Crumbling tarmac at Ilaga airstrip

The trouble out here in the mountains of Papua is that almost all of the sealed runways are of pretty poor construction. Even before the tarmac is laid, the underlying surface is never fully levelled and smoothed off. This leaves a surface with lots of undulations on it allowing rain to collect in the dips and aircraft to be thrown about after touchdown. Then the base layer on top of that is usually too thin which after a few months of laden aircraft landing on it starts to subside even further, causing the undulations to be even more extreme. Then finally the top layer of tarmac is always too thin and so doesn’t take long to start to erode away and leaving behind large pot holes and cracks, ready to burst an unsuspecting aircraft’s tyres.

Broken and bumpy tarmac at Jila airstrip

Bilogai airstrip’s crumbling tarmac before they replaced it

Saying all that, it’s not like the unsealed runways are of any better construction, in-fact they’re mostly even worse bar a select few missionary airstrips. So what’s the big deal? Why do I consider sealed ones more dangerous? Well, it’s to do with the tailwheel configuration of the Porter’s undercarriage.

In all aircraft types, the centre of gravity is between the three sets of wheels. In a tailwheel aircraft the main wheels are at the front which puts the C of G behind them.  In a tricycle undercarriage setup, the main wheels are behind the nose-wheel which puts the C of G is in-front of the main wheels. This is a crucial difference which drastically influences the controllability of an aircraft on the ground, especially when landing. 

It is always important to land any aircraft dead straight, in the centre and in-line with the runway. After touchdown in a tricycle aircraft the nose-wheel is lowered and the aircraft will track straight in the direction you are pointed. You can then use that nose-wheel to help steer the aircraft during the landing roll and maintain a straight path along the runway’s centreline.

Lovely smooth grass at Pagamba airstrip

In a tailwheel aircraft such as the Porter, the same is true. You land on the mains and when the tailwheel touches the ground (usually all at the same time), you will track straight in the direction you are pointed. However, because the C of G is behind the main wheels any slight deviation from straight, and the tail of the aircraft will want to swing out further. To help circumvent this the tailwheel is always locked straight ahead for landing.

However, this means if you don’t touchdown dead straight, or you go over an undulation, the aircraft will spear off in that direction. This requires some quick work with the toe-brakes which are mounted on each rudder pedal and allow the pilot to apply the brakes to either the left or right main wheel. This individual braking action allows a swift adjustment to the yaw to bring things back straight. On an undulating runway, it can be a bit like doing a dance on the brake pedals to keep things straight.

Now, the main reason things are so much harder on a tarmac or sealed surface compared to a grass or dirt airstrip is due to the levels of grip available. Any aircraft on landing will have a certain amount of momentum which on the whole will carry you in a fairly straight line down a runway provided you’ve got the touchdown correct. On a grass or dirt airstrip, that momentum can actually help keep things straight, even if you hit undulations or didn’t quite touchdown straight because the wheels don’t have so much grip on the surface so can slide about a little. On tarmac, the wheels do have grip and pull the aircraft in which ever direction the three wheels are pointed. It’s this reason that most pilots learning to fly tailwheel aircraft will learn on a grass airfield before moving onto tarmac ones. It’s simply more forgiving.

Lovely hard non-sealed surface at Gisilema airstrip

So places like Ilaga with it’s broken tarmac, numerous undulations, potholes and faster landing speeds due to it’s 7500ft elevation actually do give me much more grief than somewhere like the newly constructed Gisilema airstrip with it’s 195m of smooth dirt. Just because an airstrip looks undramatic compared to another, doesn’t mean it’s any easier to land on!

Current grass airstrip at Mapnduma with the new sealed one under construction (sad times!)

Not the most comforting list of alerts…

Now just to clear this up before you read on, GPS is our primary form of navigation out here in Papua. It allows us to follow safe, pre-calculated routes, made up of multiple tracks that help us to navigate the challenging environment of mountains and tricky weather. That’s not to say we follow them religiously, as often the en-route weather gets in the way but without GPS flying in Papua would be another level of challenging.
 
Anyway, this little incident happened on my first day back flying last week, having been sat at our maintenance base for a few day awaiting the completion of some paperwork for our newest Pilatus Porter. I’m always a little wary flying an aircraft that’s not flown for a few months, especially when it’s been sitting on the ground in the tropics (hot, humid, salty air); past experiences have taught me it’s always going to involve various bits of avionics failing on you. However, the ferry flight from Biak to Timika went fine and there was little warning of what was to happen the following morning.

