Why you shouldn't roll an Aircraft not cleared for Aerobatics
Posted on 01 August 2015 by Des Barker
An instructor at a Gauteng based flying school was asked by some of the aspirant pilots attending ground school, why they cannot roll the Cessna 208 Caravan that was to be their conversion training aircraft. Names like Bob Hoover and Ted Johnston were bandied about in an effort to 'name drop' and intimidate the instructor into considering rolling the Caravan.
This type of question immediately sends warning signals to an instructor pilot regarding the thinking and the state of mind of the students. In mitigation, the students tell the instructor that they don't quite buy his explanation of the illegal operation of an aircraft, structural issues and the insurance problem in the case of an accident. "Listen, we've seen video footage on YouTube of the prototype Boeing 707 barrel roll performed by test pilot Ted Johnston over Lake Washington, so why can't we straight roll the much smaller and more agile Caravan?"
As a side issue, it is necessary to note that even if test flying and pilots were somewhat different back then, Boeing was stunned and not overly excited by what Johnston had done, but whatever the exact conversation was like, he came close to losing his job. Ironically, the roll seems to have impressed the US military who immediately contracted Boeing for the KC-135, but that's beside the point.
In another case, a colleague at work inquired about a 'flip' during which the pilot conducted straight rolls in the Cessna 172. His words were: "Last month I had the exciting experience of flying in a Cessna Skyhawk during a roll. The manoeuvre was well executed by the pilot, but I was a little concerned about the potential stress on the aircraft, given that there were four people in the airplane. The airplane was visibly in excellent condition, but it was loaded with 650 lbs of people and about 1/4 capacity of fuel; no baggage. During the roll, I experienced what 'felt' like +2 to +2.5g, somewhat less than what I experienced in a high-bank turn in the same airplane during the same flight". (Obviously a well-calibrated passenger) The pilot said that he had performed the manoeuvre probably one hundred times in the same airplane with one and two occupants, but never with four.
So what's the problem you ask? Well, the Cessna 172 is not certified for aerobatics, so it is here where the problem starts, but the root cause is actually the mindset of the pilot.
ISSUES AT STAKE
An artist's impression of the by now famous Boeing 707 roll flown by Boeing test pilot, Ted Johnston.
The first thought that comes to mind is: "What was the pilot thinking?" Through his actions he "went out on a limb", instantly becoming a test pilot and flying passengers outside of the cleared envelope. Question: How many of those reading this article would have placed their passenger's lives at risk to satisfy their ego as being recognised as a great pilot by the poor uneducated souls on-board"?
"The pilot reasoned that since the stress was low, another 250 lbs at that g-loading would not totally exceed the stress rating for the aircraft. I am just wondering if I was within safe limits during that flight. In other words, if I fly with this guy again, should I be wearing a parachute? Any advice appreciated".
Firstly, don't bother to ever flying with him again. Secondly, the problem with this isn't that the 172 can't do the roll within its normal category limits. The problem arises when the pilot executing the roll, somehow screws it up. If this happens, the pilot might not have any excess g-capability to play with. Couple this with the worst possible exit scenario for a botched barrel roll being a rolling pullout, and you have the perfect recipe for a possible 'over-g' condition.
SO WHY NOT?
In theory, any aircraft can perform aerobatic manoeuvres to some degree or the other. However, in practice, there are several reasons why this is not supported by the aircraft manufacturer. You need to visit the website YouTube (http://www.youtube.com/watch?v=c6dWtDk_rOI) to watch an earlier generation display pilot, Howard Thompson, display the Ford Trimotor, performing loops, rolls and barrel rolls to really understand the skills required.
