Max-Gross Weight Ops

My brother slipped me a piece of paper on which he'd jotted three numbers: 260, 240 and 180. If you haven't guessed, they're weights. Add me and the load is 860 pounds well-marbled (not 170-pound), above-average Americans. Add full tanks, 56 gallons of fuel and we would approach gross weight. I had yet to add backpacks, fly-fishing gear and food for a week, but after that I'd have to start trading fuel for payload, and worry about CG as well. Welcome to the world of gross-weight ops.

For me, the biggest concern in gross-weight operations is an unsafe CG, poor takeoff/climb performance—especially on hot days, at high altitudes and short fields—followed by structural issues (hard landings). If you fly a twin, I suspect single-engine performance could trump any of the above.

Can you safely fly above your aircraft's maximum gross takeoff weight, or MGTOW? Absolutely. It does not apply to us regular Part 91 folks, but in certain circumstances Part 121 and 135 operators in Alaska are allowed to exceed MGTOW by 15 percent. This is based on the need to carry additional survival gear in the remote wilds of Alaska.

This kicker is where we begin to see FAA add variances to accommodate risk management. Clearly the laws of physics are the same in Alaska, but the trade-off in carrying extra safety gear is viewed as more important than the structural and performance-based limits of the aircraft.

The FAA can also waiver in higher gross weights for additional fuel required for ferry flights or record setting. Sixty years ago, Jerrie Mock became the first woman to circumnavigate the globe solo in a plane. Her plane was a 1953 Cessna 180, which is now in the Udvar-Hazy Center, the Smithsonian National Air and Space Museum's annex. It's one year olderer than my 180, but equipped with a 183-gallon fuel tank, putting it 900 pounds over gross.

In 1959, Max Conrad took off in a Piper Comanche loaded 2000 pounds over gross and flew nonstop from Casablanca to Los Angeles. I am guessing Max and Jerrie had long takeoff rolls and shallow climbs. And I bet they winced at any sign of turbulence while fully loaded. They were safe, but they were flying a less-forgiving and -tolerant airframe than designers intended.

Another way to legally exceed MGTOW is via a supplemental type certificate. For example, adding more horsepower to an airframe often comes with a corresponding gross weight increase because it improves climb performance on a robust airframe that likely skated past the G-loading and drop tests. For my 180, adding 18 inches to the wingtips and some internal structural stiffening earned a 400-pound useful load increase, which helped make it possible to carry my brother and his spawn.

I've heard many pilots admitting to flying over MGTOW, in some cases routinely. Is it unsafe? Is being a test pilot unsafe? If you load a plane over gross, you are taking it outside the approved (and known) performance envelope. Flying over gross also reflects a careless attitude and defiance of the rules, both known "at-risk" behaviors that begin the accident chain. But perhaps the most irresponsible thing about flying over gross is it often involves passengers innocent of the risk. Which means the PIC is subjecting others to risk without their full knowledge or consent. If for no other reason, it is not responsible or professional behavior.

Remember that exhilaration you felt soloing? How the plane climbed like a scalded cat once the portly instructor vacated the right seat? Gross-weight takeoffs have the opposite feeling. Instead of seeing blue sky, you will be seeing tree tops for an extended period of time.

The most common issue with an overloaded aircraft is failure to launch. Even in the best of conditions, an overloaded aircraft will have a shallow, anemic climb. Add a rough runway surface, a hot day, a high-altitude airport and an obstruction, and the takeoff attempt may not end well.

The AOPA Air Safety Institute's 22nd Annual Nall report summarized the aircraft accidents (nine total; one fatal) related to weight and density altitude in 2011. One of it conclusions: "...no deaths resulted from the seven accidents arising from delayed decisions to abort takeoff attempts. Overruns tend to be safer than trying to force an aircraft to fly."

Conditioning is one reason pilots misjudge gross-weight takeoff performance. Most of our flight time is logged at well below gross weight. We get used to the way the plane jumps into the sky alone or with a single passenger. While pilots are book smart on the degraded performance that comes with gross weights, takeoff performance at gross weight is truly experiential learning.