There wasn’t even any warning I was about to lose all GPS navigation. Just a nice female voice announcing “TAWS not available” in a rather abrupt tone, followed by various CAS (Crew Alerting System) messages appearing on the PFD (Primary Flight Display). TAWS stands for Terrain Awareness and Warning System and is considered a significant enough problem that you get a spoken audio announcement it’s offline, along with the master caution light and alarm.

No satellites on GPS 1 or 2

However, although TAWS going offline was the thing I was initially being made aware off, the various other messages regarding the loss of GPS were more of a concern. A twist through the pages of the MFD (Multi-function Display) to the AUX pages revealed that the reason for the TAWS being offline, was because I now had no GPS signal on either system. As with most things in aviation, there’s more than one system for all the important bits, so to lose both GPS systems was rather unfortunate.

Garmin GPS in Dead Reckoning mode

First things first, fly the aircraft. The route from Timika to Beoga is one I’ve done dozens of times now and thus I’m pretty familiar with the area. As I was less than ten minutes out and the weather was fairly nice, with only a few clouds about, I elected to continue and land in Beoga to see if shutting everything down on the ground would get the GPS working again.

A rather nifty function of the Garmin G950 is that if it does lose GPS, it reverts to what Garmin call DR (dead reckoning) mode. To quote the handbook “In DR Mode, the G950 uses its last-known position combined with continuously updated airspeed and heading data (when available) to calculate and display the aircraft’s current estimated position.”. Clever stuff and surprisingly accurate; it remained active until I’d landed and showed us directly over Beoga.

Where are we again?

Unfortunately for me, shutting down and restarting didn’t resolve the GPS failure problems so I was faced with returning to Timika with no GPS. This time there was no dead reckoning mode so I had to go fully old school and fly totally visually, cross checking my position with the features on the ground. This is where experience starts to count, as the whole way back to Timika was a non-event really. Once the airport’s VOR was tuned in, I simply homed in for the last 30nm or so after clearing the ridge line from the mountains to the flatlands of the south.

An interesting experience having to fly without the GPS and actually good practice. I’m happy to say I didn’t feel too uncomfortable flying around without it but certainly wouldn’t want to make a habit of it. Hopefully that’ll be the first and last time I get a double failure. Meanwhile, the aircraft’s now back in maintenance to get to the bottom of why the GPS keeps dropping out.

PC-6 right wheel bogged in

Before I answer that question, I’m going to try and explain the theory on why tailwheel aircraft pull left on take-off. P-factor is just one of the things causing this, however there are a couple of other forces at play which contribute, gyroscopic precession and engine torque. For the purposes of this article, we will assume a clockwise spinning propeller when viewed from the pilot’s seat.

P-Factor

The simplest way to understand this is to remember each propeller blade is an aerofoil and when spinning they’re moving through the air producing lift, just like the wings of an aircraft (abet that lift is actually thrust here acting along the length of the aircraft).

When on the ground, tailwheel aircraft have a naturally nose high attitude. The spinning propeller (prop disk) is mounted on the front of this up-pointing nose and thus is presented to the air at an angle. Because of this, the descending propeller blade (the one on the right when viewed from the cockpit) is moving through the air at a higher angle of attack than the ascending blade. That higher angle of attack means the descending blade produces more lift (forward thrust) than the ascending blade. That extra thrust on the right side of the aircraft causes it to yaw to the left. And the faster the propeller spins, the more extra thrust is produced on the right side and the more the aircraft pulls to the left.

Once the nose is level (i.e. when you’re flying), p-factor is zero as all blades are moving through the air at the same angle of attack relative to the airframe. This is why nose-wheel aircraft are not affected by p-factor as their propellers are usually not nose high when on the ground.

Typical nose high attitude on the ground

Engine torque

Simply put, the torque of the engine spinning the propeller clockwise acts along the lateral axis (nose to tail) to cause the whole airframe to twist in an anti-clockwise direction when viewed from the cockpit. In addition to this, the left wheel is pushed into the ground more than the right wheel causing more drag on that wheel which also causes the aircraft to pull to the left. The only way to compensate for this is opposite (i.e. right) aileron. However, until you’ve got the speed up, that will have no effect on things.

PC-6’s long nose housing a 550SHP P&W PT6

Gyroscopic precession

We all remember our gyro theory right? Probably not, so rather than going into all that dry theory I’ll try to explain it purely with relation to tailwheel aircraft. To understand how this affects the departure, imagine you’re sitting in the cockpit looking at the propeller disk spinning clockwise in-front of you. Now, gyroscopic precession dictates that a force applied to a spinning disk will act at 90 degrees in the direction of the rotation.

As the tail of the aircraft is raised before rotation, you are effectively applying a pushing force to the top of the propeller disk. This force moves through 90 degrees in a clockwise direction which is to the right side of the propeller disk. It’s this force that then yaws the aircraft to the left.

