Are you ready to upgrade to a twin? In the first of our two-part series, Jim Davis takes you through some of the drawcards and drawbacks of having two engines.
have crashed two aeroplanes. The first was my Tiger Moth, which I scattered along Runway 11 at Wonderboom, near Pretoria, because I failed to brief my passenger properly. And the second was an Aztec, which I scattered along Runway 08 at Port Elizabeth because I failed to read the Pilot’s Operating Handbook (POH) at all.
They were both quite gentle affairs. The Tiger prang had little potential for serious injury. But the twin, in beautiful weather, with one engine running perfectly, came this close to dumping five of us upside-down into a housing estate. Here’s what happened.
I was taking a bunch of Germans for a flight over their VW factory at Uitenhage. The boot door was off so they could photograph their magnificent establishment.
As we started circling, with the camera’s lens perfectly aligned, I noticed that the left engine’s oil pressure was sitting on zero. This was strange because the engine had just been overhauled. I did what any sensible pilot would do – I tapped the gauge. No response, so I stared dumbly at the shiny red cowl in the hopes it would somehow confirm that the gauge was faulty.
Eventually sanity prevailed – I shut the engine down and feathered the prop. No sweat,
we were at 2000ft and 15 miles from home.
Ignoring the bitching from the Germans, I told the tower we were returning asymmetric.
We were nice and high on finals, so I chucked out the gear and flaps, and then did some more staring dumbly. I was astonished to find no greens and no flaps. The gear was dangling but not locked. I could see this in the little, curved mirror on the cowl – stuck there for exactly this purpose.
So we were about to have a minor crash – a wheels up – not the end of the world. I pumped madly on the donkeys-dong handle for the emergency hydraulics. My efforts had no interesting results. If I had read the handbook I would have known that perseverance would have saved the day. But I hadn’t, so it didn’t.
The handbook would also have told me that in losing the left engine I had lost the only hydraulic pump on board – which is why nothing worked. The book would have advised me to feel under my seat, open a hatch to reveal a metal ring on the end of a cable. It would have instructed me to pull said ring, blasting carbon dioxide into the system, locking the gear down and lowering the flaps.
All this is a rather long-winded way of telling you that before you fly a complex aeroplane you are well advised to study the POH. In fact, let’s call that Golden Rule No.1 – the POH is your bible.
Anyhow, there we were, about to block up zero-eight, the main runway. The guy in the tower didn’t want this, and he certainly didn’t want the paperwork that goes with such an occurrence, so he said seven words that nearly killed us, “Charlie November November execute a go around.”.
And this brings us on to Golden Rule No.2 – the sheep rule. To explain this I must take you back a few years to a time when I was flying a Twin Comanche with the best flying inspector the Civil Aviation Authority (CAA) had ever employed. He was an RAF-trained, ex-Farnborough test pilot named Barry Radley.
Barry posed the following question to me: “You are flying a heavily loaded, piston twin. You are on short finals with the gear and flaps out when a herd of sheep wander onto the runway. What are you going to do?”.
“Take full power for a go-around,” I tell him.
“Wrong,” he objects. “The correct answer is, ‘Sir, I am going to start killing sheep’,”.
So that’s the ‘sheep rule’. It states that under such circumstances, if a bunch of sheep meander onto the runway, you start killing them rather than attempt a go-round. Sorry bunny-huggers, but it’s the sheep or you. Now, on with the story.
So the only thing I did right that day was to ignore ATC and continue with the landing. This turned out to be only slightly messy because the mains locked and the nosewheel folded. But if I had attempted a go-around, particularly with the gear dangling, we would almost certainly have been killed.
When an asymmetric twin on short finals calls to say, “We are going around” these are often the last words that pilot ever utters. In such cases, the good engine simply takes him to the scene of the accident. I will explain why, in a moment.
Twins do have extra safety, but it’s only available to those who are experienced, current, and have a professional attitude. A low-hour, out-of-practice or undisciplined pilot is far more likely to die in a light twin than in a single.
If you are giving serious thought to converting to, or buying, a twin engine aircraft, it would be a good idea to read the above paragraph again and let it swill round in the cranium.
A couple of years before the Aztec nonsense, I was going through a cocky 500-hour stage where I knew that rule-books and checklists were for pansies. I was taking off from Kimberley in a Twin Comanche when it started trying to go sideways. Thinking that it was a dust-devil whirly-gig thing, I foolishly dragged it into the air and came within a heartbeat of making a fireball in the desert.
There were two problems: first, I had skipped an insignificant, puny little check-list item – throttle friction. So the right pitch lever crept back almost into the feather position. Second, I should have aborted the take-off at the first sign of a directional problem.
