I recently made an open solicitation for column ideas in the hopes that COPA’s readers would let me know what issues are important to them. As luck would have it, the very first response, from a reader in Golden, B.C., brings up an important topic; properly securing aircraft.
The concern here is that pilots may be tying down their aircraft less than adequately. The reader cited a number of dubious tiedown techniques he’s come across. To be honest, as I read his email I was also thinking of all the terrible tiedown jobs I’ve seen in my 24 years of flying.
There’s not really that much of an art to properly tying down an aircraft, but there are a lot of ways to screw it up! There are essentially four aspects to the tiedown process: 1) the surface, 2) the aircraft tiedown points, 3) the rope, and 4) the anchor.
If we consider the first aspect, the surface type may provide various tiedown options, or in the case of pavement, severely limit the options. If the surface is asphalt or concrete, then we are usually stuck with whatever tiedown hooks have been installed. This isn’t necessarily a bad thing however since these surfaces usually provide the most secure anchors when installed correctly.
I’ve never heard of a small aircraft pulling an anchor out of concrete, but I suspect that it has happened with soft asphalt. In fact, if the anchor is set properly in concrete, even the best cabling for the tiedown will usually snap before pulling the anchor free; both a C-130 Hercules and C-133 Cargomaster at a nearby museum have proven that possible! Steel cables snapped; anchors left securely in the ground.
Anyway, the best-case scenario for many high-wing airplanes is a 4-point tiedown to a concrete surface. These airplanes tend to move around more freely in windy conditions than most low-wing airplanes. Three-point tiedowns are good but if the wind gusts strong towards the tail, it will bang on the ground.
I tried for several years to get my airport to install some nose tiedown anchors (since my family’s C-172 had a nose tiedown ring), but never made any progress. After we had to repair the elevator and tail twice in a five-year period, we moved the aircraft to hanger storage. Most of the low-wing aircraft I’ve flown only have three tiedown point (wings and tail), and I’m not entirely sure about the rest.
Suffice to say that if you have a nose tiedown and are expecting strong tailwinds while the airplane is tied down, the nose should be tied down as well.
One note for the do-it-yourselfers; if you need guidance on how to make secure concrete or asphalt tiedown anchors, reference FAA advisory circular "Tiedown Sense"AC 20-35C. These circulars can be obtained free of charge from the faa.gov website.
Soft surfaces, such as dirt and sod, provide the option of adding that nose tiedown for those airplanes so equipped. The trick here is making a strong enough anchor that won’t pull out of the soft ground.
The traditional dog ring that I’ve seen any number of pilots using when away from homebase just doesn’t cut-it as an effective anchor. Not only is the metal not hard enough to withstand the load under strain, but they do not adequately grip the ground. Unlike a screw that has very sharp edges closely spaced, the dog anchor makes a nice smooth channel as it cork-screws into the ground; so much the easier to screw-back out under a pulling load.
Other screw-type anchors are also considered inadequate by the Experimental Aviation Associations (EAA) since they tend to loosen the ground as they are screwed in. Even if they don’t screw back out under pulling loads, the weakened ground can more easily give way. Single-helix cable anchors are supposed to be pretty good, but I’ve never actually used these to be sure of that.
According to the EAA, the best anchors are those that use multiple pins entering the ground at different angles; especially when angling the tiedowns away from the aircraft so that that pull is not immediately straight up on the anchor. I hunted down their website to see what that might look like; they have some nice graphics that show how to make your own: http://www.airventure.org/2007/planning/tying_down.html.
A word about concrete blocks for anchors; they don’t work! Think about it for a minute, if the wind is strong enough to blow the aircraft, or even try to get it airborne, a few concrete cinder blocks are not going to be one iota of help. The airplane will move and pull the blocks with it. Remember that C-130 from earlier in the article? It pulled 4x4x4ft solid concrete blocks around without any effort at all! Now if you have 4x4x4ft concrete blocks for your C-172, well, that might just do the trick!
If the anchor is good (hard surface or the right kind of anchor for a soft surface) then the next important securing aspect is the tiedown itself. According to the EAA, the recommended material for rope is UV-resistant braided nylon or Dacron. These materials won’t shrink or rot as a result of exposure to water and they have better strength. That makes them better for long-term outdoor use than manila type rope.
We always traveled with nylon tiedown ropes in case the destination didn’t have them! Many times the destination airport did not have sufficiently long tiedown ropes for high-wing airplanes, so that’s another good reason to carry your own set of adequate length ropes.
One disadvantage of the synthetic rope is that making knots that stay put seems to be harder, probably due to large braiding and a little less flexibility than manila rope. In any event, the knots I see pilots making are not the best, regardless of the type of rope they’re using.
If you’ve had a shoelace come undone, then you know what kind of knots you shouldn’t use. An anti-slip knot is preferable. Since it’s impossible to describe the process, refer to the EAA link above for instructions on making anti-slip knots.
Any old nylon or Dacron rope will not necessarily work even if you have great anchors and the world’s best knot. You wouldn’t use sewing thread to fly a kite, and you shouldn’t use rope that is too small for the weight you are trying to hold to the ground. There is of course a good deal of science behind breaking potential under tension forces so luckily we pilots don’t have to guess what size rope to use.
If you want some idea of the size rope that will provide adequate strength for the weight of your aircraft, refer to the FAA advisory circular 20-35C dated 1983. There’s a nice chart for minimum breaking strengths for fibre cordage.
In addition to a good tiedown, control surfaces should also be secured either internally or externally. Most modern aircraft have a control lock provision at the control column. However, most do not have any provision for locking the rudder. If you have an airplane whose rudder is loosely rigged so that it will flap in the breeze, consider devising some way to lock the rudder pedals in a neutral position, or adding an external padded lock between the rudder and vertical stabilizer.
Most aircraft supply companies have these for sale. Ask around before buying so that you know which type works best for your airplane.
If your airplane has no control lock for the ailerons, securing the control column with the seatbelt is one option to keep the controls from flapping in the wind, but this is not an ideal solution.
The usual method when securing with a seatbelt is to pull the control column full aft and turned all the way in one direction. This might be ok for a tailwind situation at the tiedown location, but if a strong headwind occurs most light aircraft will bang the tail on the ground in the absence of a tailstand.
A side note on tailstands; they don’t work unless they are also secured so that they don’t kick-out from one side or another. Also, don’t try to make your own tailstand unless you really know what you’re doing. I made a beautiful one only to have it rip off the tail tiedown hook from a C-172. Boy was that fun to replace!
A final word about improvised control locks: The manufacturer has usually given thought to the hurried pilot who might try flying with control lock(s) still in place. To minimize that possibility, modern control locks usually cover some critical area of the instrument panel, such as the master and/or mag switch.
Those aircraft that come with rudder locks will frequently lash them via a cable to the control column lock so they are installed as a pair, and removed as a pair. Other aircraft that are more stiffly rigged may not come with control locks of any kind. It is these aircraft that require some extra consideration as they too are susceptible to damage in high wind conditions.
Tying down your aircraft regardless of the conditions should always be done if you intend to leave your aircraft for any period of time. I’ve seen aircraft roll on flat pavement with no more than 15 knots of wind!
Tying down also allows the pilot to disengage the parking brake, which by the way, most manufacturers recommend not be left on for any extended period of time. The line people at the FBO will also want you to leave the parking brake off so they can move your aircraft if necessary.
Securing your aircraft is the last part of every flight and should be done with as much care as the flying itself. By following the recommended guidelines above (all of which are endorsed by the FAA and EAA) you can minimize the possibility of damage to your aircraft, as well as others.
This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 180 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.