Jim Davis
Descending is a remarkably complicated subject and us overworked instructors have to be able to field all manner of questions about it.
To ease you gently into it – here’s the story about Joe’s picture. It was 1964 when I clambered on to an SAA Skymaster at Eros (Windhoek) for a flight to Keetmanshoop. The captain kindly let me sit in the jump-seat. Soon after takeoff he levelled off and asked me if I would like to descend to 1000’ below ground level. I had mixed feelings about this – he didn’t appear suicidal, but hell…
Anyhow he closed the cowl flaps, hauled the power back and stuffed the nose down for a flight into the mighty Fish River Canyon.
The big radials that had been hammering out close to 6000 hp, fell silent, but for the occasional bang and boof from the exhausts. It was a magnificent, calm, clear morning – not a bump in the sky.
The previously benign captain, an ex-SAAF guy, adopted the wild and gritty look of a Brooklands Bentley driver. He took a feverish grip on the string-bound yoke as he wrestled 35 tons of machinery into unbelievable bank angles and then heaved back so we all sank deep into our seats. This was necessary to negotiate the twists and turns of the canyon. I had to wonder how the pax enjoyed the roller-coaster ride.
What a wonderfully thundering experience that was. It must have been one of the last of the barnstorming flights that airline pilots could occasionally get away with. Taking a dirty dive into a valley is now frowned upon.
Some years later I remember Scully Levin giving us a talk on what was involved in calculating the top of descent (TOD) in a modern jet airliner. He explained that it’s now a matter of pride for airline pilots to pull the power fully back – roughly a hundred miles from destination, and only use it again for taxiing.
Actually that’s not quite true because they need to use a fair amount of thrust on final to maintain the glide-slope in the landing configuration; and in case of a go-around. But you get the idea – it’s a glide approach to a field that’s out of sight over the horizon.
So why glide? Well first, they are recovering some of the time and fuel they used in the climb. Second, jets are unbelievably thirsty at low altitudes, so a powered descent – like we use on cross-countries – eats into fuel reserves, and the airline’s balance sheet. And finally, it makes sense to stay above the weather as long as possible to give everyone a smooth ride.
Why did Scully carry on about how difficult it is to work out a precise TOD? Well, because you have to take into account the forecast winds at various levels during the descent. Then you don’t know exactly how far you are going to descend.
Sure you can use the DME – but that only gives the distance to the beacon – so you need to estimate your own track miles and make allowance for the circuit. Next, you need to keep a sharp ear out for any other traffic that might conflict with your immaculate plan. And finally when you have it all figured, ATC changes your routing.
Technology has taken some of the grind out of this. Now, on your Nav Display, you have a VNAV profile which puts a nice little icon on your track to tell you when to become a glider. Actually it tells the auto-pilot, so you sit on your hands and watch das blinkin-lights while it all happens on its own.
Out of interest, a normal glide in a 737-800 is at around 280kts indicated. Then at 50 DME it must come back to a max of 250 kts. Then 210 kts at 15 DME. Your end-of-decent point is usually, where you intercept the glide-slope on the localizer. This is normally at 8 miles, or about 2500’, where you should be at 180 kts or less.
Electronics have largely taken the guesswork out of this, but the surprises come when ATC suddenly changes your routing. They may give you a short-cut in which case you must instantly increase your rate of descent. Or they may give you speed or altitude restrictions.
So it often becomes the big deal Scully was talking about.
A quick sidetrack. I was coming back from London one night in an A340. We were working our way round the tops of some Charlies that were causing turbulence and lightning just south of Paris. The Captain, who was an ex-pupil of mine, explained that if he typed the new track into his magic box; the MCDU (pronounced macdoo), the autopilot would simply follow this around the build-ups.
Then he pointed to a knob on the MCP – Mode Control Panel on the glareshield, and said, “Of course I can fly it manually by turning this.” He was serious.
