by Paul Bertorelli
When I was a freshly minted private pilot with all of 70 hours in my logbook, I didnt have much money to fly for the simple reason that I was a lowly Spec 4 in the U.S. Army. Most of the flying I did was in the back of military transports where, on one eye-opening takeoff, I learned an unnerving lesson in management of weight and balance.
Along with a dozen other troopers, a bunch of ammo crates, pallets of cement and a stack of pierced steel plate, I was packed into the back of an Air Force C-123, a perfectly obese airplane that actually began its design life as a glider. Being a pilot, I seated myself up front, right behind the raised flight deck. The pilot was a grizzled-looking major, the co-pilot a baby-faced second looey. Clipboard and pen in hand, the co-pilot scribbled furiously on his pad, gazed uncertainly at the major and said, Sir, I think were 9,000 pounds overweight. With a look of utter boredom, the major said something that began with son and ended with, on this runway, well probably be fine.
I thought, Probably be fine? Are these guys nuts? But Spec 4s dont get in the faces of majors so I strapped in and had a nice cup of shut-the-$%#@-up while the Air Force taxied boldly to the runway. On the takeoff roll, the C-123s radials roared, its jet helper engines screeched and the airplane shook and vibrated – in other words, a normal takeoff. It hopped off the ground in about a third of the available runway and we flew on to our destination. I later learned that if the co-pilots numbers were right, the 9,000 pounds put us 15 percent over the C-123s 60,000-pound gross weight limit.
No wonder the major found it a yawner. He had probably seen or done worse and lived to tell about it. But thats military flying, at least on the trash-hauling side of the house. In civil flying, we do things differently because although flying to the mother-in-laws house for Thanksgiving dinner might be thought of as combat, its not the kind where people shoot at you.
Thou Shalt Not
Flight training is peppered with prohibitions and safety provisos of all sorts, some taken more seriously than others. Pilots who are taught to avoid stalls fly hair-on-fire approaches and run off the end of short runways. They have strange notions about leaning engines and some quiver at the mere mention of icing or thunderstorms – thereby losing the utility of those expensive airplanes as a result. In my experience, adherence to weight and balance restrictions has almost religious overtones.
And thats a good thing, frankly. The lighter an airplane is, the better it performs. It climbs and cruises faster and glides better and slower if the engine quits. Theres rarely any percentage in carrying more weight in an airplane than you have to because it eats into margins you may not wish to give up. On the other hand, blind adherence can lead to silly judgment calls.
An acquaintance once offloaded six gallons of gas – 36 pounds – to drop a Cherokee below gross weight. Since the FBO had no provisions to de-fuel – most dont – the fuel was siphoned into two plastic gas jugs, introducing the very real risk of explosion and fire due to the fumes and static electricity build-up. I later pointed out to my friend that the 36 pounds represented 1 percent of the airplanes total weight, an amount so paltry that measuring its impact on performance would likely be impossible.
But a sin is a sin and for him, adhering strictly to the FARs and operating limitations of the POH was more important – but perhaps riskier – than accepting a microscopically small performance hit caused by being 30 pounds over gross.
And at this juncture, lets point out the relevant regulation. Its FAR 91.9: …no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry. When you exceed the airplanes published POH gross weight, you bust 91.9.
Getting Away With It
Yet pilots do it all the time. At this very moment some airplane somewhere has just taken off over gross weight and will just as surely land without incident. Yes, its a violation of the FARs and, no, you shouldnt make a habit of that. On the other hand, realistically, if over gross operations are done sensibly, the risk is acceptable.
We wont insult you by moralizing about this and cautioning you to never do it. Shocking as it may seem, if you want to bend the rules, thats your business, not ours. Just dont bend metal in the process and you do that by applying that ever-important thing called judgment.
How? By understanding what operating over gross weight does to airplane performance and what that loss of performance means, if anything. Sometimes the performance degradation is trivial, sometimes it isnt. Judgment comes from knowing the difference.
An aircrafts published gross weight is determined by the certification basis, either the newer FAR 23 or the older CAR 3 regulations. It is, to a degree, a trial-and-error process based on estimates made during the design phase.
The gross weight is usually set at the maximum allowable to meet certain basic performance requirements, including climb gradient with the engine(s) operating normally and the 61-knot full-flaps stalling speed. There are also structural considerations related to spar loading and landing gear structure.
When you exceed gross weight, the airplane may no longer perform to the minimum standards set out in the regulations. That doesnt mean it cant be flown with acceptable risk, however, just that it no longer conforms to POH claims. The danger is that youll probably have no means of calculating just how much youve eroded the performance due to the higher weight.