With the Porter, this is less of a problem as we tend to depart in a nose high attitude but still worth considering on departure. P-factor is by far the biggest force causing the leftwards tendency on departure.

Lined up for departure at Idedua, Papua

So, going back to the original question, what happens when departing from a short, very muddy, slopping airstrip with zero grip in a large, powerful taildragger?, in the case of Kegata it gets pretty hairy, very quickly is the answer!

View looking up Kegata as the occasional cloud rolls across

The weather here in Papua these past couple of weeks has been very wet with a lot of rain falling every day. As a result we’ve been forced to close a few airstrips as they’re just too dangerous to operate from when the ground is so wet. Kegata has just become the next one on the list until things dry out.

The landing at Kegata was uneventful although the turnaround resulted in getting stuck side on to the airstrip requiring half an hour of digging to un-stick the left tyre. As Kegata has a decent slope to it, that meant a lot of fuel slowly transferred from the right wing tank and into the left tank. The Porter has no way of preventing this as both wing tanks are interconnected via a collector tank.

Digging the left wheel out

The usual way of departing from a slippery airstrip in the Porter is to start with the nose pointing to the right, to help counteract the p-factor and engine torque. However, on this departure I didn’t have it pointing quite enough to the right, as I was soon to discover. As I started rolling and increased the power, the aircraft began to slide left not helped by the extra fuel in the left tank.

Now, there’s a split second window once you start rolling in which you have two choices, continue the take-off or abort. The later would have resulted in the aircraft going off the left side of the airstrip. Once rolling down the slope with zero braking action there is no way of stopping. So, with full right rudder, toe brake and right aileron we slide down the airstrip fully sideways as I increased the power enough to get us airborne but not so much as to pull the aircraft off the left side of the airstrip.

Parked up at Kegata, Papua

It was a major lesson in the effects of p-factor and engine torque. Chatting with one of Papua’s most experienced Porter pilots, he mentioned he usually starts with the nose pointing 20 degrees off to the right on really slippery slopes. That result’s in a fairly straight take-off roll, abet fully sideways, which in his words is “scary as hell!”. I’ll remember that next time!

http://www.swaynemartin.com/share-your-story/share-your-story-matt-dearden-indonesian-bush-pilot/

For those of you still chasing the dream, don’t give up! And check out Swayne’s blog for more inspiration and articles on how to achieve that dream.

Pilatus PC-6 Porter left wheel stuck in the mud at Tuput, Papua

Those of you who follow my Facebook page will have already seen a sneak preview of this little incident. Unsurprisingly, it’s not exactly uncommon for over two tonnes of aircraft to get stuck in the mud occasionally. I’ve been fairly lucky thus far, in that all of my mud vs. aircraft problems have been relatively easy to resolve. I know fellow pilots who have not been so lucky and had to spend hours digging themselves out of places. Nothing quite so drastic for me this time thankfully.

Stuck in the mud

Last week I had a charter from the tiny village of Gome to the even tinier village of Tuput. Tuput is on the high plateaux at nearly 9000ft elevation and a few miles from Agudugume. It’s not a place we go to very often and no-one from our company had been there for a few months. So before landing I made a low pass over the airstrip to check it’s condition out and all seemed fine with no visible obstructions, ditches or water logging problems.

Stuck in the mud

Landing and taxiing were all fine and the airstrip was surprisingly dry. The wheels weren’t digging in and braking action was very good. So it was a surprise when I came to turn around at the end of the airstrip and got stuck. The left wheel corkscrewed itself a good four inches into the ground and no amount of power was going to bring it back out again. So I accepted defeat and shut the engine down. With the passengers off loaded, along with all their baggage, the now lighter aircraft should be easier to extract from the grips of the mud.

Using the tow bar to straighten the tailwheel

First thing one needs to do to get unstuck is to straighten up the tailwheel. As I got stuck towards the end of turning, the tailwheel was at nearly 90 degrees to the fuselage. The Porter has a handy tow bar stored in the rear empennage which attaches to the tailwheel for manouvering the aircraft on the ground. It’s important to ensure the tailwheel is unlocked and in steer mode before using it though.

Tailwheel straightened up

Once straighten out, it was a case of digging out the left main wheel and laying down some stones for added traction. Now everything was prepared, there were two ways to escape the mud. One option is to gather as many local villagers as I could and get pushing. This can work but as there was only half a dozen or so folk around, and I was already lined up for take-off, I opted for 550SHP of Pratt & Whitney PT6 to get it out.

