PILOT TIPS: Are you ready to fly a twin?
Are you ready to upgrade to a twin? In the second of our two-part series, Jim Davis takes you into the cockpit for some practical twin-engine flying.
Before vaulting into the left-hand seat, let’s quickly go over all the good things and golden rules that we looked at in the last issue. If you want a thorough refresher, it might be an idea to re-read part one of this feature now.
Remember, a twin flies pretty much like a big single, most of the time. You only need to be twin-wise when one engine starts playing up, or lets go completely – but then you really do need to know what you are doing. A multi-engine pilot who is out of practice is a very dangerous animal – he is far more likely to kill people than a rusty single engine jock.
Vmc is the slowest speed that full rudder will counteract asymmetric power. It’s variable and it depends on:
With un-turbocharged twins, Vmc gets slower the higher you go – less power gives you less turning force. But beware when you get to an altitude where Vmc is the same as stall speed. It’s not a lot of fun – running out of rudder control just as you stall can cause a few fragrant moments. So while it might seem like a good idea to have plenty of height when you practice Vmc demonstrations, remember that you really don’t want to practice them at the same time as practicing stalls. Sensible instructors either pull on a notch or two of flap, or simulate Vmc by limiting rudder travel with their size 12s.
Remember that while turbocharging is good because if you lose an engine you want the remaining one to have plenty of umph, the downside is the more power you have the higher your Vmc. Nobody said this would be easy!
Flaps increase Vmc because they move the centre of pressure aft, giving the rudder less leverage. So flapless take-offs are generally best – you get less drag and a lower Vmc.
C of G
If you have an aft C of G, Vmc increases. Again, because of a shorter rudder moment arm. So put the fat buggers in front.
If the wheels move back as you retract them, they take the C of G with them – increasing Vmc. So be careful just after lift-off – if you are close to Vmc, retracting the Dunlops might push you over the edge.
The heavier your aircraft, the higher the Vmc. This is because asymmetric blade effect creeps in to spoil your day when you have the nose up and low airspeed. Basically, the thrust moves to the right-hand side of each propeller disk (if the prop turns clockwise when viewed from behind). This means the right engine has a greater turning moment than the left one. So the left engine is called the “critical” one because if you lose it you are stuck with the right engine making a serious effort to turn you. To eliminate this critical engine problem, many twins now have counter-rotating props, with the right-hand one turning anti-clockwise. Dig out the last issue of Australian Flying for the full story on asymmetric blade effect.
Feathering the dead engine
A windmilling prop causes massive drag, which calls for huge rudder input. It obviously increases Vmc. If you feather the prop you set the blades edge-on to the airflow and reduce the drag to almost nothing.
Angle of bank
If you bank the aircraft, up to 5°, towards the live engine you reduce Vmc by as much as 10 to 15 knots. Although this puts the ball out of the middle by about half of its diameter, it reduces drag by preventing a sideslip. (When you are asymmetric and the ball is in the middle you are actually slipping towards the dead engine. It’s all explained in our previous issue.) Bank into the live engine not only prevents a drag-inducing sideslip, it helps you keep straight because it gives a sideways component to lift, which reduces the amount of rudder you need.
Vmca is the red-line on the ASI. If all the above conditions are at their worst, this is the highest speed VMC will be.
On one engine the ball tells lies. If it’s in the middle you are sideslipping, causing serious drag and increasing Vmc by 10-15kts. It should be half a ball out towards the good engine.
Six golden rules
1.The POH rule (read it): The POH is your bible – you are doomed to damnation if you don’t study it.
2.The Sheep Rule (kill them): If you are on finals, heavy, asymmetric, with everything hanging out and a bunch of sheep drift into the field – do not try a single-engine go-around – kill sheep rather than people.
3.The Airline Pilot Rule (copy them): Take checklists as seriously as airline pilots do.
4.The Crab Rule: If it starts going sideways abort your take-off immediately.
5.The Houses Rule: At Vmc, when the chips are down, throttle back a bit and let the houses get bigger, rather than let get them inverted and bigger.
