LOAD 'EM UP, FLY 'EM OUT...MAYBE By Anders Christenson CFI, FAA Designated Flight Examiner April, 1983 Being a pilot examiner you cannot afford the embarrassment of flying over max gross. "But all seaplane pilots have to fly over max gross or they could not afford to operate their air-planes". That is not an unfamiliar comment to me. This paper is not a lecture in an attempt to change the ways of many seaplane pilots. I will discuss the problems of' control with over max gross weight and out of center of gravity balance. Then let each pilot approach his decision with some new knowledge. He then may not be ignorant of what may happen although he may still be arrogant. In other words, to be forewarned, may be forearmed. In order to control an airplane about its pitch axis, there must be a certain amount of down-load created by the horizontal stabilizer. This is necessary to counter the weight forward of the center of lift on the wings. That center of lift, by the way, must always be behind the center of gravity (C.G.). The amount of down-load necessary for control varies with different types of airplanes, but it is always required. For the sake of explanation I'll state that 200 pounds down-load is required with C.G. in mid range. This 200 pounds is created by moving air over the horizontal stabilizer and elevator at sufficient speeds. That occurs on the first part of a takeoff with a combination of propeller blast and increasing forward airplane speed. The signal to us that we are creating enough down-load is when we can control pitch both up and down. Now let's talk about how C.G. affects amount of download required. Here is a typical C.G. envelope.
Now let's get reckless and move the C.G. aft of the aft limits just a fraction of an inch. Now the horizontal stabilizer is required to lift some of the weight of the airplane. Enough speed must be gained and maintained in order to gain and maintain pitch down control. With this condition the center of lift is ahead of the C.G. It is the most dangerous loading condition. This aft loading results in a takeoff pitch attitude existing during the taxiing and takeoff phase. The airplane may become airborne before the tail can provide enough lift resulting in ever increasing uncontrollable nose up pitch ending in a stall with no nose down recovery possible. So how might you use this new found knowledge since you might still fly your airplane beyond the aft C.G. limits? While taking off, your floats are coming out of the water and you are unable to pitch the nose down1 even with full forward control wheel. CHOP the power right now. You have loaded the airplane in such a manner as to make control impossible. A seaplane that is loaded over its max gross is very likely also balanced aft of Its aft C.G. limits. Some seaplane pilots approach their overloading with some forethought. They extend their loading C.G. envelope in the following manner.
They continue the forward C.G. and the aft C.G. lines until they intersect. The best I can say for doing that is they are at least aware they are operating their airplane beyond the manufacturer's limitations. Part of my aviation services include acting as a Pilot Expert in aviation accident litigation. When we look for the accident cause factors, there are often many FARs that may be broken that are not considered as even part of the cause. However, if the accident airplane was loaded beyond its C.G. limits, that fact is always the main contributing factor. You won't be able to rind an expert who will testify that an airplane that is loaded over max gross and / or out of C.G. limits will handle predictably. They will say only that such a situation is predictably unpredictable. "Serack now sat motionless in the cockpit for as much as 10 minutes, according to witnesses. As far as they could tell1 the engine remained at idle. He had the appearance of a pilot thinking about the tasks that lay before him1 or perhaps mustering his courage. An aircraft departed ahead of him. another aircraft taxied up behind him, and now Serack swung out on the runway for takeoff. Since he now told the tower it was to be the biplane's first flight and he would remain "close to the airport, it appears clear that a take-off as opposed to a high-speed taxi test-was intended. Without appreciable delay, the plane accelerated slowly forward1 broke ground gently and was airborne. The climb speed seemed slow to a witness in the tower, who noted what appeared to be pitch and roll changes "as a pilot might do when reeling out the controls." But the movements might also have been considered jerky and Mechanical. Over the departure end of the runway at perhaps 500 feet. the plane turned crosswind and "although the turn was shallow, the aircraft abruptly lost some altitude, but reacted to this and climbed back," the witness related. The plane turned downwind and had a similar altitude loss, but climbed and eventually got to about 600 feet AGL (substantially below pattern altitude). The tower controller had now begun calling to ask the pilot's intentions1 and whether he had a problem. On the down-wind, the plane was seen to make abrupt and somewhat severe pitch changes. After the controller's third call, the pilot answered, saying he "had a little bit of trouble and would come on in," the controller later reported. There would be no further transmissions from the pilot. At this point, the controller radioed that a Fairchild F-27 was on final, but if the biplane pilot could turn base now, he could land first, otherwise9 he would have to extend his down-wind. The biplane turned base without comment. The F-27 pilot radioed that he would go around if the other plane was having trouble, and the propjet now commenced a go-around. Meanwhile, on the base leg, the biplane was descending when