You owe it to yourself and your passengers to not be caught off-guard. There is no mistaking the sharp-edged awareness of an approaching 50-foot obstacle or the dawning reality of the need to use two-thirds of the available runway. It isn't something we typically practice, but when you fill the seats and tanks, that conditioning is something you'll experience first-hand.

One gross weight fallacy is the mistaken belief that because it is certified and the legal measure of aircraft performance capability, it is somehow always achievable and therefore something a pilot is entitled to. Emphatically, it is not. Maximum operating weight is basically what it takes to get safely into and out of a given airstrip. Sometimes maximum operating weight is the same as MGTOW but not always. More often than not, the limiting procedure is the takeoff and departure climb.

"The maximum allowable weight for an aircraft is determined by design considerations," according to the FAA's Weight and Balance Handbook. "However, the maximum operational weight may be less than the maximum allowable weight due to such considerations as high-density altitude or high-drag field conditions caused by wet grass or water on the runway. The maximum operational weight may also be limited by the departure or arrival airport's runway length"

When I took my brother and his sons into the Idaho wilderness this summer, I initially dropped them off at Cabin Creek (I08), a rugged Forest Service airstrip. It features a one-way landing with no go-round possibility, but its relatively long runway (by backcountry standards) goes uphill, which makes it great at arresting the energy of a fully loaded plane. Cabin Creek is within my 180's max operational weight and max gross weight for landing, and also for a gross weight takeoff when the air is cool and density altitude is low.

When I joined them a few days later, I landed upstream at a more challenging airstrip. Compared to Cabin Creek, it is rougher, more obstructed, and much shorter, roughly 900-1000 feet. On takeoffs, you'd better be in the air at 700 or the creek bank and trees will offer you an unwelcome view. In my plane, near-gross landing would require an early touchdown and a lot of brakes, but full gross takeoff is not an option. Ever.

Given the short and challenging nature of this strip, I decided to land with less than half-full tanks. That would ensure I could comfortably depart solo, just my backpack and me. The plan was to join my above-average family for a couple days of fishing, then send them back downstream and pick them up at the much longer Cabin Creek airstrip. Unfortunately, I had not planned for the challenge of stream crossing. My brother and his sons had such a harrowing experience fording the swollen stream to join me at my plane-side campsite there was no way they were going to ford the stream again for the hike back to Cabin Creek. It was a dilemma. Abandon them, head to a town to find someone with a higher performing plane, or take them out one at a time.

I decided on the last option, but decided I'd first take out their gear, about 150 lbs. If that went well, I would come back and start taking them out to Cabin Creek one at a time, first the 180-pound nephew, then the 240-pounder and finally the 260-pound patriarch. Apart from the distances, the most notable effect of the ever-increasing weight was the increasingly shallow climb angle.

After the morning of shuttles to Cabin Creek, my family entourage retreated to the lodge at Sulphur Creek (ID74). The next morning I ferried the packs out to Challis, Idaho (KLLJ), and called a buddy with a 182 to haul them home to Idaho Falls. I returned to have a filling breakfast, which Sulphur Creek lodge is known for, and watched the planes leaving as the summer morning grew warmer. It came time to leave. After departing the 1800-foot long Cabin Creek runway with the same passenger list the day before, I felt pretty comfortable with the 3300-foot runway at Sulphur Creek. It should be a breeze, I thought.

When we left Cabin Creek fully loaded at 4200 feet elevation at 7 the morning before, the outside air temperature was in the low-50s F. High barometric pressure meant density altitude and field elevation were roughly the same. That max-gross weight takeoff from an 1800-foot-long runway should make the 3300 feet available seem luxurious.

My feeling was reinforced when I took off from Sulphur Creek in the early morning to shuttle packs. The air was cool, the plane was light, and I climbed straight out and was able to clear the ridge at 8500 feet without even trying. For my gross-weight departure, however, temperatures were climbing into the mid-60s, pushing density altitude beyond 7000 feet.