When I taught him to fly a Cessna 150 in Port Elizabeth, I’m sure we used to associate manual flying with working the levers and pedals. Silly me. To him, manual flying now means twiddling a knob.
Anyhow let’s get back on topic and look at descending in proper aeroplanes with propellers and pistons.
Unfortunately, before we look at the various ways you can descend, I must tell you about a nasty called flutter. This is because any descent has the potential for increasing airspeed – possibly to the extent that it can cause this life-threatening condition. So here we go.
Flutter
You may remember that on 1 April 2010 a young charter pilot was descending from FL95 into Swakopmund in a Cessna 210. Apparently, without warning the aircraft suddenly came apart and the separate bits variously fluttered and plummeted down over a large chunk of territory.
Two months later a pilot and his navigator were descending their Flamingo towards an air-race checkpoint near Bella-Bella when it also broke up in flight.
From pilot reports no significant turbulence existed in either case. Aerodynamic flutter, caused by excessive speed, had been the culprit in both events.
Now I have just read that Jimmy Leeward’s Mustang crash at Reno was caused by worn locknuts on the elevator trim. These allowed flutter to develop. The trim failed and subjected the aircraft to +17G, which incapacitated the pilot.
Both of the South African pilots seem to have been doing what we all do – using the descent to make up for speed lost in the climb. We like to think we are safe as long as we keep below Vne – the red line
Sorry folks, this is not necessarily true. If you want to let the ASI needle move towards the business end of the dial you had better understand exactly what’s going on. It’s a complicated subject which I have covered Before. The Gleitch won’t give me the space to go through it all again, but here are the basics:
Flutter is what your washing does in the wind. Under the right circumstances parts of your aircraft can do the same. It might be a trim tab, an aileron, a tail-plane, or a whole wing. It can be anything from an almost unnoticeable buzz, to a major terrifying event which shakes the aircraft so violently that it rips itself apart. It may last for several seconds, or for only a fraction of a second, before it causes a catastrophic failure.
When the conditions are right the slightest thing may start it. It could be minor turbulence, or a twitch of the control column, or almost nothing.
To further complicate matters, flutter depends on TAS (True Airspeed) – NOT the indicated airspeed you are looking at on the ASI. This means that a particular indicated airspeed on your dial might be quite safe at sea-level but lethal at 12,000 ft.
Fortunately, unmodified, everyday aircraft, in good condition, have a decent safety margin built into the Vne limitation – as long as they are flown within their certification limitations – including maximum altitude. However any sloppiness in the control system can cause flutter at speeds well below the red line.
There is a whole set of complicated, and sometimes vague rules for gliders at high altitude. And other rules for NTC (Non-Type Certified) aircraft.
That’s just a general warning that any descent has the potential for increased airspeed – so keep that in mind when you plan to go downhill.
Basically you have two ways of making the aeroplane descend.
- You can glide
- You can descend with reduced power.
Glide
Instructors, please teach your pupil to glide. Ideally, demo it on her first flight. Then keep doing it as often as you can. This is pure flying. It’s the most elegant form of flight. It teaches her airspeed control. It teaches her judgement. And it teaches her that flight is about aerodynamics – not about engines. You want her to understand in her soul that an engine failure is an inconvenience – not a death sentence.
In Jim’s ideal flying school you will do glide approaches for most of your flight training.
Those of you who learned to fly when there was less traffic, and the plugs didn’t foul every time you closed the throttle, will remember that those wonderful glide approaches were the normal way of doing circuits and bumps.
You did a standard circuit, but when you turned on to the base leg you didn’t throttle back to start your descent. You maintained circuit height until you judged that you could easily glide for a touchdown a couple of hundred meters into the runway. You kept a beady eye on the runway and watched the drift in order to judge the wind. When the time was right you used carb heat and throttled fully back. You also used a touch of left rudder to keep the ball in the middle and you held the nose level until you reached your glide speed. Then you lowered it into the glide attitude and trimmed any pressure off the stick.