This gives us two golden rules: Golden Rule No. 3 – the airline pilot rule. Take your checklists as seriously as an airline pilot.
And Golden Rule No. 4 – the crab rule: if there’s a problem keeping straight – abort your take-off.
I’m not trying to frighten you off multi-engines. I’m saying that, as with instrument flying, you are entering a different, and far more demanding sector of aviation. Only you know whether you have the maturity and discipline to handle it safely.
Most of the time flying a twin is much like flying a heavy single. But if one engine quits you find yourself in charge of a tiger that’s just waiting for the chance to rip your head off. This introduces us to a thing called Vmc and it is the number one problem with twins. Here’s what happens.
Vmc (Velocity minimum control)
Say the left engine stops, the aircraft will try 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 climb or just maintain height on one engine, you will find you have to use more and more rudder. Eventually you will have full right rudder and she will start turning left.
This is the tiger that’s about to bite your head off – it’s the beginning of the end.
Unless you wake up smartly and take control of the monster it will hurl itself upside-down and into the ground before you know what’s hit you.
The speed at which you run out of rudder and start losing directional control is called Vmc. More about this in a moment.
This is what happens. As she starts to turn, 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 do something smart within milliseconds the tiger wins.
The NTSB says twins have fewer engine failure-related accidents than singles. However, when there is an engine failure accident, it is four times more likely to be fatal if you are in a twin. Singles mostly remain shiny side up, while twins go in inverted or vertically.
So here’s Golden Rule No. 5 – the houses rule. If you hit Vmc – reduce power. The longer version says: When the chips are down throttle back and descend – it’s better that the houses get bigger rather than inverted.
Part 23 regs, under which most aircraft up to 12,000lbs are certified, tell us how much safety we can expect from two engines. Twins fall into two weight categories – above and below 6000lbs. And a further two groups: those with 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 pitifully small. At 5000ft density altitude (with the aircraft clean and the dead engine feathered) their rate of climb is calculated 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 only betters this by a miserable 9ft/min.
The Aztec, which is less than 6000lbs and stalls below 61kts, doesn’t need to climb at all under these conditions. In fact it manages 50ft/min. It is one of a whole bunch of light twins that are allowed to go downhill if an engine stops.
More thought provoking is the fact that no Part 23 twin is required to climb in the take-off configuration. This means that if one of the donkeys dies at the worst possible time – just after take-off – you are definitely going downhill. That is unless your aircraft is certified to carry 10 or more trusting souls.
The bottom line is that when you lose half your power you scupper between 90 per cent and 110 per cent of your climb performance. It’s like carrying your dead mate across the desert, rather than walking next to him. So that’s the bad news. If you still want to fly a twin you need to understand a bit of technical stuff – particularly Vmc.
Remember I said earlier this is the slowest speed at which you can keep straight on one engine by using full rudder (you can also use up to 5° of bank into the live engine).
If you get to the stage where you have full rudder and she starts turning the quickest way to 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.
So it would seem that the obvious thing to do is avoid slowing down to Vmc, in exactly the same way that we all avoid slowing down to stall speed. But I’m afraid it’s not that simple because Vmc, unlike stall speed, is not a fixed number – it depends on a whole bunch of factors, 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.
Things that affect Vmc
1. 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 this is a bit of a double-edged sword because you will eventually reach an altitude where Vmc is the same as stall speed. This is obviously not a cheerful thought – losing directional control and stalling at
the same time.
And this leads to another snag. Vmc demonstrations are part of type conversions. This leads to an unhappy situation where you know the aircraft is on the verge of misbehaving in a big way – so you want to put as much space as possible between yourself and terra-firma as possible. But if you do so you are making the situation worse by introducing a stall into the mix.
At the other extreme, the FAA used to call for training Vmc demonstrations to be done at low altitude, because that’s where the good engine is producing most power. This rule led to Piper’s magnificent Twin Comanche getting a bad name. Because it was the cheapest new twin it was the most popular one for multi training. Its slightly twitchy wing didn’t help matters.
So we had low hour pilots practicing a dodgy manoeuvre at low altitude in a slippery aircraft. Not a recipe for enhancing the Twin Com’s reputation.
And to further complicate matters, if your twin is turbocharged, which seems like a good idea, because if you are on one motor you would like that one to give you as much power as possible, wouldn’t you? Well yes, but remember that the more power you have on the good motor the more it tries to turn the aircraft and therefore the higher your Vmc.