Lip in-front of the wheel smoothed off and stones added for traction

The aircraft rolled out of the hole with surprisingly little power and I departed without issue. All in all, pretty much a non-event and just another little tale to add to my life story out here in the jungles of Indonesia.

My flight bag and it’s contents

A pilot’s flight bag is a very personal thing. Some things we have to carry by law but there is plenty of scope to pick out those things you just can’t do without on a daily basis whilst flying. Here’s my list of things I just can’t do without!

1) The bag itself – Mine’s a bicycle courier style bag with a single large pouch to house the big items and two smaller zipped pouches, one inside and one outside. It’s about the maximum size I could get and still be able to store it under the pilot’s seat in the Porter.

2) iPad – As a pilot there’s plenty of waiting about (for weather, refuelling, loading etc) so this handy gadget keeps me busy in those idle moments. It also has GPS which could be handy should the aircraft’s decide to throw a wobbly.

3) Bose headset – Noise cancelling a must in the noisy cockpit of the Porter.

4) Passport – You never know when you might end up in another country.

5) Commercial pilot’s licence – Indonesian of course along with my radio and medical papers.

6) SAS survival guide – Handy pocket book should the worst happen.

7) Pocket survival kit – Standard issue military spec. kit to which I’ve added a mini-Maglite torch, butterfly stitches and antiseptic wipes. The aircraft has a much larger and more extensive kit stored behind the front passenger seat.

8) Raybans sunglasses – Used daily to protect my eyes from the hot, tropical sun.

9) SPOT GPS tracker – Used to send out distress signals of my exact location. Although all our aircraft all have a built in ELT and are tracked by satellite, this is my own hand-held device which utilises a different satellite network. More details here.

10) Notebook – My little reminder of various things like airstrips, minimum en route altitudes between places, frequencies etc.

11) Spare glasses – Being short sighted I usually wear contact lenses, so I am required to carry a second pair of glasses should an issue arise.

12) GoPro spare battery – Although single battery is good enough for a day, I do occasionally forget to charge it up so a backup is always handy to have.

13) GoPro remote control – So I can start and stop recording at will (saves hours of boring cruise footage to edit).

As well as all the above, I always carry a Leatherman on my belt along with my trusty Lumix GF-1 camera and a pen to fill in the aircraft’s paperwork after each flight.

Unloading in Ilaga

Don’t let the above photo fool you though. The weather has been pretty typical wet season weather this week. Certain parts of the highlands have had 14+ hours of rain every night which has made the ground incredibly soft on many of the airstrips. The worst affected that I’ve been to so far, has been Wangbe.

Landing and taxiing tracks in the mud at Wangbe

Now this airstrip has always been tricky to operate to/from thanks mainly to its unevenness. There are many bounders and a pronounced crown both of which do their very best to throw you off the sides when landing or taking-off. As the village is located in the valley at the bottom of the airstrip I have to land up-slope, try and turn around on the slope at the top without sliding off the sides and then taxi as slowly as possibly back to the bottom to unload. That taxi downhill with a fully laden aircraft is incredibly tricky when the ground is akin to a bog. There’s almost zero braking action and if you start to go a bit too quickly, you can end up skidding with both wheels locked and reverse thrust to slow you down again whilst attempting to use differential braking to stop you sliding off the sides. Did I mention the ditches on both sides?

Turning round in the mud at the bottom of Wangbe

At least the bottom part is flat so turning around again isn’t too tricky although there’s still almost no braking action here either. To get a 2.4 tonne tailwheel aircraft to turn around on boggy ground you need to have some momentum. If you stop and try and turn you’ll get stuck. My preferred method is to get a bit of speed up, reduce power to idle and apply right rudder and toe brake to bring the aircraft towards the right-hand side of the airstrip without going off the side and being careful to allow enough room for the tail to come around. Then add in some power and push the control stick forward to get some weight off the rear of the aircraft whilst pushing hard on the left rudder and brake to get the rear of the aircraft to swing around behind you. At this point the tailwheel is in full caster (like a shopping trolley) so once through about 130 degrees of turn, I get back on the right rudder and brake to slow the swing down and straighten up again before reducing power back to idle again.

Agadugume floatplane airstrip

Another airstrip affected by wet weather is Agadugume, although it does not have problems with mud but does suffer from flooding after recent rain. The big thing to consider here is the increased ground roll for take off, especially as it’s at nearly 9000ft (read my article on density altitude to understand why that’s a problem). Trying to explain to the locals why you can only take five of them out when there’s seven or more seats is pretty much impossible and can result in some heated arguments between them as they decide who gets to fly.

