GERHARD VAN EEDEN – THE OWNER OF MADIBA BAY SCHOOL OF FLIGHT IN PORT ELIZABETH.
“It’s not going to last. You know that, right?”
“Yes, well I guess we’ll see.”
That was how conversations went when I arrived in Port Elizabeth with our first Sling in December of 2012.
WELL, IT’S 10 YEARS ON and our fleet of six Slings have accumulated in excess of 35,000 flight hours. ZU-IAE has 7,300 hours on it, making it the highest hour Sling in the world.
This article tells the story of our experience of keeping our Slings airworthy. And that really is a convoluted tale, because we pioneered the Rotax 912iS engine at the same time, so let’s start with that.
The 912iS engine’s problems
On paper the iS has triple electrical redundancy and it should never fail. Will I fly it across the Atlantic though? Ha! Don’t be silly! No. Yet I would do it behind its more humble older brother, the 912ULS carburettor fed version of the same engine.
Why is that? Well Rotax has still not managed to completely sort out the stator (alternator), which is prone to heat damage and is a single point of failure. When it gets damaged and short circuits, the engine runs off the battery, until that goes flat….and then it’s a case of no electricity, no engine.
Stator failure does not catch you by surprise though. It has heaps of tell-tale signs that become increasingly evident over time. Though we dutifully train for engine failure, it isn’t something that I ever seriously worry about with the iS, but as for the Atlantic.…she’s a big place.
‘they crash really well’
So why put up with the complex iS engine when the simple ULS is more reliable? Well on a good month at Madiba School of Flying our injected iS engines save me R50 000 in fuel compared to ULS engines. I will forgive it many things for that. You just can’t argue with 14 litres per hour in the cruise. In practice, we taxi a lot at FAPE and we spend a lot of time in the circuit, so real life fuel burn at the school is 10 litres of car fuel per hour.
Another reason for loving the iS, is that the computer records every spurt, fart and hiccup the engine makes. Every MPI begins by plugging a laptop into the engine and downloading the logs for examination, and there’s a program that displays the information in graph form. It’s easy to overlay graphs for comparison, and anomalies stick out.
This is all good and well for a flight school with dedicated maintenance personnel standing by. If I were going to own a Sling for personal use out in the sticks, I would most definitely choose the ULS engine.
Gearboxes
Gearboxes on the iS engine have proven to wear. The issue was substantially remedied when, around 2017, Rotax introduced a specific oil feed directly onto the gears. Even so, the gearbox makes for rough starting of the engine, especially on the first start of the day. So rough that the airplane reminds one of nothing so much as a dog shaking off water.
Some of our aircraft are worse than others, and we can’t figure out why. It’s so bad that you can’t run traditional gyro instruments. We once removed a non-working A/H and broken bits of gimbals came tumbling out. Instrument panels crack and our MGL screens break at their mounting points. Recently one of our instructors came in saying he had smelled a burning smell in flight – and it wasn’t and oil smell. Our very experienced engineer noticed that the exhaust was up against the cowling.
He then noticed that the engine wasn’t sitting straight in the airframe, so he removed the cowlings. Closer inspection revealed that one of the high tensile engine mount bolts had snapped. I am not exaggerating when I say the iS shakes on start-up. Hung starts produce prolonged shaking, but I don’t want Rotax to hate me, so I will stop there. They have been very good to me.
Rotax has now offered me two free gearboxes to test. They have never stopped developing the gearbox and I won’t be at all surprised if the shaking thing is now behind us. I didn’t tell them I was writing this. Through all the years, this is how Rotax have done things. They just don’t stop until it’s right.
We have also now begun rewiring the starting circuits of our aircraft to bring them into line with the latest directives from Rotax, as endorsed by Sling Aircraft. It gives the engine’s computer more time to come on line prior to engine start and though it is early days on a brand-new engine that we have just flown, I am choosing to be optimistic. So far, so good.
Shaking Apart
The solution to the hammering the gyros were taking was to replace the DI and AH with Garmin G5’s which have no moving parts. Cost for the two units fitted was R100 000. Eish! We were under pressure to have a suitably equipped aircraft for night/instrument training though, so we went ahead and spent the money.
When we upgraded the second aircraft, we fitted MGL’s Blaze singles. We tried the AH, DI and Altimeter as a trial, and we have been very impressed. Though the Garmin display is similar to what I had in the Beech 1900D that I used to fly, the MGL units are perfectly good, and they cost only 5% as much as the Garmins. Sometimes local is lekker.
‘It will coldly and dispassionately kill you’
There’s one more thing; Call me old fashioned, but my mind works better in pictures. I get more information from a single glance at the faces of my six little analogue friends than I do from thinking about the numbers on those precise digital displays who each demand my brain’s focused attention.
Our MGL Voyager screens have done 35 000 hours with us. When they break down, we send them to Cape Town and the guys at MGL fix them. The displays cost only R5000 to replace, so it’s easy to keep the units in great shape.
