Aeroprakt A22/A32 Weight & Balance calculator – update


UPDATE – by popular demand, the Weight & Balance calculators now enable fuel to be entered in litres – the calculator will work out the fuel weight and give you a gross weight and CofG as before. Hope you find that more useful.

We have developed a simple MS Excel spreadsheet to enable A22 and A32 owners to calculate easily their weight and balance before flight. For ease of use, all cells except the fuel volume in litres and weight values for the basic empty aircraft, pilot, passenger and baggage have been locked,

All you need to do is enter the fuel in litres and kilogram weight figures for your aircraft and the sheet will calculate your gross weight and the CofG position for you.
If all is within limits, the gross weight and CofG figures stay green. If they are outside limits, they turn red.
Simple!

Please email me if you want a copy for your A22/A32Weight & Balance calculator – please specify if you want an A22L Foxbat, A22LS Foxbat, A22LS Kelpie or A32 Vixxen sheet as they are all slightly different.
peter@foxbat.com.au

Ballooning & floating?

Probably the most common comments I get from student pilots – and quite a few experienced pilots too – are about their perceived skills needed to land a light sport/recreational aircraft. In many cases, pilots make comments like: “I pulled back on the controls to flare and the aircraft just ballooned” or “it just seems to float and float along the runway; it just doesn’t want to land”.

Both of these events when landing an aircraft – ballooning and floating – have their own dangers for the pilot, which if not anticipated and handled correctly can result in a bent aeroplane…or worse.

So here are a few tips on how to get it right.

In simple terms, almost all balloons and floats during landing are caused by excess speed over the landing threshold. Unfortunately, many instructors have a habit of telling their students to add 5 or 10 knots to their approach speed ‘for safety’. In reality, in light sport aircraft in ‘normal’ conditions, they are often actually reducing the margin of landing safety by doing so. And this habit of adding speed to the book figure becomes instilled as a very hard-to-break habit. My own pilot training, now many many years ago, involved adding approach speed in certain circumstances and, even now, I have to fight the impulse to add speed when landing in the A22LS Foxbat and A32 Vixxen.

Let’s go ballooning
So, what’s wrong with more speed? There are two main reasons but first, remember light sport and recreational aircraft are very low weight (read: low inertia) aircraft. So, like a small car, these types of aircraft will change direction much more quickly than a limo, a ute or a truck. Not that I’m suggesting your average Cessna/Piper etc are trucks…. As a result, when landing, the controls are much more effective than bigger GA aircraft. At only slightly faster speeds the controls are even more powerful, so if you are too fast when you pull back to flare, the aircraft will not just flare, it will start to climb again, even with the engine at idle. This is called ‘ballooning’. When you go ballooning, the impulse is to push the nose down to reduce the sudden climb. Unless you are very quick (and/or experienced) you’re likely in for a bent nose leg and/or busted propeller. Another alternative, just holding back the controls during the balloon, can result in a stall from an ‘unsuitable’ height above the runway, leading to a (very) heavy landing, which could damage the landing gear or worse.

How about a bit of floating?
Next reason why too much speed is dangerous: even if you flare correctly without ballooning, the aircraft is still going too fast to land. Instructor: “Just try to skim the runway; don’t let the aircraft land; try to keep it flying as long as you can, slowly pulling back on the controls until the aircraft slows and the main wheels touch down”. This is all absolutely fine, unless you are carrying excess speed, in which case you’ll end up flying a long way down the runway before you touch down. And skimming along the tarmac (or grass) at relatively slow speed for a few hundred meters at just a few feet of height is tricky enough for an experienced pilot, let alone a novice. Throw in some cross wind, and/or a gust or two, and the risk of disaster rises exponentially! After a period of ‘skimming’ without landing, there is a huge temptation to let the nose drop a bit (or worse, push it down), just to get the wheels on the runway, and this can have two potential results: (a) because you’re still going too fast, the nose wheel touches down first and you’ll bounce/balloon, or (b) the impact will bend the nose leg and maybe bust the prop – if you’re lucky.

There are remedies for both ballooning and floating after they start but the easiest solution is not to let them happen at all!

