September 24, 2021, ©. Leeham News: Last week, we looked at the necessary Production Certificate for the production of our airplanes.
Now our program plan has approached the mid-phase of the Post-Launch phase. We start assembling our test aircraft to get to flight tests.
We plan on using two Flight Test Articles, FTAs (OEM speak for test aircraft). We will also produce two ground test airframes, one for the static test to clear our flight test clearance (Limit load and Ultimate load tests) and another one for the cyclic tests to clear our fatigue envelope.
So in total four airframes are produced, whereof the second and third are stuffed with systems to make them flight test aircraft.
In general, we need to order the long lead-time items a year before we need them but these are the first pre-production items where all involved will learn how to produce the parts. We shall, therefore, add at least 50% lead times for the production of the parts, if not more.
Experience from the industry shows you need to spend about three to four times the manhours on the early items if these are new designs. For system parts that are derivatives of something that already flies, we can put the rule at double the time for the early production runs.
So we need ample pre-planning and margins for both calendar time and resources consumed, both at our suppliers and for ourselves at the assembly of the FTAs.
Why do early examples take such extra time? Because after each produced item in for instance the wings, it needs to be carefully inspected. Any deviations from the released drawings have to be discussed with the designer.
Is this deviation, that can come from thermal effects during the milling of the part from a big Alu block, acceptable? Are these dimensions and their probable tolerances going forward, critical, or can we live with it as is?
All this checking of the thousands of parts forming a structure sub-assembly takes time. The designer might have to go back and do a simulation if this is OK or not. We won’t get an answer before next week, so we continue with the next part and so on.
Eventually, all the subassemblies, sections, and systems arrive at our Final Assembly Site (FAL). If we have planned ahead well we can assemble the parts using production tooling and alignment methods, but many times these are not yet finished and we spend extra time to align everything to get a straight flying, representative aircraft.
As systems are integrated into the assembled structure for the aircraft, we also install all the flight test equipment. For FTA 1, which is our flight envelope clearance aircraft, load measurement strain gauges are glued in place and accelerometers for flutter measurement are mounted in wings and tailplanes. An air data boom is installed to collect data such as sideslip and angle of attack information. An adjustable ballast system may be required to simulate various center of gravity and weight scenarios.
To check the exact behavior of the aircraft in the air, extra gyros and accelerometers are put in different places of the aircraft. We install a number of CCD cameras to check the operation of our movables and the landing gear operation and how the wheels are spinning when we land (to check the ABS operation), etc, etc.
Before our first flight, our Safety of Flight process needs to be established and completed. This can be a long process collecting all of the loads and safety analysis, materials testing, component testing, system testing, and ground testing that show the aircraft can meet the intended flight profile.
We also typically need pilot-in-the-loop simulations, operating instructions, and maintenance instructions to ensure all parties are ready to support. Limitations are established to constrain the flight envelope until additional data can be gathered, including preliminary flight data.
This all comes together in our Safety of Flight board, where company leaders such as our chief engineer and head of airworthiness sign off on the safety of the FTA 1.
The first phase of the flight test is done in our responsibility, and the regulator is primarily just observing our progress. However, their level of involvement will depend on the country we’re flying in and where we want to fly as they will want to ensure we do not pose a risk to the general public. This is controlled through requirements to receive regulator documents such as an Experimental Airworthiness Certificate or a Special Flight Permit.
Later, once we are satisfied we have worked the kinks out of our design (including any aero fixes we need, like wing vortex generators to get acceptable stall behavior) we will apply for Regulatory Flight Test Clearance. This is when the Certification flight tests begin, with the Regulator’s personnel present during the flights.
There are quite a bit of flight tests we need to do before we can start flying Certification flights tests. Here is a list of typical phases we need to pass before we can ask the Regulator to be involved:
FTA 1 tests:
Once we have progressed to the level that we have checked all aspects of the aircraft operationally and we are convinced we can demonstrate compliance with the Certification regulations for our class of aircraft (Part 23), we can apply for the start of Certification flights. More on this in the next article.