- Second Century
- Issue #10: Fuel planning for electric aircraft ⚡️⚡️
Issue #10: Fuel planning for electric aircraft ⚡️⚡️
👩✈️On the menu
While fortunately many companies are investing in building fully electric aircraft, there are still unanswered questions about their operational capabilities.
Manufacturers publish numbers about the projected range in a straight line. However, when an aircraft has a range of - for example - 700 kilometers, doesn't mean that there can be 700 kilometers between departure and arrival.
Fuel requirements are one of the main reasons for limiting the practical range.
This issue will discuss these requirements and how they will affect electric Regional Air Mobility (eRAM) operations
Starter: The legal requirements
Main course: In reality
Dessert: Don't set the bar too high for choosing an alternate
The legal requirements
One of the most important factors in flight planning is fuel planning. The International Civil Aviation Organization (ICAO) has strict fuel requirements that all airlines of member states (ICAO is part of the United Nations) need to apply to. The minimum amount of fuel on board consist of:
Taxi fuel: the amount of fuel needed to get from the parking position to the runway before take-off and the other way around after landing.
Trip fuel: the amount of fuel needed to get from take-off to landing at the destination.
Contingency fuel: it is possible that your flight does not exactly go as planned. It happens frequently that you have to fly a less favorable altitude or speed, thereby burning more fuel than expected. To take that into account you have to add at least 5% of the trip fuel or 5 minutes of fuel required to fly at holding speed.
Alternate fuel: it could happen that you are not able to land at the planned destination due to weather or a sudden closure of the airport. To cover for that, extra fuel is required to fly to an alternate airport. How much that is depends on the distance from the planned destination to the planned alternate.
Final reserve fuel: This amount of fuel is required in case you have to hold before the approach can start. Jet engine aircraft have to take 30 minutes of holding fuel into account and aircraft with reciprocating engines 45 minutes.
These are the minimum requirements. This means that when the flight didn't have to hold or divert, the fuel remaining after arrival is at least the alternate and final reserve fuel combined. Which can easily be 45 minutes of flight time (15 minutes to divert + 30 minutes holding) or more.
It happens that this extra fuel, that you often don't need, can be more than the taxi and trip fuel combined, the fuel that you actually used. Especially when flying to an isolated airport where the closest suitable alternate is far away. It could even be the case that the nearest alternate is the departure airport. Whatever the operational situation is, these fuel requirements apply.
🍲 Main course
It is easy to see that complying to these requirements will be a limiting factor for electrical aircraft. The biggest issue they face at the moment is the limited range. And these requirements won't help. Whether the propulsion energy source is fossil fuel or electricity, you need take the factors above into account. You must take into account diverting to an alternate, or save to hold before landing.
The trip fuel or contingency fuel requirements won't be the problem. You need that to get from A to B. The idea that eRAM will focus on smaller airports will limit the amount of energy that is required for taxiing.
One requirement that is highly influential is the alternate fuel.
Choosing an alternate
Alternate fuel is for diverting to an airport other than the scheduled destination during the flight. The alternate airport. As stated in the Starter, the amount of fuel you have to take into account for flying to an alternate differs. It depends on the availability of suitable airports in the vicinity of the destination airport.
The definition of a suitable airport differs per operator. The primary concern is always the same: the ability to safely operate to and from that airport. Think of runway length, the availability of emergency services and navigation equipment. In addition, there are requirements regarding weather conditions. An airline is not allowed to create a trip planning using an alternate with bad weather forecasted.
Some requirements depend on the type of aircraft. Bigger aircraft need bigger runways and bigger parking positions. They also require more fire fighters to be present during the landing. And when the aircraft doesn't have it's own integrated staircase for people to get in and out (which is often the case with passenger jets), a local handling agent must be able to provide one.
From a commercial perspective more limiting demands can apply. Airlines prefer to use alternates that are already part of their route network or the network of a partner airline. In that case, the operator knows that the airport provides services that comply with the operator's standards. It can also make it easier to provide transportation to the originally scheduled destination for the passengers when it won't be possible to fly the aircraft there anytime soon.
All the requirements for choosing a suitable alternate could also apply to electrical aircraft. Unfortunately, electrical aircraft have one more limiting requirement to take into account: recharging.
Refueling is of course important for jet aircraft as well. However, the fact that kerosene has been aviation's lifeline for many years made it possible for this fuel to be widely available. It will take decades before electric charging infrastructure is as common as fuel trucks are. Even in area's where electric aviation will grow more quickly.
eRAM will start with routes between airports that have proper charging equipment available. Based on investments by commercial or governmental parties. Operations will start by connecting two airports with the right charging capabilities. Meaning that the only fully equipped alternate is the departure airport, cutting the effective range of the aircraft in half. After a while there will be small networks of fully equipped airports, located a few hundred kilometers away from each other. This will make it easier to plan longer flights, but there will be some difficulties in the beginning.
In addition, it will be important to have an industry-wide standard for charging infrastructure. An electric aircraft can only be charged by using charging equipment that is certified to be used for that aircraft type. It would be a shame if an airport wouldn't be a suitable alternate because the charging equipment does not match the plane. Standardization of equipment is key.
Don't set the bar too high for choosing an alternate
It sounds strange, but that may be the only short-term solution. Picking an alternate solely based on where the aircraft can safely land as close to the scheduled destination as possible. Maybe the airport doesn't provide the quickest ground transportation to the scheduled destination (because it's located on a different island for example). There may be no qualified maintenance personnel at the airport to do an overnight check. Or it doesn't have the right charging infrastructure to quickly recharge the aircraft.
Having to divert to such an airport would be unpleasant for passengers and could severely mess up the remaining flight schedule for the airline. However, considering that diversions are rare it could be a risk to take. It would increase the operational range of the aircraft.
The upside of the initial eRAM-flights is that they will be relatively short and executed by small aircraft. It is easier to predict the weather an hour in advance than 10 hours in advance. So when a flight only takes one hour, there are reliable predictions about the weather conditions at the destination and the alternate. Making it easier to pick an alternate close to the destination or decide to cancel the flight.
Because the aircraft are relatively small, they won't be requiring a long runway or large parking position. Most of the models that are being developed at the moment don't even require a handling agent to provide stairs.
Electric aircraft are different than kerosene fueled aircraft. And I can imagine that different types of propulsion will have different types of rules. However, there are no indications that the fuel (or energy) requirements will be different for electrical aircraft in the near future.
Looking at the fuel requirements stated in the Starter, which ones would you think can be amended to better apply to electric operations?