6.The Tigers Rule: Only fly in tiger territory, below the red line (Vmca) if you understand tigers.
Before we go further, let me give you two tips that will save you a load of money, and make you a far better multi-engine pilot.
Firstly, get hold of the POH (Pilots’ Operating Handbook) a few days before you plan to have your first flight. Read it from cover to cover and understand the systems – the hydraulic system, the electrical system, the vacuum system, and most important of all, the fuel system (including cross-feeding).
And secondly, turn up early for your flights and make friends with the cockpit. Spend as much time as you can sitting in the driver’s seat and getting all touchy-feely with everything in the cockpit. Your hand should be able to go straight to the cowl flaps or the alternate air levers without looking for them. Practice your round-the-cockpit checks (discussed later). Practice your pre-take-off vital actions and your engine failure drills until you can do them with your eyes shut. Say, touch and do – muscle memory works like a charm. Knowing your way around the cockpit and being good at these procedures are excellent life insurance.
Okay, now you have got the basics wrapped up it’s time to head for the aircraft with a good instructor. To avoid all the PC nonsense like, “he or she will explain to his or her pupil…”, our instructor is a sheila, and she is called “her” or “she”. Fine?
This sheila, because she is good at her job and wants to avoid any nonsense in the air, will brief you that during a simulated engine failure she will sing out, “You have a simulated engine failure. I want you to identify which engine has failed but don’t feather it”. Or whatever else she wants you to do.
I once forgot to include this vital briefing while converting an overweight schoolteacher to a Twin Comanche. This resulted in some thought-provoking moments when the right engine failed over the trees just after take-off. He thought I was an evil bastard and had cut the motor, and I thought he had done something terminally stupid. So we wasted vital seconds discussing these opinions before I realised that the aircraft had indeed betrayed us.
Again, it’s mainly like a big single, but there are a couple of things that are specific to twins:
Props and nosewheel
Have a very careful look to see whether the nosewheel is ahead of or behind the props. Why is this important? Well, imagine what happens when you come to a bit of a step up, say from the tar to the grass. If the nosewheel is ahead of the props then you want to go up the bump square on – so the nosewheel lifts the props before they get there. And if you are going down a step, then you would like the nosewheel behind the props. Give it some thought, particularly when taxiing over uneven ground.
The dihedral of the wings tilts the motors inwards, so many twins have different left and right dipsticks. They should be marked L and R on the caps. Obviously getting them in the wrong engines can be expensive and dangerous.
Twins often have a baggage compartment in the nose. Make very sure that it is shut and locked properly. There have been a number of cases of baggage escaping into props, with disastrous results.
Make yourself a round-the-cockpit sequence for checking everything. You will use it several times on each flight. I like top-to-bottom-then-left-to-right-and-right-to-left-across-the-panels (see diagram above). It leaves nothing to memory. After you have stopped and started engines in flight the cockpit becomes chaos. Your round-the-cockpit check is the only way to make sure you put everything back to where it should be.
Beg your instructor on bended knee to use the words ‘left’ and ‘right’, not any of this nonsense about ‘port’ and ‘starboard’ or ‘No.1’ and ‘No.2’ engines. You may understand them on the ground, but in the air it only takes a moment’s confusion to cause real trouble.
Follow the POH. Do everything slowly and thoroughly. If your hands flash round the cockpit flicking switches you’re going to make your instructor hate you. Set the idle revs carefully. With the noise of two engines it’s easy to have one engine sitting at 1700rpm without even noticing it. Once both engines are idling at the correct revs, do another round-the-cockpit check. There are engine instruments to check, avionics to switch on, gyros to set and so on.
Taxiing needs care. Forward visibility is good, but watch out for tie-downs, gravel and so on, under the props. Remember the relationship between props and nosewheel that we discussed earlier. For tight turns use more power on the outside engine to help you round. Don’t let your speed run away – two engines idling can cause you to build up quite a canter on a smooth surface.
You will also notice that the engines, being out on the wings, have quite a bit of inertia when you are turning, so she tends to carry on turning further than you want. Also, there is virtually no slipstream over the rudder, so it will be less effective than you expect.