Calculating the correct threshold speed
Which is where we get back to speed. There’s a GA rule of thumb about landing speed over the threshold. This says you should aim for about 1.3 times stall speed in landing configuration. As an example, with a stall speed of 45 knots the aim is (technically) 58.5 knots over the threshold – which is usually rounded to 60 knots. With low-inertia light sport aircraft, which have lower landing speeds, it’s probably safer to go for about 1.75 times stall speed, as wind gusts can be a much higher proportion of approach speed. So, for a stall speed of 28 knots (A22LS Foxbat) the threshold speed should be about 49 knots – which is exactly what the pilot manual gives. Note – this is 20 KNOTS above the stall speed!! If you come in at 55-60 knots over the threshold, you are flying about twice as fast as the stall speed – no wonder the aircraft is difficult to land!

What a drag
There are big differences in drag between aircraft. And drag affects how quickly the aircraft slows down when you throttle back for landing. The more the drag, the quicker the aircraft will slow down and vice-versa. To some extent, high-drag aircraft are easier to land than their more slippery siblings. As you cut power and round out to land, they will slow down more quickly, so if you are a few knots over the correct speed, they will help you out by slowing quickly. However, the more slippery the aircraft, the more accurate you need to be with the threshold speed; this is because if you are faster than you should be, the speed will not wash off quickly and ballooning and floating become much more likely.

As a comparison, our A22LS Foxbat is much much draggier than the A32 Vixxen. This is clearly evidenced in the fuel economy and cruise speeds. While the book figures for landing threshold speeds are much the same at 49 knots, coming in at 55 knots in the Foxbat will still allow you a reasonably easy landing. Try it in the Vixxen and because of its low-drag airframe, you’ll probably do a lot more floating. Add yet another 5 knots ‘for safety’ and even the Foxbat will take a while to land and the Vixxen will take you all the way down the runway into the fence at the end.

Landing weight
There’s an important additional piece of information needed here – the landing weight of the aircraft. All manufacturers quote stall speeds at maximum gross weight – for light sport aircraft, this is 600 kgs. If the stall speed is 28 knots at 600 kgs, it will be noticeably slower at (eg) 450 kgs actual weight, which in an A22LS Foxbat equates to the aircraft with one pilot and 50 kgs (70 litres) of fuel. In fact, it could be as much as 3-4 knots slower. Re-calculating the approach speed for this weight: (eg) 25 kts x 1.75 = 44 kts.

Hopefully, instructors  teach their students properly about the difference weight can make to stall – and thus landing – speeds. This is particularly important for light sport aircraft, where the pilot, passengers, fuel and baggage make up a much bigger proportion of the weight and therefore have a much more significant effect on speeds than heavier GA aircraft.

Wind
Finally, a point about wind. I’ve often heard it said you should add 5-10 knots to your approach speed if the wind is across the runway and/or gusty. The idea being that if the wind suddenly drops during your approach, the aircraft is still going fast enough to keep flying above stall speed. In heavier GA aircraft, this may well be valid, as using the throttle to regain speed to arrest the momentum of a sudden descent takes time. However, modern light sport aircraft are much more responsive to throttle than their older GA counterparts, so I would never add more than 5 knots to the ‘book’ approach speed in a cross or gusty wind and use the throttle to stop descent quickly if a sudden drop occurs due to a gust.

In summary – read the pilot manual for your aircraft to check the threshold speed for that specific type – do not rely on rules of thumb, like “all aircraft are OK at 60 knots” down final and over the threshold. If the manual gives 49 knots at gross weight stick to it and – if it’s a light sport aircraft – even a bit slower if you do not have a passenger and/or lots of fuel. If you don’t stick to the book speeds, you are looking for trouble and for sure, you’ll end up ballooning or floating and sooner or later you’ll bend something. Hopefully, not yourself or your passenger!

KievProp propeller balancing

Here’s a great little video about how to statically balance your KievProp – ie off the aeroplane. I suppose the technique could be used for any type of propeller. It certainly works here!

As a matter of interest, you can also dynamically balance your propeller – ie when attached to the aeroplane, with the engine running. However, this does need a piece of electronic kit (which is not cheap and simple) which in principle works in much the same way as the static balancer.

Overall, for an inexpensive and excellent balancer, the static approach works well.

PS – be sure the carburettors are correctly balanced (ideally with the engine running) and the blade pitches are all exactly the same…not even a skerrick out!

As usual, click on the picture or the link to get you to the video.

How to destroy your aeroplane…

…and have a lot of fun, in a few easy steps.

