Article appeared in Flying Magazine February, 2015 by Martha King –
“Calm winds, clear skies, bright sunshine. It’s a wonderful day to fly.” The Flight Service Station specialist was delighted to be giving such a favorable weather report. “No,” I thought to myself. “It’s a lousy day to fly.” This surprising thought illustrates that there are special considerations and risks for every aircraft you fly.
You see, on that day I was flying the Skyship 600 blimp—over 6 stories tall, nearly two thirds the length of a football field, and weighing over 15,000 pounds without the lifting gas. On landing, my job would be to deliver this behemoth gently into the hands of the 13 or so ground crewmembers who would then use the handling lines and gondola railings to walk it over to attach it to the mast. If I did a bad job of this I put the ground crew at risk by dragging them across the landing area as they struggled to bring the airship to a stop.
Calm winds made my job much tougher. As in any airplane, the elevator and rudder in the Skyship work with the relative wind to control the aircraft. In an airplane there is always at least 40 or 50 knots of relative wind until the airplane is on the ground. When landing in a blimp I progressively slow the aircraft, to come to a stop ideally just as the lines fall into the hands of the crew. As the blimp slows there is less and less air over the control surfaces and less and less control. Steady winds of about 8 to 10 knots allow some measure of control even when the groundspeed is zero.
With no wind, when the blimp comes to a stop there is no air over the control surfaces and no control. If things go wrong, the way to get control again is to do an emergency takeoff to get air over the controls. Before I flew a blimp, I was fond of saying there is no such thing as an emergency takeoff. Now I know better.
Sunny skies present their own problems. They create thermals and shifty winds—always bad for blimp control. Plus, they create what is known as “superheat”. When the blimp is in the sunshine, the helium gets warmer than the surrounding air, creating additional lift. This lift is neither a good thing nor a bad thing. The problem is on sunny days the amount of lift from superheat can change dramatically as you fly under clouds or increase the airflow over the envelope. Changing lift makes a blimp much harder to control.
Getting the lift just right for various phases of flight requires a lot of careful planning. During preparation for takeoff, the ground crew raises the blimp off the ground and temporarily lets go. Then my job is to observe what happens. If the blimp just floats, the lift from the helium and the weight of the blimp are equal. If the airship gently descends to the ground, the aircraft weight is more than the lift provided. I need to estimate the amount by which the weight exceeds the lift in kilograms (the aircraft was certificated in England). Conversely, if it slowly floats up, the lift is greater than the weight and once again I need to estimate the amount in kilograms.
Now comes the tough part. Everything comes to a halt as the crew chief looks me in the eye, expecting my hand signal telling him the amount of ballast I want to add or remove in kilograms. Now the pressure is on. The crew chief stares at me intently while the 12 other ground crew, the copilot, the passengers, and seemingly all the rest of the world, wait while I try to figure out an answer.
I will need to consider the weight of the fuel we will burn during the flight, and the weight of any moisture on the envelope and how that might change through the day. But the trickiest consideration is estimating what that superheat will be throughout the day.
I need to take all of this into consideration, and give the crew chief that hand signal telling him how many 10-kilogram bags of ballast I want loaded and in what compartments. If I get this calculation wrong, we will all pay for it with an aircraft that is hard to handle. This does not make a cameraman or TV director happy when they are trying to shoot an event like the U.S Tennis open, the Kentucky Derby or the Super Bowl for national TV coverage. Or worse yet I will put the ground crew at risk on landing with a blimp that requires excessive speed to control.
Another consideration special to blimps is management of envelope pressure. While in an airplane the lifeblood of flight is airspeed, and in a helicopter it is rotor rpm, in a blimp it is envelope pressure. A blimp has no rigid structure and the shape of the blimp is maintained by envelope pressure. Too much pressure, and to protect the envelope (which cannot stretch) valuable and necessary helium is automatically released. Too little pressure and I risk loss of control from having the control cables come off of their pulleys.
To manage air pressure there are two airbags, called ballonets (another aviation word from the French). The ballonets increase and decrease in volume to keep the proper pressure on the helium as it expands and contracts with altitude and temperature changes. As the blimp climbs to lower pressure and the helium expands, air is released from the ballonets. As the blimp descends to higher pressure and the helium compresses, air is pumped into the ballonets. The air comes from scoops behind the propellers supplemented by electric fans. I control all of this by continuously manipulating valves and switches.
Through the years I have been privileged to fly aircraft utilizing nearly every method mankind has devised to leave the ground. In addition to airships, I have been lucky to fly balloons, airplanes, gliders, helicopters, gyroplanes, weight-shift-control trikes, and powered parachutes. I guess that leaves out rockets.
What I learned from this is that everything that flies has its own set of risks, and it takes special knowledge to know those particular risks and how to mitigate them. In very many cases, the risks are not intuitive. Who would ever think that calm winds and clear skies could be a problem for a blimp pilot? And you simply wouldn’t know about envelope pressure unless you were taught it.
Sometimes your experience at flying one aircraft can make you a risk factor in another aircraft. While pitching up to flare during landing is a good thing in an airplane, it can cause a tail-strike in a 200-foot-long blimp. In an airplane, in the event of an engine failure, using forward control pressure to maintain airspeed is good, but in a helicopter, forward pressure on the cyclic will decrease precious rotor rpm.
In a weight-shift-control trike, the control movements are opposite those of an airplane. To pitch up, you put forward pressure on the control bar. To pitch down you pull back on the control bar. On the ground, in order to turn left you step on the right pedal, and vice versa. This makes it tough for an airplane pilot to learn to fly a trike safely. And about the worst landing I ever made is when I tried to flare our old Falcon 10 with the same sight picture I had earlier in the day in a trike, where my rear was about 6 inches above the runway.
While the differences and limitations between categories of aircraft are dramatic, every aircraft has its own set of capabilities and limitations. One day an airplane collector decided John and I should sample every airplane in his collection. On that particular day there was a steady, unchanging crosswind. Every airplane responded differently to the identical conditions.
For instance, what was a problem crosswind for a Culver Cadet was hardly an issue for a Cessna 195. Clearly the amount of risk associated with a given set of conditions depends on the aircraft as well as the pilot. Through the years I have learned the necessity of thinking carefully about the risks associated with each aircraft and situation, and what I bring to each.
Oh yeah, there is something else I have learned from all of this. I am not all that good at a whole lot of stuff. Because for everything I have learned to fly, there is someone else who flies it full time, all the time—far outshining me. I also discovered that contrary to what I thought, there is at least one time an emergency takeoff is in order, even if nobody is shooting at you.