Are you ready to upgrade to a twin? In the first of this two-part series, Jim takes you through some of the drawcards and drawbacks of having two engines. And in our next issue he will put you in the cockpit for some dual on how to manage a multi-engine aircraft. 

I have crashed two aeroplanes. The first was my Tiger Moth which I scattered along Runway 11 at Wonderboom because I failed to brief my passenger intelligently. And the second was an Aztec which I scattered along 08 at Port Elizabeth because I failed to read the POH at all. 

The Aztec came close to killing five of us. We were on an asymmetric final approach, with the gear dangling but not locked, when the tower gave us some really poor advice – they suggested we do a go-around. 

I suspect this was based on ATC’s aversion to filling in accident report forms – in triplicate – using carbon-paper (yesteryear’s Alt/Command C) rather than on any traffic requirements, or aerodynamic insights may have had. 

So, almost accidentally, excuse the pun, this brings us to the first two rules of twin flying: 

Never let ATC tell you how to fly the aeroplane. This is known as the never let ATC tell you how to fly the aeroplane rule. 

A go-around under those circumstances is a terminally bad idea. This is known as the sheep rule. 

Let me explain the sheep rule: A couple of years earlier, I was doing a renewal in a Twin Comanche, with the famous ex-RAF test pilot, Barry Radley, who was with the DCA (now SA-CAA) at the time. He posed the following question – which I got wrong. “You are on final approach in an asymmetric light twin, configured for landing, when a bunch of sheep drift onto the runway. What are you going to do?” 

I told him that, depending on the load and the density altitude and the aircraft’s published single-engine service ceiling, and so on, I would take full power retract the gear and flaps and do a go around. 

He viewed me from beneath his shaggy eyebrows with uncensored disgust, “No Davis, the correct answer is, ‘Sir, I would start killing sheep.’” 

We called our betters ‘Sir’ in those days and we respected their opinions. This was a good thing because, a couple of years later Mr Radley’s sheep rule saved my life, and the lives of four fat Germans. 

He explained that doing a go-around in an asymmetric light twin is sometimes possible – but never advisable. If you continue with the landing you may damage the aircraft, but if you try to keep it flying you are likely to invert yourself into the ground. 

So, the rule is simple: kill sheep – not people. (Sorry bunny-huggers, I actually know many sheep who are nicer than people.) 

Now back to the Aztec, ZS-CNN. It was a sad aeroplane who, because it never flew long between mishaps, became known as Charlie Never Never. 

The engine failure was not my fault, but everything else was. I could have fixed the dangling, but not locked, undercarriage if I had taken the trouble to understand the two emergency extension systems – but I didn’t. 

As we touched down on the grass the main wheels locked into position and the nosewheel folded back. The nose dug in and most of the Germans shot forward and tried to squash me against the windscreen. One tumbled out of the baggage door and hopped around on one foot complaining. 

To add to the chaos Ian Ritchie, the maintenance guy, pitched up at the same time as the fire brigade. Their agendas could not have been more different – the brigade was keen to douse any chance of a fire, but Ian, who was faced with the prospect of rebuilding the aircraft, would have preferred a jolly conflagration. So, while the firemen were unrolling hoses, Ian was waving his arms at them and shouting, “Far cough, far cough – let the bastard burn!” 

I am trying to point out that this fairly harmless and even remotely amusing incident, was a wonderful alternate to crashing, upside-down, into the military base, while trying to achieve the impossible in the form of a go-around. 

The bottom line is that twins come with extra safety, but it’s only available to pilots who behave like professionals. Undisciplined pilots should stick to singles. This is not my opinion – the stats say that you are approximately twice as likely to die if an engine fails on a twin. 

Let me say that again – if you are in a light aircraft that has an engine failure, you have a much better chance if you are in a single faced with a forced landing, than if you are in a twin that’s capable of making it to a safe landing place. 

So, you may well ask, if the twin could make it to a safe landing place, why would it kill people? The answer is simple – although the aircraft may be up to the job, many twin pilots are not. 

Think about this – how many twin accidents have you read about where the aircraft crashes under control with the wings level? It seldom happens. When a twin crashes after an engine failure, it’s because the pilot has lost control and gone in inverted. 

Flying a twin is like instrument flying – you need to be well trained and current to do it safely. When I say current, I mean current on engine-out ops. I have noticed that twin fatalities around the world have mainly been with experienced twin pilots at the controls. They were simply not current on engine failure procedures. 

If you don’t have your finger out during an EFATO in a twin you will almost certainly die within seconds. With a C172 you are faced with a landing in uncertain terrain at 48kts – a very different story. 