Then there is the real issue, the actual flying skills required versus those possessed. Most of us like to think we are hotshots but don't really know what we don't know. To just try to roll or loop the aircraft not cleared for aerobatics, has cost pilots their lives. Hotshots are advised to go online to YouTube (http://www.youtube.com/results?search_query=Partenavia+Crash&search_type=&aq=f ) to watch the video of the Partenavia's wings both snapping off at the engine mounting while pulling up for a loop. If that doesn't convince you that the structural limits are real, nothing will. The problem is that a little knowledge can be extremely dangerous and listening to likeminded amateurs is a guaranteed 'loss of life'. Remember that aircraft are certified according to regulations pertinent to the 'average pilot'. You may not be average, you may even be below average, so how would you know whether you could hack it?
The Alenia C-27 Spartan on flight demonstration by test pilots at an airshow. (Alenia)
We need to start with the psychological state and mindset of the pilot that would want to become a test pilot by conducting manoeuvres for which the aircraft has not been certified. Within the military training system, instructors are continuously on the lookout for trends in which eager young aspirant pilots have confidence that far exceeds their abilities or their knowledge. History is replete with accidents in which inexperienced pilots killed themselves, their passengers or members of the public by losing control of an aircraft in which illegal manoeuvres were conducted. The pilot fraternity worldwide has not yet learnt from these lessons because such accidents continue to happen. The threat to pilots by their own egos, is real.
The question about the maturity and the psychological status of the pilot and his need for recognition, remains a challenge for human factors scientists trying to find out what it is in the human ego that will make pilot's risk not only their own lives, but those of others in an attempt to impress. Why else would one want to become a test pilot instantly and carry out manoeuvres that highly experienced test pilots, because of an aircraft's particular handling qualities and performance, have recommended not be conducted. Why else would one want carry out manoeuvres that engineers have proved are beyond the structural limits of their designs?
Let us start with the ability of the powerplant to handle negative G. Is the fuel and oil system of the aircraft fitted with a negative g capability to continue feeding the engine during negative g flight? If not, how is the 'hotshot' pilot to guarantee continued operation of the engine during aerobatics since negative g, in one way or the other, is part and parcel of nearly every single aerobatic manoeuvre? If it is not flown perfectly, negative g will in most cases certainly be required to save 'botched' or 'overcooked' manoeuvres.
What about the mechanical design of the fuel system. Acceleration forces due to harsh manoeuvres, could cause fuel within the tanks to 'slosh' to one side and shift the centre of gravity, thereby causing asymmetric loadings about all three axes which could change the stability and control characteristics of an aircraft significantly. Anti-slosh 'baffle plates' are fitted to some aircraft designed for aerobatics to prevent this phenomenon. These devices complicate the fuel system design and in several cases, designers may elect to rather go without such systems and not authorise the aircraft for aerobatics.
Just maybe a relatively small issue could be that the restraint system is not designed to restrain the pilot or passengers during unusual attitudes. There are two risks involved in rolling a Caravan type aircraft. If the nose is not pulled high enough above the horizon before executing the roll, the pilot will have to push over the top thereby subjecting the airplane, and worse yet the passengers, to negative g's. Since the aircraft is probably not equipped with 5-point hooker harnesses, the probability of people coming out of their seats, exists. It gets pretty difficult to fly with someone lying on the yoke, not to mention loose objects the passengers invariably carry along with them on recreational flights.
Entering the inverted flight phase drives lift downwards and requires expert handling skills to co-ordinate the pitch attitude with elevator and rudder to prevent scooping while all the time counteracting adverse yaw with rudder.
Then of course, there is the aerodynamic response of the aircraft about all three axes. Several general aviation aircraft are designed with limited pitch authority to prevent pilots achieving high angles of attack and thereby avoiding stall/spin conditions. By the same token, this restricted pitch authority may be inadequate to pick the nose up during a 'scooped' straight roll at high angles of attack.
One of the problems of piston engined aircraft such as the Caravan, is that the inertial roll axis and the aerodynamic axis are not aligned and the aircraft does not perform a roll about the longitudinal axis but rather some compromise 'corkscrew' between the two. Large amounts of adverse aileron yaw may even cause the aircraft to go side on during the rolling manoeuvre with a subsequent loss of energy and may be accompanied by a large nose down scoop, recovery from which could cause a structural damage if the airspeed built up beyond 'corner speed. Worse though, in some aircraft designs, there could be insufficient roll authority to counter the build up of adverse yaw which could cause the roll to stop at some point in the manoeuvre and with the nose pointing down, if the engine hasn't cut from negative g, the airspeed increases rapidly to Vne and possibly even higher.
