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Canopy Retention and Security
By Tony Bingelis (originally published in EAA Sport Aviation, October 1978)
A SUDDEN LOUD in-flight noise is your first awareness of the failure of the canopy latching system. Following that initial startled reaction, you instantly learn that . . . the canopy latch failure is a temporary scary inconvenience, or . . . that it heralds the winds of doom. Whatever your lot, you realize suddenly that you no longer have the opportunity of improving a poor installation . . . only the necessity of coping with a bad situation.
If you haven't already heard it, it is true. Too many homebuilders are experiencing the trauma of in-flight lost canopies and sprung latches. The consequences have been severe. Injuries, wrecked aircraft and deaths are being directly and indirectly attributed to these mishaps and it makes you wonder why. How does it happen? What can we do about it?
THOSE ONE PIECE BUBBLE CANOPIES
We all know that a full-blown canopy makes a nice looking installation. Installed as a single unit it makes up both the windshield and canopy, enclosing the entire cabin area in an efficient and aerodynamically clean cocoon. It may, however, be more dangerous than efficient - if the canopy is not equipped with a reliable, positive latch installation.
Especially vulnerable are the pusher configured aircraft equipped with this type of canopy. Should such a canopy come open in flight, it might be torn from its hinges and be blown thru the propeller. But even if the canopy is not lost, attempts by the pilot to resecure it could lead to a loss of control of the aircraft and disaster. But, as vulnerable as pushers are in this kind of situation, conventional tractor aircraft designs would not be much better off. They too, are susceptible to structural damage from lost canopies. Aircraft, I am told, are difficult to control when the tail surfaces have been damaged or lost.
The forward portion of one-piece canopies must be carefully fitted during construction to close tightly and prevent the entry of air at any speed. This is an essential precaution regardless of whether the canopy is hinged so that the front end rises, military style, or hinged along one side to provide a flip-over installation. Although it does complicate the installation, and the final installation isn't as aerodynamically pure as a single unit canopy, there is some structural advantage and added security in having a permanently affixed windshield separate from the opening portion of the canopy.
Undoubtedly, one reason canopy latch installations do not get as much thought and attention as the construction and installation of other components, is because the canopy fitting is among those final tasks to be completed. With the completion of the aircraft at hand, the urge to get it airborne becomes quite overwhelming. I guess most of us subconsciously try to compensate for this condition but will nevertheless hasten the installation of a none too satisfactory temporary latching device with the intention of working out an improved latch later. This is faulty reasoning and we know full well that if the latch works well enough initially, it probably will remain a permanent feature of the airplane . . . unless, of course, it sometime later decides to leave the aircraft without your permission.
For the most part the latches used on homebuilts are generally effective mechanically provided they are properly made and properly installed.
Builders, with projects under construction, who want additional information on the installation of canopies and latches might be interested in reading the previous articles printed in three parts in the December 1974, January 1975 and February 1975 issues of SPORT AVIATION, entitled aptly enough, The Installation of Sliding Canopies, Doors, Latches and Stuff'.
Those of us with completed aircraft might do well to reevaluate our canopy and cabin door in-flight security aspects. For example, does your canopy, or door latching, installation have the following features?
A CHECK LIST FOR CANOPY SECURITY
A good easy to grip inside handle is essential.
A positive latch or locking bolt that exerts a clamping force on the canopy.
A safeguard against inadvertent unlatching in flight.
A check list reminder to latch the canopy and/or warning buzzer (or light) activated when the throttle is opened and the canopy is not secured.
Hinge retention relies on bolts or machine screws rather than wood or metal screws or pop rivets.
A poorly fitted canopy is a problem in itself as it is difficult to secure, and when coupled with an improperly aligned latch becomes a potential hazard source in flight.
Sometimes the problem is as simple as an insufficient penetration of the latching bolt or plunger. Other times the hole into which the lockbolt is shoved is too large, or unreinforced, and cannot take much stress. A related difficulty is caused by the misalignment of the latching bolt and its receptacle. A canopy should not have to be forcibly distorted in order to latch it.
Still another problem. The canopy may lack rigidity and because of its flexibility, it might spring open unexpectedly.