Our Garmin radios and transponders are so good that it’s easy to forget to mention them. In all this time I have only had one transponder breakdown on me. That really helps when you are trying to keep aircraft in the air.
Okay, so back to our instrument rattling gearboxes. Gearbox inspections are required every 600 hours on the iS. They mostly make it to their 2000-hour service life now.
Rotax have replaced gearboxes and stators free of charge under warranty. Honour is a value they adhere to. It’s been 10 years though, and one would have hoped that the issues would have been sorted out by now. The engine costs half a million Rand. For that money I would like an engine that does not rattle my bones in the morning.
Rotax engines run to 2000 hours without wear. The only thing that they absolutely will not forgive you, is exceeding maximum RPM. Do that to a Rotax 912, and it will coldly and dispassionately kill you. It doesn’t take long for damage to set in.
It’s hard to over rev an engine fitted with a trusty old fixed pitch Warp Drive prop, but there are horrible electrically controlled, flight-adjustable props on the market that have killed many a Rotax engine and sometimes the people who flew behind them as well. There are propellers specifically for Rotax engines which use the traditional, tried and tested, hydraulically controlled constant speed method to control the prop, so you won’t have unscheduled over speeds. It doesn’t even cost any extra compared to the electrically controlled units. Sling Aircraft will fit the prop of your choice.
Covid and the war in Ukraine have been tough on the whole world, and perhaps it contributed to Rotax struggling to produce engines for a while. This resulted in three of our aircraft being grounded for an extended period of time with time-expired engines, but engines are now slowly starting to trickle through again. For a while now, there has been no engine oil available from Shell, which is the Rotax recommended oil.
We treat our 2000-hour engine replacement cycles as an opportunity to overhaul the airframe as well.
Madiba is structured around an expected 450 hours per month, and that model requires that we employ maintenance staff on salary rather than to pay an AMO a per-hour rate. That translates into a R55,000 fixed overhead per month, but the upside is that we can maintain our aircraft to our heart’s content. It’s hard to imagine how we might otherwise have adequately maintained the airframes.
Our Slings work hard. They are banged into the runway day after day and year after year by those whose eyes and minds and hands have not yet reached consensus on how to give expression to their desires to re-unite their craft to terrafirma.
Lightweight Airframes
So how did our airframes do? Well, they proved to be surprisingly resilient. I have to add that we operate out of FAPE with its long tar runways.
The skin of the aircraft is thin. Empty weight is 400 kg and all-up is 700kg. That means two guys each weighing 100 kg can take off with full tanks and fly from PE to Harare non-stop and have 3 hours in reserve. The downside of the thin skin is that flaps are vulnerable to people stepping on them, and flight controls are prone to hangar rash. Replacing dented skins is relatively inexpensive though and it’s quick and easy to do, thanks to the use of pop rivets.
The main landing gear is composite and I have had to replace at least six of them through the years, which means they are good for around 6000 hours. If you consider that at the flight school we do five landings per hour, and that replacement cost for the gear is around R35 000, then that translates roughly to R1 per landing. That’s not bad.
‘regular maintenance is essential‘
Flight controls are operated by push rods that run through bushes. When you replace these bushes, the aircraft just feels like it’s factory new again. The difference in feel really is fantastic.
The original push-button flap switches didn’t last, and Sling replaced them with sturdy rotary switches. You have to change the instrument panel though, or put up with an ugly patch. Sling Aircraft kindly provided us with the CAD drawing of the panel and so we are able to make the required changes.
Fuel tanks have been leaky; not so much dripping fuel, but leaving ugly brown stains around some of the rivets. We generally disassemble ours completely and clean the skins to original aluminium before re-applying PRC (very expensive, terribly gooey stuff, that seals holey aluminium sheets) at major inspections. This also provides an opportunity to inspect wing spars. I always marvel that a spar that has done thousands of hours can look like it came out the factory that same morning
Corrosion
At Madiba we operate in the salt air of our coastline, yet we have had no real corrosion problems, due to the fact that all skins are alodined. One of our dashing young hour builders managed to fly so low over a salt pan that he inadvertently splashed down into the 40% salt solution. We washed that little plane down with plenty of fresh water, but for the longest time you could run your finger in places and it would come out white with salt. At the last major inspection we tore it down to kit form and really looked for corrosion, and found that it was limited to just a few mild steel brackets. I test flew it after the rebuild and honestly, it’s the best Sling I’ve ever flown.
Climb-out after takeoff was at 110 kts and 700 ft/ min with a brand-new engine! I had the engineer on board as well to monitor engine parameters and to help with swinging the MGL compass. The pictures in the article are of this aircraft, ZU-IBZ. You would never say it has 5000 hours on it.
‘banged into the runway year after year’
Last year the factory considered a possible maximum hour service life for the airframe. When the first aircraft left the production line, no one imagined that there would be Slings with more than 7000 hours. I thought about that number. In the course of maintaining our aircraft, none of them still have all their original parts, and some of them have been substantially rebuilt. If for example it was thought that maybe the wing spars were getting on in years, we could buy a new set of factory-built wings, which costs a mere fraction of the purchase price of a new aircraft. The same goes for any component on the airframe.
