have to accept the risk and fly towards a pickup point that is within range. In the process of doing this, we have learned the flying tricks of monitoring the vacuum and leak detect voltages to minimize impact on the electronics.
RU17 then sent us a second message from the surface that included the following lines.
sensor:m_vacuum(inHg)=8.12776288306713 5.158 secs ago sensor:m_leakdetect_voltage(volts)=1.05079366266727 52.628 secs ago
The good news was the internal vacuum was holding. That meant the airbag was holding. The airbag has to inflate and deflate for every surfacing. Thats a lot of motion over 5 months with the potential for wear points. If we have a leak, the airbag is always the first suspect. But the internal vacuum was holding, so the airbag was likely intact. That's very good, because the airbag is also our life ring. Just like your lifejacket, it gives you that extra added buoyancy to keep your head (in our case the communication antenna) above water.
The bad news was the leak detect voltage was down to 1.05. Usually the voltage on the leak detect sensor is rock solid around 2.48 volts. Any rolling drop of water may move it a few hundreths or even a few tenths. But here it was already down to 1.05. Even though we know there is no direct correlation between the amount of water inside and the magnitude of the voltage drop, and that by definition, any leak in a submarine at sea is bad, the severity of this drop in voltage stopped me dead. That one number spelled trouble.
Since we now had RU17 at the surface, we may as well spend a little Iridium money and start downloading the engineering files to determine what happened, and when the leak detect was triggered. The crew at Rutgers downloaded the most recent "dbd" file from RU17 with all the engineering data. Here is the plot they sent to us in Italy. It shows a trace of depth (black) and leak detect voltage in red. Note that the leak detect is multiplied by 10 so it can be displayed on the same axis. The plot indicates that as RU17 started this segment, it was heading down towards 100 m depth and the leak detect voltage remained constant at 2.48 volts. RU17 hit the inflection point at about 98 m, and started heading back up. About 11 minutes into the ascent, at a depth of about 47 m, the leak detect voltage changes, and within 2 minutes drops to about 1.1 to 1.05. RU17 triggers a leak detect abort, and heads to the surface.
With this news the rescue mission was on. It was now monday in Europe and we could not have been in a better place. The entire European glider community was assembled in Italy. Clayton Jones from Webb Research was with us. We downloaded more data from RU17 to try to get a better handle on why we were leaking. Nothing in the previous records looked odd. Emanuel Coelho from NRL Stennis was with us in Italy, and he helped us with contacts in the Azores. The University of the Azores research vessel, the R/V Arquipelago (http://www.horta.uac.pt/) was perfect and was available. It was a 1.5 day steam from its home port to RU17's present position.
RU17 was drifting north. The altimtery below showed the situation. It was in the northward currents between a warm eddy to the east and a cold eddy to the west. It would continue to drift north and then whip around one of these eddies. If it stayed with the warm eddy, it would whip around closer to the Azores. If it stayed with the cold eddy, it would drift farther away. So drift was setting a time frame for us. We had about a week before RU17 would drift to the northern side of these eddies and turn east or west. Since RU17 looked to be favoring the cold eddy and the turn to the east, we wanted to be out there before it made that turn. Chip was in the U.S. and could fly out to the Azores to meet the boat. Dave was in Italy with us and a full load of glider recovery gear for the EGO glider school. He could leave from Italy and head to the Azores. Whoever got there first would head out.
That left us on shore to monitor the trends. But the news was not good. The leak detect was not holding steady. Instead it continued to drop. By tuesday it was down to 0.42 volts. We were getting very good at coming up with scenarios that could explain the continuing drop in voltage that did not include the obvious explanation that more and more water was somehow getting in. We were at dinner on tuesday night when Hugh called from Rutgers. We had lost communications with RU17. Oscar and Josh later told me that in that one second, they watched me age 10 years. RU17, my constant companion for 5 months, had gone silent. And we were quickly running out of tricks.
Our next hope was the emergency ballast weight. If RU17 looses both communications and gps signals, it assumes it is somehow stuck underwater, and it blows its emergency ballast weight. 500 grams of lift. In a world measured in grams, this provides a big push to the surface. If the seawater had not damaged RU17's electronics, and it could still think, in 16 hours it would eject its ballast weight and return to the surface.
It was a restless night for most. Email traffic persisted through the night. People would wake up, trade emails, and try to get back to sleep. No sign.
The 16 hour time period without communications was set to expire the next day, Wednesday, just before the lunch break at the EGO meeting in Italy. We all gathered around the Dockserver station we set up for NATO for use in the glider training school. 16 hours came. no comms from RU17. 17 hours came. no comms. We went to lunch.
There were still several scenarios that could have RU17 at the surface, but with a damaged Iridium satellite phone so it could not call us. In this case the emergency ARGOS location system is on a separate circut, and the next time a satellite passed overhead, we could get an emergency transmission with a location. We have also found gliders by this method in the past. The Argos system gives you an approximate location within a few kilometers with a few hour delay. Anyone trying to find something in the ocean with this information knows how difficult it is. But once you get close, we have something called a Gonio. Its a direction finding antenna that you tune to the ARGOS broadcast frequency that you can then use to steer the boat in to close the last few kilometers on the glider. We keep 2 Gonios in the lab for just this purpose. They have paid for themselves many times over. But after another full day of monitoring ARGOS, still no reports from RU17.
The EGO meeting and glider school ended yesterday, and now we are back home in the States. We'll continue to watch and wait through the weekend. Yes, stranger things have happened. We always keep in mind the Oregon State experience where a lost glider suddenly reappeared several days later and phoned home. We'll let these next few days pass, and see if we get anything. If nothing new happens, we'll likely close out this mission sometime early next week. At that point we'll celebrate RU17's many successes, and start the build on the next glider that our students will launch towards Europe in 2009.