A number of in-flight canopy problems could have been resolved had the pilot been able to close the canopy or door from within. In some instances there was no handle or knob or grip of any kind to grasp. Without that sort of aid, even a 97 pound weakling is no worse off than a robust lad with sinews to spare. Sure, many canopy and door installations do have inside handles to aid in closing the canopy (or door). However, some of these things are so small and so difficult to grasp that they are more for show than for convenience.
The inside handle installed to aid in closing and latching the canopy should be large enough to grasp easily and with sufficient mechanical advantage to insure that securing of the door or canopy is possible even under adverse conditions.
An old fashioned "U" handle (as used on kitchen drawers and screen doors) would make a very effective and decorative aid in closing canopies, doors and hatches. Think about it.
Consideration must also be given to in-flight loads and the slipstream's effect on the canopy. There is, as you know, a pronounced tendency for bubble canopies (also for bulged windows and doors) to be sucked away from the aircraft by the lift forces generated by the slipstream. This tendency results in increased wind noise caused by the canopy being lifted away from its normally snug fit. These lift forces also cause a continuous load to be imposed upon whatever latching mechanism is installed. The latch design should therefore, compensate for this condition.
There are builders who believe that no canopy latch is necessary with a bubble type sliding canopy because aerodynamic forces will cause it to slide shut automatically. That may be true but unless a latch, which is capable of exerting a clamping force against the weatherstripping, is installed, you are going to have one noisy airplane.
One of the most frequently employed basic locking devices for a hinged canopy consists of a plunger which engages a hole in the stationary portion of the windshield or fuselage structure. The plunger (rod/bolt) should be beveled or tapered to provide a wedging effect, when in its latched position, and have sufficient penetration so that slight distortions of the canopy in flight will not cause its accidental release.
The plunger (sliding bolt) provides positive latching for the canopy and is most effective when the design incorporates dual forward and aft sliding bolts which are simultaneously activated by a single lever. Long tandem two seater canopy installations may require a third latch located at the canopy's mid point.
WHAT ABOUT CABIN DOOR SECURITY?
Low winged aircraft with cabin doors hinged at their forward edges are relatively free from door latch deficiencies. Should such a door come unlatched in flight, no problems will ordinarily be encountered except for the unnerving effects of wind noise. Attempts to close and latch the door, which will have probably been sucked outward a couple of inches, will often be unsuccessful unless you have a good inside handle to pull on while relatching the door. Of course, if the latch has failed, the only recourse is to return for a landing.
Cabin installations with doors on both sides are sources for another potential problem. They are an invitation to the curious snooper at Fly-Ins. He will open any door he finds unlocked and then forget to relatch it before he wanders off to bestow his thoughtlessness on others. The pilot-owner (later) unfortunately, may not remember to check the far opposite door before take-off and there you have an in-flight incident, if not an emergency, in the making. A very good reason for including your canopy or door latches on your cockpit check list.
High winged aircraft equipped with doors are probably the safest of all installations under the conditions of overlooked latching. (No fair including the ol’ J-3 Cub). These doors tend to stay closed or slightly ajar in flight if inadvertently left unlatched. No in-flight, emergency conditions should result. Here again, a good inside handle to grasp would make relatching easier.
I do not intend to infer that poorly designed and fitted doors, on either high winged or low winged aircraft, cannot be torn from their hinges. This can happen to any unlatched door. However, even under such conditions the pilot can escape serious consequences if the lost door does not destroy the tail surfaces in the event of an unprogrammed departure. It is, therefore, far better to have adequate hinges and good latches than to worry later. In-flight failures of the door type canopy (or cabin configurations) is relatively unheard of and rarely of serious consequence.
Good workmanship and a good fit of the canopy latches is essential to your safety in flight. Poor or inadequate canopy retention in flight could lead to disaster. And, I am sorry to say the record shows that this is not an overstatement of fact.
This may be the time to rework your canopy latches as you may have been promising yourself for a long time. Perhaps it is only the wind noise caused by a latch that doesn't compress the weatherstripping or one that exerts no clamping action whatever. Perhaps it is a latch which is hard to work . . . or perhaps you have one that is too bulky and often gets bumped into the unlatch position. Whatever the problem, if one exists, why not apply a little thinking and designing effort to improve the situation.
A review of the thoughts expressed here and in the previously mentioned articles should offer ideas applicable to just about any canopy or door situation you have to cope with.