Aircraft Maintenance: Gross weight increases
General aviation aircraft designs are products of a series of compromises, balancing trade-offs for speed, payload, and efficiency (not to mention cost). Regardless of your choice in aircraft, chances are you’ll eventually yearn for a little more performance in one of more of those areas.
When it comes to increasing the aircraft’s useful load, your options are limited to:
- Reducing the empty weight of the aircraft
- Increasing the maximum gross weight of the aircraft
- Taking the aircraft to Alaska…
There are a number of aircraft modifications available that increase the MGTOW of the aircraft, and some may surprise you. For example, supplemental type certificates that allow owners to add tip tanks to many aircraft also come with an increase to the aircraft’s legal MGTOW, and thus increase the associated useful load of the airplane. At first glance, that seems counterintuitive. After all, you’re adding weight to the aircraft in the tip tank structure and additional fuel, but you’re not adding more power to lift it. However, the devil’s in the details. Ultimately, the STC holder needs to demonstrate that the aircraft can meet the requirements for the higher weight with regards to climb performance, cooling, and structural safety.
We recently worked with General Aviation Modifications, Inc. (GAMI) to increase the MGTOW of SocialFlight’s A36 Bonanza by 250 pounds. Since the aircraft was already equipped with a Continental IO-550B, GAMIjectors and GAMI’s cooling mods, the practical portion of “installing” the useful load increase was an act of paperwork made possible by the GAMI’s STC.
As we waited for delivery of the letter that would make it possible for our plane to carry 250 pounds more, the boys and I joked about how the UPS driver would looking carrying a package that weighed negative 250 pounds… However, jokes aside, there’s a lot involved in getting approval to increase the MGTOW of an aircraft.
The GAMI STC requires three things to be in place before you can get the increased load: First, the aircraft must be equipped with Continental’s 300HP IO-550B engine. Next, GAMI’s balanced fuel injectors must be installed. The combination of the engine and injectors increases the power available for climb. Finally, GAMI’s cooling mods for the No. 2 and No. 6 cylinders must be installed to ensure that the engine has adequate cooling to support climbout during operations at the higher weights.
GAMI performed extensive engine/airframe analysis and testing in the process of obtaining the gross weight increase STC. They also used one other certification tool to get approval at the higher weights: They changed the category of the aircraft as part of the STC. The Bonanza is certified from the factory in the utility category, which provides for load factor G-limits of -1.76 to +4.4. When operating above the original factory certified MGTOW of 3,600 pounds (for our A36), the GAMI STC puts the aircraft into the normal category. This means that when we are loaded above 3,600 pounds, up to the new MGTOW of 3,850 pounds, the aircraft has reduced load factor G-limits of -1.52 to +3.8 (normal category limits).
Regardless of what the paperwork says, the aircraft should always be flown based on the performance charts for safe landing and takeoff distances for the loading and conditions of the day. Having more useful load is only as useful as your actual height above the trees on climbout! That said, the GAMI STC is an invaluable addition to our aircraft, which has abundant power to lift that extra 250 pounds of growing boys, folding bikes, and all of the video gear required to document our next flying adventure for SocialFlight.com! Until next time…Happy Flying!
Jeff Simon