After making sure everyone was belted in, I firewalled the throttle. The plane accelerated, and accelerated, but did not lift. I kept waiting for the dramatic climb I had experienced in the morning. It didn't happen. I was at full gross. I needed all the runway I could get and a bit of ground effect as well. The plane eventually came up, I nursed the flaps out and instead of turning toward the ridge, I pointed the nose downstream and eventually into a spiraling climb. We enjoyed the view of the river and canyon below, and eventually we had the altitude to clear the ridge. My departure that morning with just the packs had given me a false sense of performance.

Even after the performance lessons I had gotten the day before, I was still surprised at how flat and anemic my otherwise-peppy 180 is when it's fully loaded and the day is warmer. I was beyond glad that I had called in a favor to get the packs ferried separately.

The most significant effects of gross-weight operations are experienced at the beginning of a flight rather than the end. It drastically increases your takeoff distance and reduces both the rate and, as a result, the angle of climb. To put it into perspective, a Cessna 206 at gross weight flies like a 182. On a hot day, it flies like a 172. If you are at a high-altitude field on a hot day, it will fly like a 152.

In the end, my above-average family called for some above-average planning. At full gross, it is no big whoop to depart my 9000-foot-long home runway, but it was not an option with the 900 feet available at a back-country strip. If you are going into short strips it is reasonable to leave things behind and come back later for the rest of the load. The lesson I learned is if you fill your plane with above-average people, expect below-average performance.

Maximum gross weight is a certification standard. It's parameterized around both the structural and flight characteristics of aircraft. Unless you are involved in aircraft certification, you're unlikely to have seen or read relevant sections of the FARs, but some of it should be familiar to you. For example, you should know that, at gross weight, the wings should stay on and the airframe should remain undamaged when the airframe is stressed to as much as 3.8G and 4.4G for Normal and Utility categories, respectively. Those standards—structural airframe G-loading limits—are in FAR Part 23, which defines how to test and prove an airframe is up to the task.

Part 23 also states that the landing gear must survive a specified drop test for static and dynamic loading. There are several different methods for this but, as the name implies, it actually involves dropping the loaded airframe from a specified height a certain number of times. One drop test must simulate a descent at 1.2 times the maximum designed vertical descent rate, assuming the lift and aircraft weight are equal. An even more brutal test is the landing gear drop test at 1.5 times the load limit. If the landing gear holds, the plane is good to go.

Finally, the formula for defining maximum certified gross weight includes some performance numbers for climbout and balked landing scenarios, including parameters for single-engine performance in a multi-engine airplane. The maximum gross weight basically requires an aircraft to be able to eke out a 1.2-3.5 percent gradient climb depending on whether the scenario is a balked landing or a single-engine climbout).

What does all this mean for you the typical pilot thinking about loading an airplane to its limits? One thing it means is a well-maintained certified aircraft is capable of gross-weight operations subject to those limits. For example, your aircraft can withstand G loading, land pretty hard and not suffer damage at its gross weight.

None of this, however, means its performance will be adequate for what you have in mind.

"Don't rush," said Pete Nelson, owner of Middle Fork Aviation, a Part 135 operator flying gross-weight loads into the Idaho backcountry for a living. "You will get to altitude eventually, but if it's hot and you're fully loaded, you have to be very patient.

"The only thing a gross weight plane does well is descend, so don't turn toward the hills, or a ridge or higher terrain until they are below you," said Nelson. "When climb performance is low, all your escape routes are below you. Your out is knowing where the descending terrain is.

"You stall at a faster speed at gross weight," said Nelson. "So you have to land a little faster, which means more momentum to arrest in the flare and rollout. When you're heavy, you don't want a hard landing. If you get too slow because you want to land short, you run the risk of getting a porpoise or oscillation started."

Mike Hart is an Idaho-based commercial/IFR pilot with more than 1000 hours and proud owner of a 1946 Piper J-3 Cub and a Cessna 180. His is also the Idaho liason to the Recreational Aviation Foundation.

This article originally appeared in the September 2014 issue of Aviation Safety magazine.

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