What is the purpose of the landing gear hydraulic accumulator?
Such a good question. Really, in some ways, a meta-question. I stare at it on the pink 3×5 flash card I have made and search my mind for the answer.
Long time readers of blog may recognize this image, which I took as Dexter and I approached Denver, Colorado in the summer 2013. It was the end of a long day of flying, which began in Ontario, Canada. It’s possible the more conservative choice would have been to sleep in Nebraska, but both of us enjoyed the motion across the country and being striking distance from home the next day seemed like a great trade off for one more hop in the plane. I admit that I was a little anxious during the last twenty minutes of the flight. Darkness fell as a huge summer thunderstorm slid out of the Denver area to the north. I was already at 10,500 feet, so my engine did not have as much power as it does at sea level, and the controls are a little less effective when the air is thinner. In addition to the plane being away from the center of its performance envelope, I could no longer see the ground. I began to see the lights of houses, but I didn’t know if there was anywhere to glide to if I lost the engine.
We were fine. It was a wonderful night in a very comfortable hotel and a delicious breakfast the next morning before returning to Santa Monica in a couple hops. But while I sat in my comfortable, leather-covered seat two miles above sea level I wondered if I could be less anxious.
I felt that way again during Thanksgiving break later that year when we flew to the Sundance Resort. Landing at Heber City, Utah is non-trivial for a pilot. It is really high altitude (see the above explanations) and to enter the traffic pattern you have flown through a mountain pass that, as Nell said the first time, makes it feel that the sides of the mountains are a few feet from the wing tips. There’s not a lot of space to maneuver in the valley, there are gliders, charter jets, and no air traffic control tower to help you out.
None of that bothered me. I was on my most alert, put the wheels down in the first quarter of the runway and we were stopped before the halfway mark. The fellow after me went around in his Bonanza.
But on the way home, I rode out of that mountain pass wishing for a little more climb performance, and on the long passage over the floor of the desert from Salt Lake City, past St. George, and down to Las Vegas, I was a little anxious. I saw the families in their minivans and station wagons returning from their turkey dinners, sliding along on the Interstate. I scanned for places I would glide to if we lost an engine and a lot of the way the only realistic option seemed to be a piece of flat desert alongside that Interstate. But we’d need to be far enough from the highway that we wouldn’t be tangled in electric or telephone lines. Would we simple hike from the plane to the road and wave someone down? What if one of us was hurt? What if we got turned around and weren’t sure where the highway was? I didn’t like our options.
This past summer I dropped Dexter in Iowa City and did some solo flying around the countryside. At one point I was motoring along at cruising altitude and realized that there were very few lights down there. In fact, I wasn’t sure I could pick out anywhere that I would want to glide to if the engine quit. When I feel that sort of anxiety in the plane it is usually fleeting. I tap my foot a little, look at some of the nearby airports on my iPad, and put on some music. Lucinda Williams is good for that, she’s got her own problems and isn’t bothered by my worries about the airplane, old tractor engine technology, or places I can glide to.
But that was enough. When I got back home I got my multi-engine rating. I wasn’t sure that we were going to get a multi-engine plane, no more than I was sure that we were going to get a plane when I went down for my first lesson in April of 2005. I wanted to know how much harder it was to fly an airplane with two engines.
It was hard. I failed my test to see if I was allowed to do it, but then I passed after working at it a little more. But I didn’t know if we really needed a different plane and other than renting a Twinstar to take a fun flight with Susan Dost, I didn’t do anything with my new rating.
And then on one of our flights returning from the Bay area Nell asked, “What would be our next step up in an airplane? Have you thought about that?” So I compiled a book of ten planes that could replace the trusty Diamondstar. Nell looked through it, read all the careful charts of comparison and said, “Which of these would you pick if it were just your decision?” At that moment I was leaning toward the Twinstar, which was not a big change in speed (thirty knots faster), seating (the interior is identical to the Diamondstar), or comfort (it isn’t pressurized), but did change our flying in a few ways that I felt, and feel, are really important.
1. The biggest one is the second engine. There are a lot of planes that we could look at that do that, specifically a Beechcraft Baron or similar plane from Cessna. Having a second engine would eliminate all of the anxiety visited upon me during those trips. I no longer would worry about flying across the water to Catalina and how cold the Pacific is. (An online pilot friend had a Cessna twin and had three engine failures, every single one of them over Lake Michigan, one at night.)
2. We would step up to a more advanced autopilot. The KAP140 that we fly in the Diamondstar (or, rather, which flies us), is technology from the late 1970s, which you can tell from the nice, warm red LED display. Even more advanced autopilots like the STS 55X model found in the Cirrus SR22 has led to the death of a handful of people, so being early 1990s doesn’t help. (Cirrus has since replaced the STS with the Garmin GFC700, which is what we would have in the Twinstar.) The super-modern autopilot would keep us safer.
3. We would no longer burn AvGas. This is a 100 octane fuel which still contains lead as an antiknock agent. I have never been that happy about being part of the 186 million gallons of AvGas burned in the United States (even though that’s 0.14% of the total motor fuel burned in the country, I know the environmental harm that lead caused here and wished I had a way to skip out of that pie chart.) AvGas is probably going to be phased out by the EPA, and that’s going to present a problem for the people that still have tractor engines and need it as their fuel.
4. I would no longer be responsible for monitoring and managing the engine, which was never something I was very enthusiastic about. Again, I feel like this is a solved problem; we know that computers are good at these sort of tasks and we should really turn the work over to them. Instead of three levers to operate our source of thrust, I would have just one. Actually, I’d now have two, with the two engines, but if we moved to a Baron I’d wind up with six. Simplicity adds to safety.