Pre-take-off checks (vital actions)
Use a home-brewed written checklist. POH checklists are fine but they don’t include such things as setting frequencies, flight instruments, passenger briefings etc.
In a twin there’s too much to do, so don’t try to commit it to memory – you must have something typed out.
Now is the time to sit up and pay attention – this is the most important part of your whole conversion. Plan for an engine failure on every take-off. This means that before every take-off you must scratch your head and think carefully about the following:
On a hot day, a high altitude airfield can have a density altitude above the single-engine ceiling of many twins.
Load has a massive effect on single-engine performance. A Seneca I’s single engine ceiling increases from 3650ft to 10000ft if you reduce your gross weight by 360kgs!
A decent headwind not only gives you a shorter ground run but it improves your obstacle clearance – which is seldom a problem in a twin unless you are on one engine.
Can you land straight ahead on the runway after an engine failure?
Wet or dry, grass, gravel or paved all affect not only your acceleration but also your braking if that becomes necessary.
Even a 1° slope may be more than your single-engine climb gradient.
What are the landing options past the end of the runway?
The blue line on your ASI gives the best rate of climb speed. But your best angle of climb can be more important if you are trying to clear the gum trees. Look it up - it’s in the POH.
Here are two examples using figures Seneca II handbook figures.
You are taking off from Orville at gross. It’s 3550ft amsl, and has 1740m of tar. The OAT is 45°C which gives you a density altitude of 7500ft. If one engine fails you’ll climb at 150ft/min – if the engines, airframe and props are in good condition, and your handling skills are pretty sharp. The POH says that you will use 620m to get to 50ft. This leaves you more than a kilometre of runway ahead.
I would plan to climb to 1000ft agl at the best rate of climb speed of 89kts. If an engine fails soon after lift-off with no usable runway ahead I would go to 76kts (best angle of climb) and use local knowledge to reach the golf course or whatever. If I couldn’t clear obstacles and maintain Vmc I would reduce power and land ahead regardless of terrain.
And, most important, I would get all this in my head before even starting the engines.
The second scenario is you are in the same aircraft, at the same airfield on a chilly 0ºC morning. There is only yourself and one passenger, no baggage, and you have two hours worth of fuel. The POH says you will be at 50ft 300m after releasing the brakes, and you will climb at 500ft/min on one engine, and almost 2000ft/min on both, so you have many options.
Here’s an example of a typical self-briefing before take-off:
- There’s no crosswind so if I have a directional problem I will abort immediately.
- I will rotate at 71kts (red line + 5) and I’ll retract the gear when I have positive climb, and I will accelerate to 89kts (blue line).
- If I have an engine failure below 76 knots (best angle of climb) I will be prepared to throttle fully back and land if directional control becomes a problem.
- If I have an engine failure above 76 knots, I will:
1. Fly the aircraft – maintain direction with rudder, and at least 76kts;
2. Power up – all levers fully forward;
3. Gear up;
4. Flaps up;
5. Identify – dead foot dead engine;
6. Confirm –throttle back suspect engine;
9. Bank up to 5º towards the good engine; and
10. Plan – (eg I will do left-hand circuit on to zero-five).
It’s a very good thing to say this, or similar, out loud just after you’ve finished your take-off vital actions. It sticks in your head better if you do this. Don’t be embarrassed about it – if you are alone, no problem. If you have a passenger, explain that this self-briefing is what professional pilots do.
You won’t remember all this under pressure. Do the sitting-in-the-cockpit-muscle-memory thing so often that it becomes second nature. It works even better if you say it out loud. Your brain remembers and reacts better to voice commands – even if they are your own.
The Seneca II POH recommends that after lift-off you leave the wheels down until there’s no useable runway. Most pilots prefer to retract the gear as soon as they have a positive climb. The first method avoids doing an embarrassing gear-up on the runway, however the second means the aeroplane is flying properly, and in a strong position to accelerate and climb away if an engine fails. The choice is yours. I prefer to get the gear up as soon as possible.
