Sunday, January 11, 2009

175, 150, 125, 148, 27.5, 28.5, 24, 20, 11.7, 10/11/12, 1090, 765, 30/60/30/60/30/30, 20/120/20/120/20/60

Numbers numbers numbers! One of the great joys (insert sarcasm) of learning to fly a new airplane is memorizing numbers. Just about anything having to do with flying involves numbers, which need to be memorized because often times things are happening so fast that you just don't have enough time to go look them up. That old excuse "I don't know what it is, but I know where I can find it", won't work when it comes to these numbers. 

In a Cessna Caravan, a single-engine airplane, one of the most important numbers you should have in your head is the best glide airspeed. This is the speed at which, if your engine fails in-flight, you'll get the maximum glide range. Obviously, when you've been unwillingly turned into a glider due to an engine failure, you're going to want to maximize your glide range in order to maximize your options for landing sites. If you're lucky, you'll be able to find an airport within your gliding distance, or a road, an open field, golf course, etc.  So there you are fat, dumb, and happy cruising along at 15,000 feet and 150 kts over an 8,000-foot mountain range, when suddenly the high-pressure fuel pump, of which there's only one, decides to come apart. Now your heretofore ultra-reliable PT6A-114A turboprop engine has become a quarter-million dollar paperweight.

From the pilot's perspective, it won't be obvious what caused the failure. It's clear what didn't cause the failure though. Since there was no obnoxious aural warning, no sudden nasty bang,  and the engine flamed out, rather than rolling back to idle, we know that the engine's not on fire (good-although an engine on fire will typically keep running for a while), it didn't have some catastrophic mechanical failure that would preclude an attempt to restart it (maybe good), and it's not a Fuel Control Unit (FCU) malfunction (bad-because that one's easy to deal with and your engine will keep running).

Being a pilot of sound judgment, the first thing to do would be to fly the airplane. In this case, it would mean trimming the airplane (or using the autopilot) to start slowing to your best glide airspeed, 95 kts at max gross weight, decreasing by 8 kts per 1,250 lbs below max gross weight. This is important because with the airplane cruising at 150 kts with cruise power, the elevator is trimmed for that high airspeed and power setting. Once that thrust goes away, the airplane will naturally want to maintain its trimmed airspeed of 150 kts. Without thrust and without any input from the pilot, the airplane will pitch down and start a fairly rapid descent, right into that 8,000-foot mountain range. Simultaneously with trimming and slowing to 95kts, a turn would be started to get away from the mountains and start heading for the nearest airport or other suitable landing site.

This is where that 95kts becomes really important. A mountain range doesn't present a lot of options for landing an airplane. At an altitude of 15,000 feet, you're 7,000 feet above the mountains, which at the proper glide speed will allow you to glide approximately 15 nautical miles before hitting the terrain. That's a considerable distance, and it may just be enough for you to clear the mountain range completely. If you were to clear the mountain range completely, your 15,000 feet of altitude would allow you to glide 32 miles before reaching terrain at sea level. The chances are very good that you'll find a suitable landing site, or even an airport, within 32 miles of your current position. 

Okay, back to the story, things are happening fast. The instant the engine starts to lose power, the left hand is flying the airplane, slowing it to 95kts, turning away from the mountains and towards the nearest runway, and the right hand is pulling the power lever to idle, and then turning on the engine ignition. Oh wait, the ignition switch is on the left side of the panel, so let go of the flight controls to turn it on, or switch hands, grab the yoke with the right, and flip the ignition switch on with the left, then switch hands again. Push the power lever up with the right hand and see if the engine's accelerating with the ignition on. Nope. Check the engine instruments, is it still running at low idle? Or is it flamed out? If it's at low idle due to a FCU malfunction, the engine instruments should show Ng RPM of 48%, fuel flow of 80-110 pounds per hour (pph), and 500-600 degrees Interturbine Temperature (ITT). No time to look up those numbers either.

In this case the engine's not running, so it can't be a FCU malfunction, so there's no point in trying to use the Emergency Power Lever to control the engine. Time to attempt a restart, which I'll talk about in detail in another post. But for now, there's more numbers to recall. During a start, fuel should be introduced when Ng RPM reaches a minimum of 12%, and the engine instruments should show ignition within 10 seconds of introducing fuel, indicated by increasing ITT. If ignition doesn't occur after 10 seconds, the fuel must be cut off again as it's just pooling in the engine, a big hazard. After ignition, as ITT increases and the engine accelerates, the ITT must be monitored to ensure it doesn't exceed 1090 degrees C for more than 2 seconds, otherwise engine damage will occur. Finally, Ng RPM should stabilize at a minimum of 52% if the start is successful. All of this occurs in the span of several seconds, so there's no time to refer to your books.

While troubleshooting the engine, ATC needs to be informed of the situation so they can get other airplanes out of the way and possibly get search and rescue crews going, in case that airport is out of reach. Chances are that we're already in contact with them, but if not, we'd have to set an emergency code on the transponder, 7700, and change to the emergency radio frequency, 121.5 to make contact with somebody, both numbers that need to be pulled up quickly.

Getting closer to the runway, start thinking about extending the flaps. The Caravan has 3 flap settings, 10, 20, and 30 degrees. The maximum speeds for extension of each of those flap settings is 175, 150, and 125 kts. Since we're gliding at 95 kts already, those limit speeds won't be a problem. In preparation for touchdown, the speed needs to be reduced to a speed appropriate for a landing without engine power with the flaps fully extended, 80 kts. Be considerably faster than this on final, and the airplane will have to be much speed, float in the flare, and possibly overshoot your landing point. Hitting the ditch at the far end of the landing area is just as bad as hitting the ditch just before the landing area. Be much slower than this, and the airplane may hit hard as there isn't enough airspeed to get a good flare.

It takes a considerable amount of effort to cram all these numbers into your head since it's just rote memorization. Once crammed in there, they also tend to be the first things to fall out if you don't fly for a while. Which is why staying current is so important for pilots. There's so much information that needs to be crammed into your head and available on short notice, that you need to review frequently. Sure you could go flying without knowing the numbers. Happens all the time. Most of the time, you know just enough of the numbers that you need on a daily basis and that gets you by just fine. But then worst case, there may be a situation where if you don't pull them out of your head immediately, it can lead to airplane or engine damage, or getting stuck over a mountain range in a glider.

In case you're curious, here's an explanation for all those numbers in the title:
175 - Max operating speed
150 - Max speed flaps 20
125 - Max speed flaps full
148 - Maneuvering speed at 8750 lbs
27.5 - Bus volts when on the standby alternator
28.5 - Bus volts when on the main generator
24 - Minimum battery volts to start the engine on the battery
20 - Minimum battery volts to start the engine using an external power cart
11.7 - Minimum TKS fluid required for dispatch into known icing conditions
10/11/12 - Configuration for an instrument approach, 10 degrees flaps, 1100 torque, 120 kts
1090 - Maximum starting ITT
765 - Maximum climb ITT
30s/60s/30s/60s/30s/30min - Starter duty cycle limits on the battery
20s/120s/20s/120s/20s/60min - Starter duty cycle limits on the external power cart

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