When you realized you were a bit high you would use some flap to steepen the glide. You progressively used flap. Each increment would steepen the descent to bring you down for a gentle landing near the beginning of the grass runway. No tar, no VASIs and no PAPIs.
Sideslipping was a normal part of any approach that was too high. We will deal with that wonderful skill a bit later.
If you had used carb heat – we generally didn’t need to on Cherokees – you would put it off at about 100ft.
After a bit you got pretty good at glide approaches. So when they started teaching you forced landings – well you really couldn’t miss. Forced landings usually call for a longer glide, so you have to warm up the engine, and give it a blast of carb-heat, every thousand feet or so. You should also close the cowl flaps, if you have them.
Okay, so here’s the way we should teach students to start a glide:
- Make sure there’s no conflicting traffic
- Use carb-heat if the POH recommends it
- Richen the mixture
- Smoothly come all the way back on the throttle
- Use enough left rudder to keep straight
- Hold the nose in the level flight attitude
- When the airspeed reduces to the best glide speed lower the nose into the correct attitude
- Trim the pressure off the stick
- Relax and enjoy the sensation of pure flight
To maintain the glide:
- Keep a good lookout for traffic
- Don’t look away from your aiming point for more than a few seconds at a time
- Remember engine warm-ups and carb-heat
To recover:
- Put the carb-heat off
- Richen the mixture
- Smoothly apply full power
- Use enough right rudder to keep straight
- Raise the nose into the level flight attitude. In some aircraft at large flap settings you may need to use a huge amount of forward force on the stick to prevent the nose from moving above the horizon
- When you reach cruise speed reduce power and adjust attitude accordingly
- Tidy up – trim, mixture, power-setting and cowl-flaps.
Descent with reduced power.
This will be your normal descent to destination – say on a cross-country.
The question is where, when, and how much, should you reduce the power.
Ideally I like to plan a normal descent to destination on a cross-country at a bit over 300 fpm. This gives you a reasonable increase in airspeed without running into the yellow on the ASI. It’s also nice and easy to work out your TOD – you are going to take about three minutes for each thousand feet.
So if you are descending to Virginia from say FL 95 you need to lose about 8500’, to get to circuit altitude. This should take you around 25 minutes.
If you normally true out at say 120kts, you can probably work your descent at a TAS of around 140kts. Which means you would start your descent 60nm short of Durban. These are thumb-suck figures – they can’t be exact because of winds and so on.
It is common for new pilots to misjudge this and start their descent way too late. This means ears-popping, shock-cooling and excessive speeds.
First let ATC know you are at TOD. Richen the mixture a bit, ease the nose down a fraction, let the IAS increase by 15 – 20kts. Come back slightly on the throttle to prevent the revs running away. When everything is settled – trim the elevator and the rudder.
Keep an eye on the engine all the way down – tweak the mixture a tad richer every thousand feet and make sure the revs stay where you want them. As you get lower, any particular rev setting produces more power than it did at altitude. So you need to gradually reduce the revs during the descent.
If you expect to be distracted by nav, traffic or pax, give the mixture an extra dose of richness – you don’t want a lean cut at low altitude. Also remember carb heat. I have had icing in a Cherokee 235 on the Natal coast when the OAT was over 35°C. It’s the humidity that does it.
And finally, let’s quickly sort out a long standing discussion. In piston-engine aeroplanes we control the airspeed with the elevator and the rate of descent with the throttle. I did touch on this earlier, but there’s a bit more to it.
Jet pilots do it the other way round, and there are good reasons for this. Briefly, because they are so heavy and carry so much inertia, it takes too long for changes in nose attitude to alter the airspeed. If they want to go faster NOW, thrust is what will do it for them.
Back to our sort of aeries, the only time you will ever break this stick-for-speed/power-for-height rule, is if you are low and slow on final approach. Obviously you can’t put the nose down to regain airspeed – so you have to use power. But this is not really something you need to think about – it just comes naturally.
Next time we will look at climbing and gliding turns and the disappearing art of sideslipping.