Line up and stop briefly. Rolling take-offs are for cowboys. If it’s a prop-friendly surface, hold her against the brakes and increase power smoothly on both engines. Make sure that temps and pressures are in the green, fuel flows are steadily increasing and roughly equal, and power is increasing at the same rate on both motors (turbo-charged engines sometimes lag or surge). Release the brakes and go smoothly to full power. Confirm that the ASI is live and increasing.
The aircraft will accelerate and climb much quicker than you expect. Everything happens fast and you will be glad you spent time practicing checks. When you reduce power you will notice the thrum of unsynchronised engines. Do not watch the rev-counters while synching – they are not accurate enough and will confuse you. Simply move one pitch lever (not throttle) back or forward a millifrac until there is no beat. Good syncing is the hallmark of a good twin pilot.
In the climb there is no nose to use as an attitude reference. Well, it’s there, but you just can’t see it – you’ll get used to it.
Your upper-air training will include steep turns and stalls. During stall recovery don’t be too quick with the power – if it comes in asymmetrically below Vmc you can go inverted.
Much of your training involves engine-out work. Because repeated shutting down and re-starting can be hard on engines, your instructor will sometimes simulate a feathered engine by using zero-thrust settings from the handbook. You will do a Vmc demonstration on one engine. During this you will slowly reduce airspeed by easing back on the stick. You will need more and more rudder to keep straight.
Eventually you will have full rudder and the aircraft will start turning – that’s Vmc. Because you caused this situation by easing back on the stick, it’s easy to think that you should recover by moving the stick forward. If you do this you will recover – but slowly. The quickest way of regaining directional control is to reduce power on the live engine.
On many aircraft Vmc happens close to stall speed. If you stall and lose directional control simultaneously, guess what – you will find yourself spinning. For this reason Vmc demonstrations are often done with flap to keep well above the stall. We discussed this earlier.
Increased power at low-altitude gives you a higher Vmc – which means it is further from the stall speed, so we have the silly situation where it’s theoretically safer to do the Vmc demonstration at low level. Personally, I prefer to do it at high altitude and use flap to keep a healthy distance from the stall. Better still, I manually limit rudder travel with my foot when training.
If you lose an engine at any time other than just after take-off, don’t rush into feathering and shutting down. Once you have identified the engine have a calm look round the cockpit – chances are it is a fuel problem that can be fixed by changing tanks or putting on the pump.
Smooth and steady is the secret for all asymmetric flying – don’t be in a hurry. No violent manoeuvres. Plan for gentle turns in the circuit. Particularly don’t allow a tailwind on base to force you into a hammerhead and steep turn on to finals.
They used to say never turn towards the dead engine. This is not a cast-in-bronze rule, but it carries a warning – banking into the dead engine increases Vmc dramatically. Turn either way, but make it gentle and maintain plenty of airspeed.
After doing the immediate actions following an engine failure, it’s time to tidy up. Close the cowl flaps on the dead engine, to reduce drag, and open them on the live engine to keep it cool. Pull back the throttle, pitch and mixture on the dead engine so that they are out of the way. Shut down the mags, fuel pumps and other services to the dead engine.
Check that the live engine supplies adequate electrical power, vacuum, and hydraulic power. To make sure you don’t forget anything, use that round-the-cockpit check again.
If an engine fails enroute, re-plan your altitude and ETA’s and tell ATC. Also, give thought to fuel management. It’s generally best to use fuel from the same wing as the dead engine – you will need less aileron to keep that wing up.
For a single-engine landing tell ATC you are asymmetric and won’t be able to do a go-around. Do a normal circuit, or even go a little wider so you can keep the turns really gentle. Only use flaps and undercarriage when you are sure you can make the field with minimal power. Be very careful not to get into a situation where you are trying to drag her to the fence with dwindling airspeed and a fistful of sideways power.
If you have used rudder-trim to counteract the asymmetric power you will need opposite rudder when you throttle back for touch-down.
Taxiing on one engine is easy as long as you keep moving and don’t turn sharply towards the live engine.
So that’s it. Getting your multi-engine rating is a challenge – it calls for discipline and professionalism. It also makes you a better all-round pilot. But most of all it is a whole lot of fun.
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