Cloud formations on the ridges east of Puncak Jaya

Every day this week the ridges have all had some form of cloud formation on them. Usually it’s just plain old cumulus building up due to the sun heating up the wet slopes. Occasionally you get some pretty interesting formations though, like the above. I love weather and reckon if I wasn’t a pilot I’d probably be a weatherman or something!

Flying amongst the mountains and clouds

I’ll try not to bore you with the technical formulas etc but basically the higher you go, the less dense the air is. Also, the warmer the air, the less dense it is. Now, combine these two facts and you end up with some pretty thin air.

10,000ft indicated, 11700ft density altitude. Note the CAS and TAS differences

That thin air effects aircraft in two significant ways: less power available from the engine and a faster true airspeed for the same indicated airspeed. But what does that mean in reality when flying out in the mountains?

On the ground in Agadugume

Consider landing at an airstrip at 8700ft like Agadugume. Note the temperature here, 19C. That’s pretty warm for nearly 9000ft. Combine that warm temperature with the altitude and you end up with a pretty fast true airspeed. So whilst your airspeed indicator is reading the normal 65kts on approach, you’re actually doing 85kts over the ground, assuming no wind. That makes for a much faster approach and is effectively like trying to land with a 20kts tailwind (and more often than not at mountain airstrips you have a tailwind too, just to make the landing even more interesting).

Approach into Agadugume at nearly 9000ft

What’s also noticeable when landing at high altitudes is the increased engine power required to maintain a normal approach speed and flight path. Thanks to the thin air, the wings are less effective at keeping you up in the air so if you do start to fall below your desired approach profile, it can require a significant amount of power to bring the aircraft back to where you want it, especially at all up landing weight.

Lined up for departure at Kilmit

Take-offs are also compromised thanks to those hot and high conditions. The engine can be limited by either torque or temperature or even Ng (turbine rpm) which you must be aware of so as to not exceed any limitations. Even if you are able to get full power, you won’t be getting the same performance that you get at sea level. That means longer take-off rolls and reduced climb performance, both of which need to be considered if you’re on a short airstrip in a tight bowl that you need to climb out of.

If in doubt, don’t try and take-off with a full load. Although trying to explain the effects of all this to the dozens of locals trying to get into the aircraft for a ride out of their village can prove a little tricky at times!

Pilatus Porter tailwheel digging into soft clay

It’s almost impossible to judge the condition of an airstrip from the air. What looks nice enough from above can turn out to be surprisingly soft once you’ve touched down. Standing water is common but doesn’t always mean the airstrip is going to be soft and boggy. It all depends what’s underneath. And that is something you can only know from having previously been there or at least chatted to someone else who has.

The worst kinds of airstrip are the pure grass ones grown on soil/dirt. In order to help with drainage, the locals tend to build these types of airstrip with a pronounced crown. An airstrip with a crown has a side slope on both sides, so that any water runs off the centre of the airstrip and to the sides. There’s usually a ditch on both sides to catch the water (or aircraft!). This kind of runway is the worst sort for a tailwheel aircraft to operate from.

You absolutely must land in the middle of the airstrip! If you land slightly off-centre on an airstrip with a crown, the aircraft will tend to want to slide off to that side. If it happens early into the landing roll you usually have enough airflow over the rudder to correct it. However, if you start to veer off the side later into the landing roll, you can have a problem.

Wangbe on a dry day. Note up-sloping runway and crown

The technique for rescuing the situation is something you have to judge depending on how fast you’re going, how much runway is left and how close to the edge you’re drifting. Braking the opposite wheel can help but if it’s really slippery you sometimes have no option but to add power and get some airflow over the tail to help pull the aircraft back to the centre.

Even once down to taxi speed the fun continues. A fully laden Porter at all up landing weight is 2660kgs which is how we land most of the time (you don’t make money flying empty!). That’s a lot of weight on soft ground and in order to prevent getting stuck you have to keep moving when taxiing. The hardest part is turning 180 degrees on a slippery, up-sloping airstrip like Wangbe.

Turning 180 degrees at Wangbe

In order to get the tailwheel to travel up the slope and position itself behind you so you’re lined up for departure, requires powering up the slope, steering to one side of the airstrip, stamping on the opposite rudder pedal and toe brake to get the aircraft to swing round whilst pushing forward on the control stick to get as much weight off the tailwheel as possible. If you stop at any time during this manoeuvre, you’ll probably get stuck.

As you’ll be off the centre of the airstrip to do this, the aircraft will want to slide in that direction as well as back down the slope, so you might need to use a bit of power to pull yourself back into the middle after you’ve got the aircraft pointing back down the runway. It’s quite a curious feeling when all three wheels are sliding sideways and the only way to stop it is to add power and turn into the slide.