The first golden rule to remember is that when it comes to your aeroplane, you know best! Most manufacturers generally do not design aircraft the right way and if you ask around, you’ll almost certainly find an engineer who will agree with you. So – don’t waste time reading the pilot or maintenance manuals, they are only there because the manufacturer legally has to issue them and they are full of antiquated procedures anyway. These manuals don’t carry anything like as much weight as your own knowledge and experience, seasoned with a liberal dose of advice from mates, who also usually know much better than manufacturers, engineers and the like.

Here are a few more specific tips on how to destroy your aeroplane:

When you park, don’t bother to secure the controls or control surfaces. If possible ensure you park the aircraft facing downwind so that even a light wind will help you get those ailerons, elevators and rudder flapping nicely – but the stronger the wind the better! Once the control surfaces have hammered up and down a bit – allow at least a couple of hours if possible, overnight is better – the hinges, control rods (or cables) and connections will be well stressed. If you’re lucky you might even pre-fracture a connection somewhere in the system which, hopefully, you won’t find out about until you’re in the air – it’s great fun flying an aeroplane on the rudder or aileron trim, assuming they aren’t busted too! Remember, if the aircraft manufacturer has supplied a control lock, lose it as soon as you can! And it is so fiddly to tie the controls with the seat belts, it’s best not to even think about using them!

Leave the doors or cockpit canopy open at all times when you aren’t flying. This helps to air out the aircraft, particularly on a breezy day. In fact, the stronger the breeze the better, as it will likely slam the doors or canopy closed for you. If all goes to plan, it may even damage the hinges or latching mechanism – it’s a wonderful experience to have a door or canopy come open when you’re flying! All the maps fly out and sometimes the aeroplane can get a bit tricky to handle, specially when you try to land. If you are extra lucky, the door slam may even weaken or damage the door frame, helping it to unlatch in flight even if you think you closed it properly!

Always clean the screen and windows with household products – Mr Sheen, Windex, truck wash, polishing creams, they are all good at attacking aviation polycarbonate or acrylic. Even better, wait until there is a good layer of dead bugs and dust on the windscreen and then use a slightly moistened cloth – a kitchen ‘wash-up’ sponge is better – to rub off the mess. If you are diligent, you can create some wonderful swirly patterns which look great when you fly towards the sun. After a time, you’ll get used to heading away from the sun most of the time, so an unexpected landing towards the west at the end of the day will be all the more exciting, as you guess where the runway threshold is.

If at all possible, keep your aeroplane in a damp environment – ideally put a few old bits of carpet over an earthen hangar floor and ensure they stay reasonably wet. That way, most aluminium and steel parts can start to corrode as quickly as possible. Even if you own a glass fibre aircraft, a damp hangar is still a good idea; even plastic planes have metal control systems and engines. And as every boat builder will tell you, moisture attacks glass fibre too! Here’s an extra tip for glass fibre plane owners: park your plane out in the direct sun as much as possible, particularly when the temperature rises above 25 celsius. That way, the composite material has the best chance of  UV decay. A good wheeze is to paint a dark colour on white composite wings and fuselage – the temperature differential in hot sun really accelerates the breakdown of the fibres.

If you have an auxiliary fuel pump – leave it switched on at all times! Even if the pump supplier says otherwise – after all, what do they know? Remember, the pilot always knows what’s best for their aeroplane, and the aux fuel pump is a good way of ensuring continuous fuel flow to the engine. A good time to switch on most pumps is at the top of a long descent; the slow rate of fuel flow cooling the pump might enable it to heat up and, if you’re lucky, could even start melting the insulation round it. This is terrific fun, as the cabin fills up with smoke, which might be smelly but at least it generally isn’t toxic. If you really want to let rip, leave the pump on after it starts smoking and it might even catch fire, so you’ll have a real emergency to tell your mates back home about.