Consider stalling speed. It usually increases with weight and it also increases as the square root of the load factor, which is why stall speeds increase with increasing bank. Lets say your 2,800-pound airplane stalls at 61 knots flaps down and you decide to fly 150 pounds, or 5 percent, over gross. The hit on stall speed? About a knot and half. Not a big deal. But if you bump that up to 500 pounds over gross – yes, that has been done and is done in some ferry operations – the stall speed rises by 5 knots for that configuration.
Its easy to see how that could be a problem during takeoff and landing if you dont allow for it. Sometimes, the stall speed actually decreases with higher weight in certain configurations. This is true of the Beech 36 series with full flaps because the center of lift shifts, requiring less down force on the horizontal stab in that configuration.
Being overweight can have a dramatic impact on takeoff performance, and this is where trouble is most likely to arise. Takeoff margins are the make-it-or-break-it variable to be examined realistically at high takeoff weights.
Referring to the manual for a new Cessna 182S, we find that the max takeoff weight for the airplane is 3,100 pounds. At 2,300 pounds, a light weight for that airplane, the POH says well need 800 feet of runway to clear a 50-foot obstacle on a standard day at sea level. At 2,700 pounds, the distance rises to 1,135 feet and, at 3,100 pounds, its 1,570 feet. But what about 3300 pounds, which is 6 percent over gross?
The POH doesnt say but we can be sure its more than 1,570 feet. The relationship of weight to takeoff distance required is not linear but increases by the square of the added value. So at 6 percent over gross, the takeoff distance rises to 1,764 or 12 percent longer. Bear in mind that figures given in the POH are both demonstrated and calculated and this data is collected under ideal conditions, on dry runways with close to standard conditions and with new airplanes flown by expert pilots.
Some POHs provide graphical takeoff performance data – Beechcraft and Mooney products come to mind – where its possible to WAG a takeoff performance beyond the stated gross weight limitations by extrapolating the lines. But these obviously havent been confirmed by testing. Youre on your own.
Another thing to consider is structural limitations on the landing gear and wings, plus limitations related to zero-fuel weight. Further, higher gross weight may also affect flutter margins, due to the effect on stiffness caused by higher loading. These might not be show stoppers in a Skyhawk or a Bonanza, but they shouldnt be ignored, either, especially if you find yourself having to land over gross. As for flutter margins, fly conservatively below Vno/Vne, especially in descents.
Remaining within the CG envelope is a must any time but its especially critical if youre over gross and youve given up performance. Some CG envelopes are squarish at all weights, others narrow noticeably at higher weights. The 182 is an example of the latter and it also has a published minimum landing weight thats 150 pounds less than the allowed takeoff weight. In reality, that means that you cant legally land after a gross weight takeoff until youve burned 25 gallons of gas or heaved a passenger over the side.
From reviewing NTSB records, theres a discernible pattern related to accidents in which over gross operation is causal or at least mentioned. A word of caution here: Its not rare to see the word overweight mentioned in an accident report but our impression is that in many cases, the weight wasnt a factor in the accident and in some instances, weight wasnt confirmed by the investigator. Loading may have been one factor in a pattern of misjudgments.
In those circumstances where weight was a causal factor, two things often stand out: short or obstructed runways and high density altitude. This pattern is so obvious that these accidents rise to the level of what-were-they-thinking idiocy.
Had the pilots of some of these aircraft had the vaguest understanding of density altitude issues, many might never have attempted a gross-weight takeoff, never mind an overweight takeoff. In one accident, two pilots departed Telluride, Colo. – density altitude 11,000 feet – in a Champion 7GCBC estimated to be 213 pounds or 13 percent over gross weight. No surprise that the takeoff into gently rising terrain resulted in a fatal crash.
In another accident, a pilot loaded three passengers and heavy toolboxes into a Piper Arrow and departed a near sea-level airport with sharply rising terrain on a cool morning. Neither density altitude nor runway length were at issue but the terrain was. With an after-takeoff climb rate of only 300 fpm, the hills climbed faster than the airplane. Three of the four people were injured and the aircraft was destroyed by fire.
Another interesting pattern is the number of ag operators involved in what appear to be overweight-related accidents. This is not particularly surprising, since ag pilots tend to take off in airplanes heavily laden with chemicals or seed. Further, the farm roads they use as runways are often bordered by wires or other obstructions and since ag flying happens in the summer, density altitude may be a consideration.