witnesses heard the sound of power being reduced and noticed a slight flattening of the pitch attitude. An instant later, the plane snapped over into a left spin, completing nearly one rotation before impact. It hit the ground nearly vertically, the left wing striking slightly before the right. Almost simultaneously, it burst into flame. Whether Serack survived the impact: is moot; he could not have survived the fire. Investigator's Findings: The FAA investigator in charge of the crash was aided by a four-member committee from the local chapter of the Experimental Aircraft Association, in which Serack had been a member. Finally, however, the committee made a startling discovery, and one which was to figure in the NTSB's probable cause statement. Upon reviewing the aircraft's weight and balance records, it was found that Serack's calculations contained a gross error. The actual center of gravity of the aircraft upon takeoff was likely at least 1.2 inches (and perhaps more) aft of the rear-most limit defined in the builder's manual. The aft C.G. problem easily accounts for some of the pitch problems and the prompt snap-spin at power reduction. An airplane in such a C.G. condition may be marginally controllable or totally uncontrollable, depending on speed, power, and aspects of the design. As we've mentioned before (Aviation Safety1 November 1981, Page 9), even professional test pilots in factory prototype aircraft proceed cautiously on C.G. by starting out well inside an expected normal range and sliding weights aft until controllability becomes suspect. The C.G. limit is set reasonably forward of this point, and the test is over. Test pilots do not explore the region behind the aft limit, because that regime can be assumed to produce an uncontrollable airplane. In effect, Serack had gone where test pilots fear to tread. * (Author's comments) Note that test pilots, in determining the aft C.G. limits, continue moving the weights aft until controllability becomes suspect. That point of suspect is different for each test pilot. So the aft C.G. limit is set reasonably forward from what point of suspect and how far "reasonably forward"? It's all too "iffy" for us to count on any manufacturer's safety margin. "Grossly aft center of gravity" is suspected in the crash of a Piper Aerostar 602P on January 5, 1983, near Evanston Municipal Airport, Almy, Wyoming. The crash killed the lone 24-year-old pilot, who was making a delivery of oil well drilling equipment that had been loaded in the aft baggage compartment. The airman had some 1,500 total hours and 300 in type. Investigators said the pilot had departed Casper, Wyoming and conducted an uneventful IFR trip. arrived at Evanston and canceled IFR. and descended to enter the pattern. (Author's comments) Note that test pilots, in determining the aft C.G. limits, continue moving the weights aft until controllability becomes suspect. That point of suspect is different for each test pilot. So the aft C.G. limit is set reasonably forward from what point of suspect and how far "reasonably forward"? It's all too "iffy" for us to count on any manufacturer's safety margin. "Grossly aft center of gravity is suspected in the crash of a Piper Aerostar 602P on January 5, 1983, near Evanston Municipal Airport, Almy, Wyoming. The crash killed the lone 24-year-old pilot, who was making a delivery of oil well drilling equipment that had been loaded in the aft baggage compartment. The airman had some 1,500 total hours and 300 in type. Investigators said the pilot had departed Casper, Wyoming and conducted an uneventful IFR trip. arrived at Evanston and canceled IFR. and descended to enter the pattern. The pilot entered the downwind leg at about the same time as a Beech King Air, whose pilot radioed that he would follow the Aerostar when he rolled out again. The King Air pilot radioed asking the plane's position. two Mayday calls were then heard, followed by "a scream and a moan". Ground witnesses observed the plane to pitch nose-up and "wobble" out of sight before it struck the ground nose-low and left-wing-low, traveled 18 feet and struck a parked trailer, broke apart and slid to rest. Investigators found that the plane had departed with full fuel and 400 pounds of cargo. The cargo had been loaded in the aft compartment1 which is placarded for only 240 pounds. Calculations indicate the center of gravity at takeoff may have been as much as six inches aft of the rearmost limit, and was probably as bad after the two-hour flight". *1 Aft C.G. problems occur with all classes of airplanes. If we load our airplanes where the test pilots, with all of their precautions, fear to fly---at least we should know it. Now, let's do our weight and balance. Anders J. Christenson, FAA Pilot Examiner (GL-15-24) is a leading authority on airplane pilot certification. With over 25,000 flight hours and 30 years as an examiner and instructor. For an individual consultation or dual flight instruction contact Anders at Flying Cloud Airport, Minneapolis, Minnesota USA (612) 944-4036 or email. [email protected] *1. Aviation safety, April, 1983, Vol. III, No. 4, p7
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The farther forward we move the C.G..the more
down-load is required on the tail to hold the nose up. The total down-load now is greater
than 200 pounds. The wings have to lift more and performance gets worse. The stall speed
goes up because more,airspeed over the tail is required to create more pounds
down-loading. The opposite is true when we load the airplane so the G.G. moves closer to
the aft C.G. limits. Less than 200 pounds is required to offset the weight forward. The
wings have to carry less weight and performance gets better and stall speed gets lower.
However, the C.G. gets closer to the center of lift.
