Sunset High Above Palm Springs, Westbound
My first solo was almost 17 years ago, over 2000 flight hours ago. Just the flying machine and I, not a good time to lose confidence. I just pretended that my instructor was still sitting next to me…and that was enough.
While flying, I have experienced in order: a fuel pump failure, clogged fuel injectors, an alternator failure, a runaway elevator trim and a hydraulic leak. Without any declared emergencies. Checklists matter, as do backup systems.
I have also enjoyed some amazing experiences, such as flying across Australia in a Cessna 182, or cruising over the plains of Africa in an open cockpit Piper Cub. Or simply landing at an obscure airport for fuel, driving the loaner retired police cruiser crew car to town for lunch.
I regret not having taken a picture of every crew car I have driven.
Along the way I learned how planes fly, why some planes are better than others, how our nation’s aircraft eco-system works, and most importantly, the difference between flying and safe flying.
Turbojets and turboprops are aircraft that both use jet engines for thrust. The first is a “pure jet” (suck air in, push air out), the second a jet engine powered propeller. Either way, both are much more reliable than piston-engines, one reason why all commercial aircraft are either turbojets or turboprops.
Flying requires a lot more power than driving, which is why non-jet powered aircraft with piston-engines must always operate just below red-line RPM, and why their engine oil must be changed every 25-50 hours. When is the last time you changed your car’s oil? Or ran your car’s engine RPM at red-line for hours on end? Aircraft piston engines are designed to run this way, but nevertheless, there is metal-on-metal wear that must be managed. With jet engines there are no cylinders, no metal-on-metal wear and no worn-out engine oil to change.
Jet engines produce a lot of thrust, which means that they require a lot of energy in the form of jet fuel (kerosene). Any thrust producing system should be analyzed in three ways:
1. How efficient are the engines at converting fuel to thrust?
2. How much does the engine weigh?
3. What is the energy density of the fuel?
4. How reliable and safe is the system?
After the engineering comes the economics. How much does the fuel cost? Maintenance expenses? What is the useful load (i.e. how many passengers & baggage can be carried)? What is the range of a fully loaded aircraft?
Here is the quick explanation as to how aircraft fly. The engine must provide thrust to push the aircraft forward, and a lot more thrust to hold it in the air. The shape of the wing creates the downward thrust. The downward thrust must equal the weight of the plane to hold altitude. The downward thrust must be greater than the weight of the airplane to climb.
Consider the Epic E1000, a single engine turboprop that has an empty weight of 4,600lbs. Assume for this flight the pilot adds 190lbs and the 150 gallons of jet fuel (kerosene) 1023lbs, making the takeoff weight 5813 lbs. This means that the engine will need to produce much more than 5813lbs of total thrust because it needs to both accelerate in forward speed and climb at a good rate.
What is a good rate of climb? This gets us to safety and other topics.
Altitude and airspeed are a pilot’s best friends. With plenty of both, a pilot can usually cope with most problems, including if necessary, gliding to a safe landing. The quicker an aircraft climbs to a high altitude, the better. While the minimum climb rate (at sea level) is about 500 feet/sec, a better climb rate is more like 2000 – 3000 ft/sec. This faster rate of climb requires a lot of excess power.
Engine output power (and climb rate) decrease as altitude increases, as the air gets thinner. The excess power at sea level becomes just enough power to reach desirable high altitudes, where planes travel faster, and less fuel is consumed. Once an aircraft is above 30,000ft or so, it is above most weather, and smooth comfortable flights become the normal.
How pleasant to fly locally on a sunny, summer afternoon. True, but commercial airlines need to fly year-round in all sorts of bad weather. They always fly IFR (Instrument Flight Rules), required to fly in clouds, for most night flights, etc. IFR flights require an IFR certified all-weather aircraft, an instrument rated pilot, support of ATC and ample fuel reserves.
Per FAA regulations, an IFR aircraft flying to a cloudy destination needs to have enough fuel to reach the destination, fly a missed approach, fly to an alternative destination (with preferably better weather), and have another 45 minutes’ worth of fuel.
Meaning at least one hour reserve fuel, and usually more. We have learned this is a good idea the hard way.
Flying is 99% boredom, and 1% excitement/fear. The autopilot can be used for the 99%, a skilled human pilot for the 1%. After watching the video of the recent tragic Nepal Airliner crash, I am willing state that it is highly likely this was pilot error, a low-power stall. Stalling a plane is a common cause of accidents during takeoff and landing. The better the pilot, the less likely a plane will be stalled. Stalling during takeoff and landing is usually fatal.
In the U.S. we enjoy very experienced and well-trained pilots, one reason our domestic commercial accident rate is so low. Contrary to the media portrayal, even the accident rate of general aviation (private pilots like me), is at an all-time low.