If you think this is not possible, read Brian Lecomber's article on aerobatics on the Sopwith Camel ("Of a Camel and Stupidity", World Airnews, April 2008). In this article, he states that the Chipmunk completes a roll in 6 seconds, the Tiger Moth in 11 seconds and the Sopwith Camel in an amazing 23 seconds!!!! During this time, the expert pilot is required to maintain balanced flight and keep the nose attitude from scooping too far below the horizon – this requires a huge amount of motor skills and knowledge of the aircraft's handling qualities. These are typically the things inexperienced pilots don't even bother to think about.
The build up of structural forces rolling against the torque effects of the engine may work against the roll power required to complete the roll. Many inexperienced pilots consider the barrel roll to be the easiest manoeuvre to fly under positive g; remember, it is not 1g, but positive g that is required throughout the manoeuvre. However, many a pilot has 'bought it' by 'burying' the second half of the roll by allowing the nose to drop excessively during the last 180° of the pull-out. With an aircraft accelerating in a steep dive, the pilot is called upon during a rolling manoeuvre to distribute asymmetric forces to the structure which could result in twisting the fuselage, structural fatigue or even catastrophic failure.
Let us consider for a moment the wing of the Caravan. To achieve the necessary cruise performance, a high-aspect ratio wing was chosen with a NACA 230 series airfoil, similar to the 300 and 400 series Cessnas. It needed such long span flaps to reduce the stall speed to the required 61 knots, that there was little room left for ailerons. The flaps reduce the stall speed by a full 14 knots at maximum extension, 30°. Even with a span of more than 51 feet, the available aileron area simply couldn't give the roll control desired.
Cessna had built a Model 308 in the early 1950s with a wing only four feet shorter and discovered just how sluggish in roll a long wing could be. In what is still a surprisingly little known fact, the Caravan's roll control is primarily via 'slot-lip' spoilers that start moving when the up aileron deflection goes past 5°. The relatively small ailerons are considered 'feeler' ailerons and mostly exist to get rid of the dead spot in a spoiler control system when both spoilers are stowed. The result is that a pilot who isn't told about the spoilers, will never guess they exist, other than noticing there's almost no adverse yaw with roll input.
There are so many aspects the test pilot must consider before considering approving aerobatic manoeuvres for certain categories of aircraft, and energy management is one of them. Pulling up for the straight roll may seem rather mundane, but for a low powered, high drag aircraft, sufficient energy could be bled in the pull-up to make it hazardous for the pilot at the apex of the pull up, to commence a straight roll. Coupled adverse yaw plus and engine torque effects could easily transition the aircraft to departure and spin, many such cases have occurred at airshows with catastrophic results.
Rolling G Limits
With aileron applied during a 5g pull-up, the upgoing wing could be subject to 7g and the downgoing wing, 3g.
So what about rolling g, a subject not really well developed in civilian flying? Load Factor is the issue here, not weight, just the technical term, 'Load Factor'. In case you've lost the plot, there are lower g limits for rolling manoeuvres than simple pitching manoeuvres, but, POHs don't always tell you that! Contrary to military combat aircraft, the Pilots Operating Handbook for civilian light utility aircraft does not deal with rolling-g limits because the aircraft are not cleared for aerobatic manoeuvres.
Take the case where we are pulling 'g' in the vertical plane at, or near, the published 'g' limit for the aircraft. Both wings will be operating at, or near, their maximum load factors. Application of ANY aileron at this point and the load factor will increase beyond the maximum load factor on the up-going wing. In this case, the increase in the load factor is disproportionately skewed towards the wingtips where the spar and ribs take a far higher load. If you are going to do rolling and pitching manoeuvres, you NEED to know your rolling 'g' limitations and preferably unload before rolling if you're in a high-g situation and need to level the wings.
