Year 2 e-plane circuits: Does an older battery mean fewer circuits?

Figure 1: Horizontal Profile of Six Circuits in a Flight 

Circuit training is one of the core activities in learning to become a pilot. Pilots need to learn the skills to take-off, fly a standard circuit pattern, and land safely. Last year, we reported completing 5-7 circuits in a typical flight in the Velis Electro before landing with a reserve of at least 30% SOC, State of Charge, in the battery (equivalent to how much fuel remains in the tank). 

This year, the battery has aged with 140 hours of repeated cycles of charging and discharging, so we want to see whether this impacts the number of circuits in a typical flight.  

The repeated patterns of circuits are shown in Figure 1 (horizontal profile or GPS tracks over the ground) and Figure 2 (vertical profile or altitude, 250m = ground level, 550m = 1000 feet above ground level). The typical time taken for a circuit is six or seven minutes, but this can be longer if the circuit needs to be extended for other traffic. 

Figure 2: Vertical Profile of Six Circuits in a Flight

The capacity of the battery to hold and deliver electrons decreases with time, use and temperature. The analogy is that the fuel tank is gradually shrinking. This capacity is measured by the SOH (State of Health) of the battery. The onboard computer monitors the battery properties and displays a number from 0 to 100 for the pilot to use in flight planning. 100 represents the capacity of a new battery, and 0 represents the capacity of a battery that needs to be replaced (i.e. it can no longer deliver the 50 kW required for a safe and efficient take-off). Our 2023 circuit flights had typical SOH values in the 90s, while the 2024 circuit flights had SOH values in the 70s. The Pilot’s Operating Handbook, POH, notes that for every 20-point reduction in SOH, one less standard circuit (12 km at 1000 feet above ground level) can be performed. Is this the case with our Velis Electro? 

A quick look at circuit training flights in 2024 reveals that 6 is the maximum number of circuits in a flight, a reduction of one from the 7 reported as the maximum in 2023. Given that the SOH has decreased by about 20 points from 2023, this is in line with the performance expectations in the POH. 

In addition to looking at the number of circuits per flight, we want to see if the %SOC per circuit has changed. A standard circuit with a new battery uses approximately 10% SOC. As the battery is discharged from 100% SOC to 30% SOC (the recommended landing minimum), there would be sufficient power for seven circuits that use 10% SOC each. Three of the eleven 2023 circuit flights had an average %SOC value per circuit less than 10. In 2024, no flights had an average %SOC value per circuit less than 10. Instead, three flights had an average value of less than 11. This is consistent with an aging battery and reduced SOH. 

In practice, as soon as the circuit is extended, for example, flying farther to allow room for other aircraft in the circuit, it would use more than 10% SOC and fewer circuits could be completed in the flight. Hence, the range of 5-7 circuits per flight with the new battery, as reported in 2023. To look at the impact of traffic on the number of circuits in different flight school settings, we can compare flights at a very busy airport (YKF, Waterloo) and a less busy airport (YFD, Brantford). 

The comparison of circuit flights at Waterloo and Brantford offers important insights. The busier airport had fewer circuits per flight and a higher average %SOC per circuit. One reason for the fewer circuits at Waterloo was a decision to increase the target return reserve from 30% to 40% SOC. This effectively cuts one circuit from the flight.  

Overall, the difference between the two airports was small for the minimum (9 vs. 10) and average (11.3 vs. 12.6) %SOC per circuit, just a 1% difference (Table 3).  

In contrast, the maximum values were much higher at the busy airport. The highest value among the flights studied was 14% SOC for a circuit at Brantford, 21% SOC for a circuit at Waterloo in 2023, and 30% SOC for a circuit at Waterloo in 2024 (for comparison, the second highest 2024 value was 17%). The unusually high 30% value illustrates what can happen when a circuit includes an extended downwind for traffic, an instruction to climb and overfly the airfield due to traffic, and then another circuit. It effectively became the equivalent of three circuits in one. The battery had ample power available, and the e-plane landed with 47% SOC remaining. Clearly, extended circuits due to traffic will reduce the number of circuits that a pilot can complete in a flight. 

Table 3. Battery discharge per circuit by year and airport, %SOC 

The answer to our initial question is yes. An older battery with a 20-point lower SOH value will deliver one less circuit in a typical e-plane flight. The maximum in 2024 was six circuits while the maximum in 2023 was seven circuits. This is in line with the performance noted in the POH for the Velis Electro.  Similarly, the average %SOC per circuit increased from 11 in 2023 to 12 in 2024.  

In contrast to the declining SOH causing an increase of 1%SOC per circuit, it was noted that a less busy airport would reduce the average energy use per flight because there would be fewer extended circuits due to other traffic. Circuits at Brantford airport, YFD, averaged 1%SOC less than those at the busier Waterloo airport, YKF, (11.3 vs 12.6 %SOC per circuit). 

Comparing the e-plane performance at more than one airport and over more than one year, enables flight schools to imagine the performance that could be expected at their airfield.