Autopilots are a great tool until one tries to land a plane in gusty crosswinds on an icy runway, etc. Someday our robot friends will be able to do this, we are just not there yet.
Presently about 1/3 of airline pilots are ex-military, and that percentage is decreasing. While airlines have started to directly train pilots, many are recruited from the roughly 1000 flight schools in the U.S. Each flight school has a handful of instructors that accrue flight hours and experience while teaching others to fly. Every year many of these experienced instructors are picked up by the airlines and begin their career as a commercial pilot, while some the students become the next generation of instructors. The remaining students, such as I seventeen years ago, financially support the system.
This “pilot providing eco-system” is composed of many small general aviation airports, the flight schools, the affordable small piston airplanes, the mechanics, the FBO’s and of course, the FAA. If general aviation becomes too expensive this eco-system will disappear, and we should expect an even more acute shortage of well-trained pilots.
Our present pilot shortage is the direct result of many experienced older pilots retiring during Covid. Airlines are petitioning to lower the minimum required hours for new pilots (I am not supportive) and increasing the mandatory retirement age (I am supportive).
Most of us are aware of the recent NOTAM system failure that grounded all flights in the U.S. for several hours. The usage of 30-year-old computers is unthinkable in the private sector, and should be unacceptable by any government agency, except maybe the Post Office. Unlike the delivery of junk mail, safe and reliable air transportation is critical to our economy. ATC is staffed by very well-trained and professional people. Provide them with the funding and modern tools to do their job.
Once again consider the Epic E1000, equipped with a P&W PT6A-67A jet engine. This engine consumes kerosene, and outputs hot compressed air and exhaust. The hot compressed air drives the propeller, and heats & pressurizes the cabin (important at high altitudes!)
In this next section I determine how many of the common 2170-type lithium-ion batteries would be required to power the Epic E1000. You may skip the math to the underlined answers.
The PT6A-67A produces a maximum of 1200 horsepower (hp), which is the torque applied to the propeller. Horsepower is a measure of the rate at which work is done, and one mechanical horsepower equals about 746 Watts(W), so 1200hp equals 895,200W.
The energy density of kerosene is about 9722 Watt-Hours/Liter (Wh/L)
One liter of kerosene weighs 0.819kg, so the energy density becomes 11,870Wh/kg
The maximum fuel load for the Epic is 280 gallons. As one gallon of kerosene weighs 6.8lbs, 280 gallons must weigh 1904lbs, or 865kg. Implying that the stored energy in 280 gallons of kerosene is:
865kg x 11,870Wh/kg = 10,267,550Wh (yes, that is a lot of energy)
For the Epic the average fuel burn rate is 55 gal/hour, which implies a maximum flight time of about 5 hours. Total work done by the propeller during a 5-hour flight is 895,200W x 5 hours = 4,476,000Wh. Meaning that about 44% of the kerosene energy is converted to propeller mechanical energy, and the other 56% to heat (waste).
Could this Epic be powered instead with lithium-ion batteries? The same 2170-type battery used in the Tesla Model 3?
2170 battery specs:
- Nominal Voltage 3.7V
- Capacity 4.8Ah
- Weight 68 grams
The stored energy per a fully charged 2170 battery is 3.7V x 4.8Ah = 17.76 Wh
Dividing the mechanical power output of the Epic propeller by this value yield how many batteries are required, assuming that the electric motors are 100% efficient.
4,476,000Wh / 17.76Wh per battery = 252,027 batteries
Each battery weighs 70g, so total battery weight is:
252,027 batteries x 0.070kg per battery = 17,642kg = 38,812lbs
The required batteries are 20 times the weight of the 280 gallons of kerosene.
This is the challenge with battery powered aircraft. While we wait for battery energy density to increase, what can be done? What if we were reduce both the power and the range by one half? One quarter of the batteries would now weigh 9,703lbs, still 5 times the weight of the 280 gallons of kerosene.
By doing so we run into the problems of very short flights (because we still need our reserves), slower climb rates, and the inability to fly above the weather. Are we sacrificing safety and comfort?
Such planes will also be restricted in useful load, as every extra pound for batteries means one less pound for paying passengers. Will we drive up the cost to fly?
The availability of high energy density low-cost kerosene enabled our modern aviation industry, which for better or worse, made air travel in the U.S. safe & affordable for many. Will more expensive kerosene, increased carbon offset taxes, and battery powered aircraft return us to the days of the DC-3 departing from Casablanca, a mode of transportation reserved for the wealthy?
Are hybrid planes (batteries recharged by a kerosene-burning turbine engine in flight) the short-term answer? Will a scientific breakthrough provide us with low-cost sustainable kerosene? Or cost-efficient carbon sequestration? Or a dramatic increase in battery density?
Guess we will find out.