Tuesday, April 30, 2013

2013 Annual Inspection (ii)

This year's Annual Inspection, now completed, was an unusual one. And I mean that in a good way. There was very little "emergent" work resulting from the inspection and most of the work effort was focused on items properly classifiable as "maintenance". Even the one major cost item, the inspection and resealing of the propeller (first discussed here) was not the result of a "squawk". The prop was turning just fine when removed from the airframe.

If you wonder what happens when the propeller shop is told to "IRAN & Reseal" rather than overhaul, the log entry for the propeller work is instructive:

"Propeller disassembled, cleaned and visually inspected for red dye oil leak. Replaced all seals and gaskets. Reassembled, set angles, track and balanced. Propeller filled with red dyed oil and pressure tested."

The engine, at 1749.3 hours Since Major Overhaul (SMOH), seems to be holding up very nicely. The pressure drop data is perfectly acceptable (see the chart at left for seven years of data), the filter media were free of metal particles and there were no worrisome indications found by borescope. An oil sample has been sent off for spectrometric analysis but I don't anticipate any bad news from that quarter.

I flew the airplane for a 1/2 hour local test flight last Thursday evening with the only squawk being that the landing lights (both of which were replaced) were inop. As it happens, there is an in-line connector in the wire harness that supplies power to the landing lights and it has to be unmade to remove the lower cowling. In this case, it didn't get re-made when the cowl was re-installed. That minor glitch was remedied the next day and on Friday I flew N631S down to the DC area for the first time in three weeks. Then we flew back, mostly in rain and clouds, yesterday morning. No new problems showed up so I'm ready to call it done.

The only leftover item – which is an expected issue – is the need to adjust the Shadin Miniflo-L fuel computer to correctly interpret the output of the replaced fuel flow transducer. Every transducer is tested by the manufacturer to determine the number of electrical impulses it emits per gallon of flow passing through and that "K-factor" is supplied with the sensor. For this unit, the K-factor is 19.8, meaning 19,800 pulses per gallon. Right now, the computer is reading about 10% high. To correct the condition it has to be removed from the panel and reprogrammed with the new K-factor. Naturally, this can't be accomplished in situ, as that would be far too easy. So sometime soon my friends at Three Wing Aviation Group (check out their spiffy new web site!) will make the adjustment and all will be well.

I anticipate one more post about this year's Annual wherein I'll analyze the final invoice.

Wednesday, April 10, 2013

Eternal Father, Strong to Save...

On 10 April 1963, fifty years ago today at 9:18AM EST, the Sea enveloped in her cold embrace the 129 men aboard the USS Thresher and took them away from their loved ones forever. It is important that we remember.

I remember. I was 14 at the time and in the habit of listening to Jean Shepard's program on WOR radio from New York before going off to sleep. There was a news bulletin. The Navy was reporting one of its submarines as overdue at Portsmouth Navy Yard. A search was underway. I had no understanding of abyssal deeps and vast pressures, and so I went to sleep with hope. As I'd understand in future years, my hope was unjustified.

Of course, a day or so later, the finality of the loss was made clear. She was gone and her people were gone. The convening of Boards of Inquiry was announced. It was the first loss of a nuclear powered submarine, and in fact the lead ship of a new class, so naturally much was made of the disaster. But little information was forthcoming and of course the events of November of that year drove everything else from the front pages.

I really didn't think very much about Thresher until eight years later, in the summer of 1971 when I went to work as a design engineer at the Electric Boat Division of General Dynamics Corporation. Immediately, I was plunged into the design effort to incorporate modifications known collectively as SUBSAFE on the forthcoming second overhaul of the Lafayette class of ballistic missile submarines. The work embodied in the SUBSAFE program was the direct result of the loss of Thresher.

There are two aspects to SUBSAFE. One is an intensive quality assurance regime encompassing both materials traceability and rigid process controls. The other involves improvements in systems design to ensure that the casualties that are inferred to have led to the loss of Thresher can never again have similar consequences.

Based on the painfully limited evidence, those investigating the ship's loss concluded that the precipitating casualty was the failure of a seawater pipe joint in one of the ship's machinery spaces. The crew was unable to secure the inflow of seawater from this failure. The intense spray, driven by the pressure of the sea at or near Thresher's design depth, would have penetrated electrical control panels and caused the loss of propulsion. (NB: Recently a countervailing theory has emerged which posits that the seawater piping failure is neither likely nor necessary to account for the loss of the ship. The sinking, it holds, could have followed from loss of propulsion due to an electrical casualty compounded by a failure of the ballast tank blow system.)

At the time, the design and operation of the nuclear propulsion plant was governed by a philosophy that focused on saving the reactor at any cost. So any disruption of control circuitry would cause an immediate reactor plant shutdown. But this deprived the ship of the ability to use hydrodynamic forces against the effects of the flooding casualty. Would a few more minutes of propulsion have made a difference? We'll never know.