VA and limit load therefore decrease if the aircraft is rolling. The rolling motion could come from aileron deflection, or from aggressive rudder input causing a roll couple (as in a snap roll). Aircraft flight manuals that specify a maximum limit load for rolling pull-ups typically place it at two-thirds to three-quarters of the symmetrical limit load. If you settle on a conservative two-thirds, a 6g aerobatic aircraft has a rolling pull-up (and snap roll) limit of 4g. To keep things in round numbers, the "rolling" VA, as calculated for the aircraft weight, would be about 20% less than the symmetrical VA. By the same logic, a large aircraft certified under FAR 25.337(b) with the minimum allowed limit load of 2.5g would be restricted to a 1.65g rolling pull-up and there is no doubt that all pilots would require an inordinate degree of skill to prevent overstressing the aircraft by flying the manoeuvre in less than 1.65g.
The above not withstanding, manoeuvring speed is usually defined without regard to asymmetrical loads as the maximum speed at which full or abrupt combined control movements can be made without damaging the aircraft. The FAA's AC 61-23C, "Pilot's Handbook of Aeronautical Knowledge," says that "any combination of flight control usage, including full deflection of the controls, or gust loads created by turbulence should not create an excessive air load if the airplane is operated below manoeuvring speed."
Let us not forget the engineer's stress work in the design. The torque generated by an engine is contained by the engine's stress mountings on the airframe. The engineer would have designed the aircraft for a certain spectrum of stress loading which in an uncertified aircraft, would not consider torsional loads generated by aerobatics. The classic example of overstressed engine stress mountings are to be found from continuous aerobatic stress loadings on engine mounts.
There is some good news though, and one is not sure whether such information should be made public for fear of some 'hotshot' pilot finding a false sense of security, is that according to NTSB records, there has never been an in-flight structural failure of a strut-winged Cessna. The NOAA uses C182's to penetrate thunderstorms for research and with very little structural modifications, the C150 (aerobat) flies competition aerobatics. It is not only illegal, but also very dangerous to practice aerobatics in an aircraft that's not certified for it. If you need the 'rush', go rent an aerobatic aircraft; it doesn't get any better that when green is up and blue is down.
Remember that even with aerobatic experience, one can still have an off day and not quite get it right – the aircraft loss rate at low level, both during airshows and low-level aerobatics, is evidence of how regularly highly qualified and experienced pilots, 'get it wrong'. What about the loss of life and the subsequent legal hassles of paying out insurance money when the insurance company prove that the manoeuvre conducted was outside the certified envelope for the aircraft. Can you imagine the effect on your family who will be left to fend for themselves financially due to your problem with your ego? Think about it, is it really worth it to try to impress your colleagues? NO!!!!!!
In theory, any aircraft can perform aerobatic manoeuvres to some degree or the other. However, in practice, there are several reasons why this is not supported by the aircraft manufacturer.
Just because something is possible in an aircraft does not mean you should try it. Any aircraft can be rolled but, if it isn't certified for aerobatics, don't do it in an aircraft someone else might fly since the cumulative fatigue effects of your irresponsible behaviour, could create catastrophic consequences for the next pilot's to fly that aircraft. Also, do it over a large body of water so that when you make like a tent peg, you don't kill anybody but yourself and maybe a few fish.
If one really likes boring aerobatic holes in the sky, get an aerobatic aircraft especially if you find that you don't really want to travel, but just like to fly, especially "unusual attitudes". It is clear that any amateur that would even consider conducting such manoeuvres, should be taken far away from flying.
So all in all, there are many different reasons; one or all of the aforementioned may affect a specific aircraft, design. Without sounding melodramatic, whatever they are, if you perform aerobatics in an aircraft that's not certified for aerobatics, that could make you a test pilot instantly, and possibly a dead test pilot at that.