It always sounds akin to a tiki bar to me. Flight Into Known Icing. The Twinstar is certified to fly into conditions that the pilot knows may produce ice on the airframe. I have only encountered ice three times so far (on one of my IFR training flights, returning from Phoenix with Dexter, and headed to Oakland with Dexter), but there are other flights and routes that we did not fly because there was the chance of encountering it. Even in the summer, up high it gets cold and we may give up the best route over some mountainous terrain because there is the possibility of ice in the clouds above. The Twinstar carries eight gallons of deicing fluid, sort of an umbrella drink for cute airplanes. It’s sugar and alcohol that when coating a surface, keeps the water from dropping into freezing temperatures. The Cirrus planes all have it. Everyone says that “it is an escape hatch, not a bulletproof vest.” That’s fine, there are a few times when I would have been really happy to have an escape hatch available.
6. Comfort. The seats in the Twinstar can include front seats that recline. I know, coming from the world of automobiles where the least expensive Kia has reclining seats that sounds silly, but in the Diamond aircraft the seats are rated to 20g impacts, so it is no small engineering feat to make a couple that can withstand that and recline. Additionally, there is a Supplemental Type Certificate (STC) that allows Diamond to make the control stick on the right side removable. That means Nell could fly along with her laptop in her lap. Comfort is no small thing because if the plane were a little more comfortable I’d be happier to fly a leg longer than two and a half hours, which changes the places we are able to go in a single hop, and changes all of the places we might see as we go coast to coast (I’m very tired of Liberal, Kansas).
7. Speed. These are in order of declining importance, and speed isn’t really our top concern. The truth is I like being in the sky and if you told me I could get to Catalina in fifteen minutes instead of half an hour… I’d probably choose the slower option because I like looking at the landscape below as I head out there. But being a little faster is nice. More importantly, the Diamondstar always flew along at what was basically it’s top speed. Unlike a car, where people can say, “Don’t hurry, it’s not worth the risk…” the plane is going in a straight line and pretty much flat out. (This is not totally true, I run the engine at 2350 RPM and there are people who run theirs at 2450 and get a few more knots. I studied that when I first had the plane and it wasn’t worth the fuel burn.) The Twinstar has a margin, though, where it will fly happily at 65% power and at 95% power. There’s a considerable difference in speed and noise. I have a feeling we’ll be at 75% a lot of the time.
Those are seven good things. Each one of them could be answered by a different plane, but the Twinstar is the only plane that can provide all of them. And now, with the DA62 that just received FAA certification (the model I flew was visiting from Europe), a larger version of the same plane could even carry another three people.
The biggest thing isn’t on the list though, which this question speaks directly to:
What is the purpose of the landing gear hydraulic accumulator?
What is the purpose of having retractable landing gear? (Elimination of drag and elimination of a series of surfaces on which ice can accumulate.) But why complicate your airplane? I suppose, deep down, why fly?
Science, freedom, beauty, adventure: what more could you ask of life?
– Charles Lindbergh
I am not sure. But I know one thing: for the past year or so I have been looking at the bleak prospect of an empty nest, since both our sons will be away in college starting in September. Rudy will be starting his third year up at Reed in Portland, Oregon and Dexter will be at one of three places (currently undecided). I made a concerted effort starting in July to learn the latest web development framework and take another shot at being a developer. That was not a success. And I have noodled around with some design things and while it is interesting to me it is not the all-encompassing preoccupation I like it to be. That magic project has not happened upon me, the way it always has in the past.
So I was adrift and for months when people would ask how things were I would be honest and say I was hoping for something to occupy me during the critical year or so when Dexter is first away and our lives change drastically to something else. To my surprise the idle poking around to find a possible used Twinstar as our Next Airplane snagged my interest in a way that was very similar to my first experience with learning to fly.
So, for now, not on that list of seven advantages is that it has given me something new to learn, new to master, and a bunch of possible new adventures. As Mr. Lindbergh wrote, it gives me a window into science, a freedom, a connection to beauty I don’t find on the ground, and an encouragement to plan some more adventures.
The landing gear is raised with a hydraulic system, an ingenious invention that traces some of its roots back to 1795 and Joseph Bramah, the same guy that developed the flush toilet. It uses a small electric motor to generate pressure in the hydraulic lines, which then pull the gear up. (The gear is extended when the pressure is released and gravity just pulls the gear back down. That’s nice if you have an electrical or hydraulic failure since the gear will simply drop down without the hand-crank method that is common in other retractable gear planes.)
If the electric motor had to maintain the pressure in the system on its own it would be cycling on and off every few minutes. That’s not efficient. Instead, there is a pressurized gas accumulator, charged with nitrogen at 1,150psi. That helps maintain the pressure, keeping the electric motor from cycling on and off as often. It also naturally helps dampen the pressure changes that occur while operating the gear.
Because of the retractable gear, the Twinstar stands higher than the Diamondstar. The wings are two feet longer on each side, which means I am very careful while on the taxiway and when parking. My new parking spot is a “pull through” at the base of the tower and having pulled in half a dozen times I can tell it will be dozens more before I can do it without wondering if they’ve narrowed things while I was away.
Technically, we took delivery of the new plane on March 3, last Thursday.
As detailed in a previous post, I didn’t get to fly it up to Oakland as planned, but I have been trying to take it for a flight every day. And, although I have not finished all of my flashcards, I read part of my book* every day and try to learn about the plane every day. (Selecting and buying the plane was an endeavor worth its own post. John Armstrong at Lifestyle Aviation talked with me for weeks before we made an offer on a plane. And when that didn’t work out he helped find another.)
If the nest is empty, it’s probably good to spend some of the time flying.