Wherever possible take the opportunity to modify the aircraft from the condition the manufacturer specified. Here are a few tips:
– if the prop is pitch adjustable, coarsen the pitch to the maximum. You won’t get more cruise speed, the take-off and climb will suffer but hey! it will really stress out the gearbox and crankshaft
– if you can, redirect any drain and vent tubes to places that wizened old mechanics tell you. For example, carburettor vent tubes can be lengthened so they pass outside the cowling into the airstream. It plays havoc with the carburettor float levels but at least any vented fuel goes straight out into the slip stream instead of inside the cowling
– here’s a good one: replace the aviation (pink) brake hydraulic fluid with motor brake fluid (pale yellow). It’s much easier to get, it’s cheaper and the brakes will only seize up after a few weeks
– for you electrical buffs, try replacing fuses with the next size up; it helps to minimise the chance of an inconvenient fuse blowing when you don’t have a spare
– if the throttle lever seems a bit short, get an old piece of aluminium and bolt it on to make it a more comfortable length; don’t worry if it now fouls another control, it likely only does so at extremes and how often do you need those?
– wherever possible, replace fixed nuts and bolts with quick-release fasteners; they are weaker and there’s a reasonable chance they will ‘quick release’ when you least expect it, giving you some extra in-flight fun – well worth the effort to install
– a lot of people like to fit bigger tyres because they think (a) their plane looks sexier with huge tyres and (b) they think it makes it easier to land on bumpy surfaces. Because of the extra leverage from the radius of the bigger wheels, you’ll find the aircraft pulls up more slowly and/or wears out the brake pads three times as fast – either way, go for the fat tyres!

Last but by no means least, try to avoid pre- and post-flight inspections. First, they take up such a lot of time when you could be flying and, second, you never find anything anyway. If you have to check anything at all, maybe limit it to a look at the oil level – but only if there’s a little hatch to reach through; it’s such a hassle taking off the cowling to look at the engine because it never goes wrong. Does it? I mean, it’s not as if your life could depend on it….

Rotax Engines (4) scheduled maintenance update

Over two years ago, I wrote a blog post about Rotax engine scheduled – ie regular – maintenance, like oil and filter changes and other time-based servicing.

At that time, I said that the information in the Rotax manuals was ‘about as clear as old engine oil’. After several emails around that time the Rotax-Owner website confirmed by email to me that scheduled maintenance times on 912 series engines should be by reference to flight times – ie ‘wheels off to wheels back on the runway’ – as required by FAA in the United States.

However… it turns out that this may have been wrong. Current Rotax manuals, updated in the last two years, apart from a couple of small anomalies, clearly state that scheduled maintenance must be according to engine start-up and shut down times, as recorded by a reliable engine hours timer. This is further supported by Aeroprakt – manufacturers of the A22LS Foxbat and A32 Vixxen aircraft – who have aligned their airframe maintenance times with those required by the engine manufacturer.

This comes as a blow to many flight schools operating aircraft with Rotax engines, who have serviced their aircraft based on flight times – just like, I may add, the vast majority of the Lycoming and Continental engined GA fleet. At larger airports in particular, start-up/shut-down times can be as much as 25% and more, longer than flight times. When allowing for engine warm-up on cool mornings, the overall % could be even higher – substantially increasing maintenance costs.

Nevertheless, as one of my Aeroprakt colleagues put it: ‘in all aircraft, vibration from engine running is one of the major factors affecting the airframe fatigue life’. This is particularly true of the recreational and light sport category, where airframes have to be constructed to lighter standards and closer tolerances than many GA aircraft. So the aircraft may be less expensive to buy but will require more frequent maintenance. Thankfully, overall running costs in recreational and light sport aircraft are still only a fraction of those in their heavier GA relatives.

So – make no mistake, if you have a 912-series Rotax engined aircraft, scheduled maintenance must be carried out based on engine running hours, NOT flight times!

A few lines of aviation humour

What follows are a few lines of aviation humour to help you through your Monday morning. I’ve heard some before but not all of them.

Happy smiling!

  • There are more planes in the ocean than submarines in the sky
  • The only time you’ve got too much fuel is if you’re on fire
  • Flying is the second best thrill in the world – landing is the best
  • Death is just nature’s way of telling you to watch your airspeed
  • You can watch the clouds go by – or fly above them
  • An optimist invented the aeroplane, a pessimist invented the parachute
  • A helicopter – thousands of parts flying round an oil leak waiting for metal fatigue
  • The three most common aviation expressions: “Why is it doing that?”, “Where are we?” and “Oh crap”
  • Modern aerial warfare: a $70 million aeroplane drops a $350,000 bomb on a $10 tent
  • In thrust we trust
  • Engine power: lots is good, more is better, too much is almost enough
  • Pilots get paid to sit and stare out of the window
  • The emergency exit row – with great legroom comes great responsibility
  • When all else fails – use duct tape
  • When opening the overhead bins take care – shift happens
  • Loud, sudden noises in a helicopter WILL get your undivided attention
  • Latitude is where we got lost; lontitude is how long we’ve been lost
  • There are certain aircraft sounds that can only be heard at night or over water
  • No need for a checklist, I’ve got it all memorised
  • Mummy, when I grow up I want to be a pilot. Sweetie – you can’t do both
  • The flight attendant smile – fooling passengers since 1912

Carb icing or fuel vaporisation?