When I was young and going through that cocky 500-hour stage where I knew that rulebooks and checklists were for pansies, I took off from Kimberley in a Twin Comanche and lost directional control. I foolishly dragged it into the air sideways and came within a millithing of making a fireball in the desert. 

There were two problems: first, I had skipped what I thought was an insignificant checklist item – throttle friction. The right pitch lever crept back almost into the feather detent. Second, if I had been current, I would have aborted at the first sign of a directional problem. 

That’s another golden rule for flying twins. The slightest sign of directional problems during takeoff means ABORT IMMEDIATELY. It does not mean, ‘I wonder if that’s a gust of crosswind.’ Neither does it mean, ‘I will just wait and see what happens.’ It must be part of your self-briefing before you open the throttles. ‘If it doesn’t run straight as an arrow, I am going to snap these back so damn quickly you will think we have hit a wall.’ 

I’m not trying to warn you off multis. I’m saying that as with instrument flying, you are entering an advanced sector of aviation. And only you know whether you have the maturity and discipline to do it safely. 

I didn’t but I was lucky to live through that era of foolishness. 

Most of the time a twin will handle much like a heavy single, but if one motor quits the aircraft grows fangs. Here’s what happens. 

Say the left motor fails, the aircraft tries to turn left. This means you must use right rudder to keep straight. But there’s a problem – the rudder is only effective if there is enough airspeed. So, if you let the airspeed decay, perhaps because you are trying to clear obstacles, you will have to use more and more right rudder as it slows down. Eventually you will have full right rudder and it will start turning left. The speed at which this happens is called Vmc (Velocity minimum control) – more of this shortly. 

As it starts turning, the left wing, on the inside of the turn, moves slower, gets less lift and the aircraft rolls left. You counteract this by moving the stick to the right and this causes more trouble. The left aileron digs down into the airflow dragging the wing back and pulling you deeper into the turn. 

Now you are running out of airspeed while rolling and yawing left. You have full right aileron and rudder. If you don’t throttle back the good engine immediately you will roll on to your back and spin. This happens in a heartbeat. 

Here’s another golden rule: When the chips are down; throttle back and descend rather than lose control at Vmc. 

Part 23 regs, under which most aircraft up to 12000lbs are certified, tell us what safety we can expect from a twin. They are in two weight categories – above and below 6000lbs. And two groups those with a landing-configuration stall-speeds above and below 61kts. 

Only twins with a gross of over 6000lbs or a stall speed of more than 61kts have to demonstrate any ability to climb on one engine. And that requirement is almost pitiful – at a density altitude of 5000ft (with the aircraft clean and the dead engine feathered) their rate of climb comes from a formula based on stall speed. For example, a Rockwell 500S (Shrike) which is over 6000lbs and has a stall speed of 63kts, must be able to climb a no-wind gradient that translates to 107ft/min. The Shrike actually achieves 129ft/min. 

The Cessna 310, which is less than 6000lbs but stalls at 64kts, needs to climb at 110ft/min. When the aircraft is brand-new and flown by a factory test-pilot it beats this by a miserable 9ft/min. 

The Aztec which is less than 6000 lbs and stalls below 61kts, doesn’t need to climb at all under these conditions. Actually, it manages a positive 50ft/min. But is one of a whole bunch of light twins that are not required to demonstrate any single engine climb at all. In fact, they are all allowed to go downhill if an engine stops under those conditions. 

More thought provoking is the fact that no Part 23 twin is expected to do a single-engine climb in the take-off configuration unless it is certified to carry ten pax or more. 

The bottom line is that when you lose half your power you scupper between 90 and 110% of your climb performance. It’s like carrying your dead mate across the desert compared to walking with him. 

So that’s the bad news. If you still want to fly a twin you need to understand some technical stuff – particularly Vmc. Remember it’s the slowest speed at which you can keep straight on one engine. 

When you have full rudder and it starts turning, the quickest way regain control is to reduce power on the good engine – this decreases the turning force and lowers your Vmc. But it also means the houses start getting bigger. 

Vmc is not a fixed speed – it depends on a whole bunch of things, the most obvious of which is the amount of power on the good engine. 

Here are the main things that alter Vmc. Remember that a lower Vmc is better – it means the aircraft remains controllable at lower speeds. 

FACTORS AFFECTING Vmc 

Density altitude. With normally aspirated twins Vmc decreases with altitude. This is because you get less power the higher you go so there’s less turning force. But think of this: if you fly a turbocharged model for better high-altitude performance, you also have the disadvantage of a higher Vmc. 

C of G. If the C of G moves aft Vmc increases; which is bad. This is because it shortens the rudder’s moment arm. So, for extra safety seat heavy pax forward. 