The last hope would have been use of the ship's high pressure air banks to blow the main ballast tanks on an emergency basis. Testing later showed that the rapid flow of air through the system would have caused plummeting temperatures and formation of ice that would block the piping and prevent the air from reaching the tanks. With no way to deballast, no propulsion, and no way to secure the flooding, Thresher was doomed.

With the adoption of this scenario as "probable cause", the SUBSAFE program set out to address each stage of the cascade of casualties. The quality assurance aspects sought to ensure that all seawater piping systems could always withstand the pressures of the deep. But if that failed, hydraulic remote valve actuation systems were installed (that was the piece of the puzzle I worked on) to ensure that hull and backup seawater valves could be shut to secure flooding. And last, the Emergency Main Ballast Tank Blow system was completely redesigned to ensure that it would be effective under all circumstances.

Outside of the bounds of SUBSAFE, the design and operating philosophies of the Naval Nuclear Propulsion Program were modified to allow ships to maintain propulsion as long as possible in the event of a casualty even if damage to the reactor is a possibility.

Did SUBSAFE accomplish its mission? The evidence says that it did. From 1915 until the loss of the Thresher sixteen US Navy submarines were lost to non-combat causes. Afterward, the only such loss was that of Scorpion in 1968. That ship was not yet SUBSAFE certified.

I am proud to have been involved in a modest way in the implementation of SUBSAFE. And I hope that the surviving relatives and friends of those lost aboard Thresher 50 years ago can find solace in the fact that their loss led directly to a profound reduction in the risk of submarine operations.

Eternal Father, Strong to save,
Whose arm hath bound the restless wave,
Who bid'st the mighty Ocean deep
Its own appointed limits keep;
O hear us when we cry to thee,
for those in peril on the sea.

Tuesday, April 2, 2013

2013 Annual Inspection (i)

Yesterday was the first day of N631S's Annual Inspection availability for 2013. My friends at Three Wing Aviation Group will be doing the inspection and associated work package and this year Tony diNuzzo is the IA on the case. I've known and respected Tony for a long time now, and am looking forward to working with him.
Our last flight up from the DC area had been uneventful and I had very few "squawks" to report. The taxi light bulb had burned out a couple of months ago, and the landing light joined it about a month back, so both of those bulbs will require replacement. And the performance of the Shadin Miniflo-L fuel totalizer has been increasingly erratic, so we'll take this opportunity to replace its transducer with a spare that I acquired a year or so ago.

Beyond those tasks, the most notable work-item planned for this year is an inspection of the McCauley constant speed propeller. The prop was overhauled in conjunction with the 2005 Annual Inspection – the first year N631S spent with us – and so it now has about 1,200 hours and eight years SPOH. McCauley holds that the recommended TBO (Time Between Overhauls) is 2,000 hours or seven years, whichever first elapses.

I feel no compulsion to overhaul components at some arbitrarily selected calendar time or time-in-service, as I'm a firm believer in maintenance "on condition". N631S's Continental O-470U engine has a recommended TBO of 2,000 hours or 12 years. The latter mark has long passed and I will cheerfully let the former pass unremarked as well, so long as the engine is talking to me and saying, "I'm fine, boss...let's go fly." And it will do so through spectroscopic analyses of the oil, visual inspections of the filter media and borescope inspections of the cylinders and valves. Ah, but the propeller presents a different problem.

The propeller is a "black box". It doesn't speak to you and there is no way to know that it is sick until something Very Bad happens. Recently, blogger Ron Rapp did a post (which I endorse in its entirety) on Constant Speed Propeller Maintenance, where he describes some of the Very Bad Things that can happen to a sick propeller with terrifying suddenness. I take these things to heart and view propeller maintenance as something you neglect at your mortal peril.

That said, I don't want to "overhaul" the propeller. In the world of aviation, the word "overhaul" has a very specific (and expensive) meaning. It means that you drag out the manufacturer's overhaul manual and do everything that it says to do. But N631S's prop was overhauled back in 2005 and since then has accumulated about 60% of the hours McCauley assigns for TBO. It's just nicely broken in, from the perspective of hours in service, and I'd argue that an overhaul would be premature. My concern is directed more toward the notion that non-metallic parts wear and rubbery things deteriorate more in step with the calendar than with the Hobbs meter. So I've asked Three Wing to take the prop off and send it to New England Propeller (who did the overhaul in '05) with instructions to "IRAN and Reseal". That means, "Inspect and Repair As Needed (to serviceable condition) and renew all fluid seals". Once that is done, I'll happily run the prop for another 1,000 hours before sending it off for "Overhaul".

I stopped at Three Wing this morning and spoke with Tony. He'd done the maintenance run-up and found nothing noteworthy. The cylinder leakdown test had gone satisfactorily. Cylinder #2 was a bit weak at 59 psi (on a day where the minimum acceptable result was 46 psi). Tony will continue and I'll be visiting N631S every day for a while.