Over the last year, I have received a couple of reports of what was described as ‘fuel vaporisation’ causing rough running in an aircraft with a Rotax 912 series engine.

Intrigued, as there have been no other mentions of ‘fuel vaporistaion’ – let alone actual confirmed reports – in the entire 1,000+ global Aeroprakt fleet, I decided to do some internet and personal investigation on fuel vaporisation and carburettor icing (something which can, in some circumstances, give similar symptoms). As a bit of background, we currently have close to 175 Aeroprakt aircraft operating across Australia, from the searing outback summer temperatures of 45+ celsius to the cooler temperatures of Tasmania in the winter. While occasional carb icing has been reported, never has fuel vaporisation been mentioned…until this last 6-9 months.

Here’s what I found about fuel vaporisation.

Vaporisation typically happens at high ambient temperatures – websites I viewed suggested at 35-40 celsius and above. However, the likelihood of  fuel vaporisation can be affected by a number of factors: mogas vaporises much more easily than avgas; fuel under suction (eg in the line from a tank lower than the pump) will vaporise more easily than fuel under positive pressure (eg in a line from a tank higher than the pump); carburettors without vent lines are more susceptible than those with vent lines like the Rotax engine. The most typical scenario for a vapour lock is when an aircraft with a hot engine after a flight is parked and the temperature in the fuel lines in the engine bay can soar well above 60 or 70 degrees celsius, causing the fuel to boil in the lines in the engine bay – making restarting difficult. In fact, Rotax recommends that for engine bay temperatures over 45 celsius, the fuel lines and carburettors should be ‘cooled’ although they don’t specify exactly how.

It is very unusual (but not impossible) for vaporisation to occur when the engine and fuel lines are relatively cool (eg during a descent) or when cruising or climbing at normal or higher power settings, when there is a good flow of cooling air over the engine and cooler fuel is flowing from the tank(s).

Here’s what I found about carburettor icing.

Carb icing typically but not exclusively occurs at slow cruise or low power/idle throttle settings. It can occur quickly and at surprisingly high temperatures – see the graph above. For example, on descent, there is a ‘serious’ risk of carb icing at temperatures as high as 30 celsius and as low as 35% humidity. Carburettor heat can help prevent icing but once formed, ice can take a while to clear. In fact, descent into warmer air is sometimes just as effective at clearing the ice – provided the engine is still actually running. As with vaporisation, mogas is much more susceptible to carb icing than avgas; fuel with ethanol and/or other additives can be more prone to icing, as the fuel may have absorbed water. Even with carburettor heat, I was trained to warm the engine on longer descents by applying 80%+ power for at least 30 seconds every 1,000 feet of descent – and that’s with avgas, which is much more resistant to icing than mogas.

So, what is the most likely cause of a rough running engine in the following circumstances: icing? Or vapour lock?

  • descending aircraft
  • low cruise power setting
  • ambient temperature around 22-25 celsius
  • relative humidity around 55-60%

After long discussions with the Aeroprakt factory covering the reports which have been made, it is our conclusion that the reports of ‘fuel vaporisation’ causing rough running may have been mistaken for simple carb icing, which is a much more common problem. The prevention of vapour locking and carburettor ice are quite different – a fuel/vapour return line for the former and carb heat for the latter. Although it must be stated clearly that neither solution is guaranteed to be effective in all circumstances.

Some early Aeroprakt aircraft were fitted with fuel return lines, although these have in the past caused problems of their own, with excess fuel pumped overboard due to wrong tank selection. Current ASTM standards for Light Sport Aircraft do not require an excess fuel/vapour return line to be fitted. The Rotax manual, although stating a return line is ‘mandatory’ in one place, actually states that the fuel system is ultimately the responsibility of the aircraft manufacturer. All Aeroprakt aircraft manufactured after February 2015 either have a fuel return line fitted or one can be retro-fitted by owners if required.