2. Flap setting
Using flaps increases Vmc. The reason is that the flaps move the centre of pressure aft, which gives the rudder a shorter turning moment. So retracting flap is generally a good idea – it not only reduces Vmc it also gives less drag, increasing your airspeed.
3. C of G
If the Centre of Gravity moves aft, Vmc increases. Again, it shortens the rudder’s moment arm. So for extra safety of a low Vmc put the fat buggers in the front.
4. 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 – getting rid of drag by pulling the gear up may not be a smart move if it puts you below Vmc.
Increasing the weight (‘mass’, for the pedantic) increases Vmc due to asymmetric blade effect. Don’t panic – it’s actually quite simple. Here’s how it works. At cruise speed the fuselage is level and the props are square on to the airflow. So on both sides of the prop disk the blades meet the air at the same angle of attack, giving you equal thrust all round the disc. Now, if you are flying slowly, or if you load the aircraft so it needs a higher angle of attack, and has a nose up attitude, this means that the prop disks lean back.
In this nose-high position, with a normal clockwise prop (viewed from behind) the right-hand, down-going blade has a greater angle of attack than the left-hand, up-moving blade. This causes more thrust on the right of the disk, which tries to turn the aircraft left. This is also part of the reason that most single engine aircraft pull to the left while climbing.
The diagram to the left shows a twin flying at a large angle of attack. The thrust line from each engine has moved to the right, which obviously has the effect of turning the aircraft left.
If the left motor 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 and you are flying on the left one, its close-in thrust line will give you 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.
To summarise, a large angle of attack causes asymmetric blade effect to move the thrust to the right of the disk, so the problem is most critical if the left motor fails. Many twins now have counter-rotating props – the right-hand one turns anti-clock – to eliminate this problem.
6. Feathering the dead engine
A windmilling 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.
7. Angle of bank
Up to 5° of bank towards the live engine reduces Vmc by a massive 10 to 15 knots. 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, on its tummy, across an ice lake. You ask your young sister to help you move it. You both put on spikes. You go round to the front and pull forwards on the right-hand prop. Obviously the aircraft swings round to the left. To keep it straight you get your sister to push on the right-hand side of the tail. Now the aircraft slides smoothly across the ice in the direction shown in the diagram on the left.
Now let’s think for a moment. What do you imagine the ball is doing while the aeroplane slides across the lake? I’ll tell you – it’s spot in the middle. There is no bank, and no acceleration to move it out of the middle.
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 side-slips. But the ball stays in the middle!
This sideslip is extremely bad news – it causes huge drag, and the aircraft tries to weathercock to the left. You can cure both problems by banking a bit to the right. This produces a sideways force which kills both the sideslip, and the weathercocking tendency – so you have less drag and more rudder control. And what happens to the ball now there is no sideslip? It moves out to the right, because of the bank.
The diagram on the right shows how this works. Say your aircraft weighs 5000lbs, then it needs 5000lbs of lift to keep it in level flight. Let’s bank it 5° to the right and split the lift vector into vertical and a horizontal components. We lose only 19lbs of vertical lift but gain a massive 436lbs of horizontal help. More than 5° of bank is counter-productive. In fact, for most light aircraft 3.5° of bank is about right.
What is Vmca ?
Now you know about Vmc, I must introduce a new animal called Vmca. This is simply the worst case scenario for Vmc and it’s marked with a red line on your ASI. It was originally determined by the factory test pilot in a white overall when the aircraft was first certified under Part 23. It assumes that the critical engine fails under the following conditions:
- Power was at take-off settings on the good engine (normally full power).
- The aircraft was at gross weight.
- Flaps were set in the take-off position.
- The undercarriage was up.
- The C of G was on its aft limit.
- The critical engine prop windmilling in the take-off setting (normally fully fine).
- And the aircraft was banked towards the live engine at not more than 5°.
Vmca says that if everything is against you, this is the highest speed at which you will lose directional control. So if you have feathered the prop, or have less than sea-level power, or a more forward C of G, and so on, Vmc will be less than the red line.
And this brings us to Golden Rule No. 6 – it’s sometimes okay to fly in tiger territory below the red line (Vmca), but if you do it you had better understand tigers.
So that’s the basic theory. In the next issue I will take you into the cockpit so we can do some big captain multi-engine flying.
Jim Davis has 15,000 hours of immensely varied flying experience, including 10,000 hours civil and military flying instruction. He is an established author, his current projects being an instructors’ manual and a collection of Air Accident analyses, called Choose Not To Crash. Visit Jim's website by clicking here.
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Are you ready to upgrade to a twin? In the first of our two-part series, Jim Davis takes you through some of the drawcards and drawbacks of having two engines.