It is a very easy and inexpensive aircraft to replace parts on, and there are no limits in terms of what can be done. With a suitable maintenance plan there is no reason to write off airframes.
Tail strike damage stopped after addition of simple flat steel plate protector.
Crashworthiness
Over the years our students have crashed the Slings, and they crash really well. I once test flew a Sling after it’s MPI. A careless apprentice at our previous AMO had left the nut off an aileron hinge bolt. It’s a long story, deserving of its own article. Neither myself nor the two engineers who each have a lifetime of maintenance experience noticed the missing nut during the triple pre-flight inspection. I don’t know how we did that! You can see me there on the security camera footage energetically moving the aileron up and down, for heaven knows what reason. If only I had looked for the nut! But there was no record that anyone had worked on it. Well of course the bolt fell out in flight, causing the stick to become jammed in the full left deflection position. It could move freely in the fore and aft plane only. I found that rudder authority exceeded that of the ailerons, but only at almost full power and high speed. So how do you land a plane in that condition? Well the short answer is, you don’t. You crash.
A subsequent internet search revealed that just about nobody else had managed to survive that particular set of circumstances, including the test pilot for Cirrus Aircraft. All I can say is that the Good Lord saved my life that day.
The Sling took it. I walked away, which means it was a good landing. And then we went ahead and fixed it. We just kept stripping until we found that the parts were no longer damaged. That point was behind the cockpit, and I believe that is the secret of its crash-worthiness. It is the crumpling that absorbs the impact.
There are anomalies. It takes far fewer man hours to repair damage resulting from a collapsed nose gear, than it does to repair damage from a tail strike. The tie down point in the tail acts like a guillotine and shears a bunch of structural rivets which then requires the entire tail of the aircraft to be removed to enable repairs. I designed a 200 gram spring-steel tail skid in place of the tie down and we said good-bye to damaged tails. Of-course we ran it by the factory’s design department. Same goes for the nav lights. The original sets cost R20,000 and never lasted. We replaced them with lenses we designed, and LED strobes/navlights which cost R700 for the entire set, didn’t need high voltage, and were brighter.
At the end of the day, for us who use Slings commercially, there is absolutely no difference in the way we have to maintain the aircraft compared to certified aircraft. And AME’s have a singular disability to understand the difference. Maybe that’s a good thing.
Maintenance Costs
So what does it cost to maintain a Sling?
Well that rather depends on its flight hours. For the first 2000 hours it will require 10 hours of labour at MPI plus oil and filters, which will come in at less than R10,000, or R100 per hour of operation.
After the first 2,000 hours, wear and tear becomes a factor that should be anticipated and planned for. By 4,000 hours you should be prepared to do some work on your aircraft to keep it in good shape. You will be replacing the instrument panel and all the toggle switches. Front windscreens become crazed in time. They easily last 4000 hours but since they only cost R4500, replacing them then is a good idea.
Canopy rails get tired and since canopies that stick are a safety hazard, they should be replaced. There are at least two items on the airframe that crack and have to be replaced, but Sling has become highly professional at remedying those kinds of defects, and at bringing out accompanying Service Bulletins. Rudder cables need to be checked. Throttle cables got sticky and were redesigned. Brakes got metal lines near the wheels because the plastic lines couldn’t stand the heat. As Sling hours around the world accumulated, problems emerged and they were overcome.
You can’t beat the fact that Sling is local. Their AMO section has every imaginable part in stock, and couriers deliver overnight. That simply translates into less downtime. Aircraft sitting on the ground cost money.
Ten years ago, I bought my Slings for around 1.1 million Rand. Now I would have to pay more than double that for a new one. That’s fantastic news for resale. The days where you could pick up a second-hand Sling for less than a million bucks are over. These days there’s seldom a week goes by that I don’t have someone calling me to see if maybe they could convince me to part with one of mine. In fact, as I write this, it is
only the middle of this week and I have already had a phone call and an email. To put it into perspective, a Sling kit costs R900,000 and then it is not a factory-built aircraft, so it can’t be used in a flight school. Add an engine, a propeller, radio, transponder, instruments, multi-function display and the cost of building and paint… it wouldn’t make sense to sell.
In conclusion
As I conclude this rather lengthy article, I do believe that the jury may finally, and confidently deliver its verdict. After 35,000 hours on our fleet, and hundreds of thousands of Sling-hours around the world, the only competent verdict must be that Sling Aircraft, as an organisation, has grown up, and that their little aeroplane has become the world’s ultimate training machine.
It is possible to keep the aircraft in a perpetually new condition. Mine certainly are in great condition. So now I can answer those who had told me they wouldn’t last. My Slings are tough, and they did last. But more importantly, they are inexpensive and easy to maintain.