Flap setting. Using flaps increases Vmc. The reason is that the flaps move the centre of pressure aft, which again gives the rudder a shorter turning moment. So retracting flap not only reduces Vmc. It also gives less drag – increasing your airspeed. Generally, a flapless takeoff is safer on a twin. 

Undercarriage position. If the gear moves back as it retracts (which often happens) it takes the C of G with it and increases Vmc. This can be bad news if you lose an engine just after lift-off – pulling the gear up may put you below Vmc. 

Load. Increasing the load increases Vmc due to asymmetric blade effect. Here’s how it works. At cruise speed the fuselage is level and the props are square on to the airflow. So, at both sides of the prop disk, the blades meet the air at the same angle of attack. This gives equal thrust all around the disc. 

If you load the aircraft it flies at a larger angle of attack in a nose up attitude, so the prop disks lean back. This means that with a 

normal clockwise prop (viewed from behind) the right hand, down-going blade has a larger angle of attack than the left hand, up-moving blade. This causes more thrust on the right of the disk. This in turn tries to turn the aircraft left. This is partly why most singles pull left at high angles of attack while climbing. 

The diagram shows a twin flying at a large angle of attack. The thrust line from each engine has moved to the right, which turns the aircraft left. 

If the left engine fails, you are going to struggle because the thrust from the surviving right motor is far from the fuselage – causing a strong turning moment. However, if the right engine quits you are flying on the left one with its close-in thrust line, giving a smaller turning moment. So, with clockwise props the left motor is known as the critical engine. Meaning that your situation is most critical if it fails. 

Many twins now have counter-rotating props, the right hand one turning anti-clock, to eliminate the critical engine problem. 

Feathering the dead engine. A wind-milling prop causes tremendous drag and demands a lot of rudder which increases Vmc. Feathering the prop turns the blades edge-on to the airflow and cuts the drag to almost nothing. 

Angle of bank. Up to 5 degrees of bank towards the live engine reduces Vmc by a massive 10 to 15kts. To show you how bank helps, I am going to tell you something you won’t want to believe. Hold on to your hats. 

Flying on one engine is like pushing an aircraft, wheels up across an ice lake. You ask your young sister to help you move it. You both put on spikes. You pull forwards on the right-hand prop and the aircraft swings round to the left. So, to keep it straight, you get your sister to push sideways, against the right-hand side of the tail. Now the aircraft slides smoothly across the ice in the direction shown in the diagram. 

While this is going on, what do you think the ball does? I’ll tell you – it stays bang in the middle. There is no bank and no acceleration to move it out of centre. It’s exactly the same after an engine failure. If you keep the wings level the live motor pulls forward, the rudder pushes to the side and the aircraft sideslips. But the ball stays in the middle! 

You read that correctly – you sideslip but the ball is in the middle. A glider type yaw-string would naturally indicate the sideslip. 

Of course, this sideslip is bad news on two counts. It causes massive drag, and the weathercock tendency increases the aircraft’s desire to turn left. Fortunately, you can cure both problems in one go – you simply bank slightly to the right. This produces a sideways force which counteracts both the sideslip and the weather-cocking tendency – so you have less drag and more rudder control. And now that there is no sideslip the ball moves out to the right. But a glider yaw string would be happily straight, telling you that the aircraft is going where it’s pointing. 

In a nutshell, this 5 degree bank into the live engine is critically important. When you are flying on a knife-edge you need all the help you can get. Take it very seriously. If you don’t use it the aircraft will not perform according to the POH. 

The diagram shows what happens. Say your aircraft weighs 5000lbs, then it needs 5000lbs of lift to keep it in level flight. Let’s bank 5 degrees to the right and split the lift vector into vertical and horizontal components. We lose only 19lbs of vertical lift but gain a massive 436lbs of horizontal help. More than 5 degrees of bank is counterproductive. For most light aircraft 3.5 degrees is about right. 

WHAT IS Vmc? 

It’s simply the worst-case scenario and it’s marked on your ASI with a red line. Vmc was determined by the factory test pilot when the aircraft was originally certified under Part 23. It assumes the critical engine fails under the following conditions: 

  • Power is at take-off setting on the good engine (normally full power). 
  • You are at gross weight. 
  • The flaps are set for take-off. 
  • The undercarriage is up. 
  • The C of G is on the aft limit. 
  • The critical engine prop is wind-milling. 
  • You are using not more than 5 degrees bank towards the good engine. 

Vmc says that if everything is against you, this is the highest speed at which you will lose directional control. So, if you have feathered or have less than sea-level power and so on, Vmc will be less than red line. 

Golden rule: To fly in tiger country below red line you had better understand tigers. 

So that’s the basic theory of twin flying. If you think this is for you, then next month we will look at practical twin flying. You sit in the left-hand seat and I will try to explain how to do it safely. 

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