Interestingly, over gross accidents seem to be largely the province of single-engine aircraft, not twins. Does this mean that twin drivers are altar boys when it comes to weight and balance? Doubtful. What it probably means is that with both engines running, an over gross twin has more power to deal with the additional load and thus is less likely to conclude its overweight flight as a takeoff accident, as a single will if it simply cant lift the load or cant outclimb the terrain.
On the other hand, an over gross twin that loses an engine on takeoff is the stuff of nightmares and because of the higher speeds involved and the ever-present danger of a Vmc rollover, the likelihood of a survivable outcome is lower than it would be in a single-engine airplane. Many piston twins climb marginally if at all on one engine at gross weight. Its an article of faith that the lighter a twin is, the better it will perform on one engine. Again, its a question of margins. If your single-engine technique in a twin isnt all that sharp, lighter is better. And even if you are sharp, lighter is still better.
We can all agree that you shouldnt fly an airplane beyond the POHs weight limits; the logic and common sense are unassailable. We could tell you to never fly your airplane over gross, but thats not the way it works in the real world. While it may be true that you never have to fly over gross, its also true that events may conspire to force you into it. Perhaps the FBO overfuels you or passengers show up with five bags instead of three. It happens and theres no point in pretending otherwise.
If its your policy to never fly even a single pound over gross, thats commendable. Dont let anyone talk you out of it. But if you nibble past the margins from time to time and youve done so successfully, you know that any blanket prohibition against over-gross takeoffs lacks credibility. The key survival skill is to recognize when the flight you are proposing has graduated from being a bit more risky to the profoundly stupid.
Transoceanic ferry permits allow takeoff weights up to 25 percent over gross, according to one shop we spoke to that installs ferry tanks. In a 3,000-pound airplane, thats 750 pounds of additional weight – an enormous amount that the ferry pilots think of as 125 gallons of gas.
Thus loaded, those airplanes dont exactly stagger into the air but they climb sluggishly and handle like dogs. They require long runways to get airborne and the climb rate sags well below the POH numbers. But ferry pilots plan accordingly and use long, unobstructed runways where neither terrain nor density altitude are significant factors. Often, the ferry permit specifies a takeoff point that will meet these requirements.
Personally, and admitting guilt here, I have taken off in airplanes 5 percent overweight and didnt think much of it because the CG was centered up and I had a long, clear runway. If faced with the heavy takeoff or trying to drain gas without the proper equipment, Id take the overweight takeoff, hands down. (A short flight to burn off fuel is obviously another option.)
The ultimate secret to survival is to avoid high density altitude conditions and/or short runways or, above all, both together. Indeed, in these cases, the more you are below gross weight, the wider the performance margin youll have, even if it means an extra fuel stop. Or two extra fuel stops. You shouldnt even consider heavy takeoffs in high density altitude scenarios with terrain as a factor.
This sounds obvious but pick the longest runway thats into the wind. Weve seen accidents in which pilots took either the shortest available runway or took off downwind and crashed despite the airplanes being loaded only slightly overweight.
It should go without saying that remaining within CG is just as critical – if not more so – than not getting too greedy about pushing weight limits in the first place. Beyond the edges of the CG envelope above gross weight, airplanes can take on freakish handling qualities, especially when aft of the limit. If youre going to bend the rules on gross weight, dont aggravate the situation by loading aft of the limit. Take the time to check it.
Even below gross weight, make sure you understand how to ground lean a normally aspirated engine to obtain the most takeoff performance in high density altitude conditions.
Last, if you overload a twin, you are no longer in a twin but a single with an extra engine to extend the glide. Some piston twins may eke out a single-engine climb when over gross, but most will not. If you lose one on takeoff, trying to drag it around to land may be suicide.
Youll be better off closing the throttle on the good engine and picking a survivable spot to land, just as you would in a single if the engine quits. In a twin, the lower you keep the wing and power loading, the more performance margin the extra engine will provide. Another way of looking at this is that in a twin, overloading negates the advantage of the second engine, perhaps to the point that youre better off without it.
And whether a single or a twin, the risk is not just during the takeoff phase. With diminished climb rate, a little turbulence or icing that might be trivial in a lightly loaded airplane could be disastrous in an over-gross airplane. Obviously, use care in flying a heavy airplane in higher terrain, where the weather may require all the climb performance you can muster.
If it looks like flying over gross is an unappetizing option, it is. But as with everything else in the world of GA, the risk is neither absolute nor black and white. Yes, you can get away with it and many pilots routinely do in the right circumstances. But if those circumstances arent right – short runways, high density altitudes and/or absurd overloading – its simply the wrong choice.
Paul Bertorelli is a CFII, skydiver, editor of Aviation Consumer and editorial director of AvWeb.