Another guest blog from retired pilot Rudy Jakma. This on the topic of banner towing – where light-aircraft trail a banner behind them, to be read by people on the ground. Rudy has extensive experience of banner-towing, particularly from the earlier parts of his career, with around 4000 hours logged, and around 1000 banner pick-ups. His experience even pre-dates his flying career, as he worked as a 16-year old at a local aerodrome, helping to prepare banners to be picked up.
Last week, during the election campaign in the UK, a light aircraft crashed injuring the pilot and his passenger. It had been involved in a banner-towing flight on behalf of a political party, and the passenger, Nigel Farage, was an official of that party and standing as a candidate.
On Sky News, an “expert” proclaimed that towing a banner would have put the aircraft under abnormally high stress, increased by the weight of the passenger and that this would have meant that the flight (which sounded as if he meant the entire flight) was more hazardous due to the presence of the banner. Where do they get these experts from?
From the TV footage of the wreckage I identify the aircraft as a Polish-built Wilga. This is a purpose-built utility aircraft. Towing objects, be they banners or gliders, is well within its structural capability.
All aircraft used for towing must be approved according to manufacturers’s recommendation and are fitted with quick-release tow hooks. The aviation authorities will follow manufacturers’ guidelines and may impose further limitations before they approve a certain aircraft type for towing operations. This ensures that towing an object will never allow stresses to be imposed on the structure that could compromise the structure in any way, be it due to deformation, stress or metal fatigue. The limitations will be published in the aircraft operating manuals and, if deemed necessary, also posted on placards in the cockpit.
The engine will not be subjected to any excessive demand for power, During cruise flight when towing, the engine usually is running at lower RPM than required for normal cruise flight. But, as banner towing always involves a speed well below normal cruise fllght, it may well impose a higher than normal nose attitude. This in turn could in some aircraft cause the engine to run hotter than normal. In such cases, manufacturers of airframe and engines may specify installation of an extra oil cooler and / or modifications to the engine cowling if the aircraft is used for (prolonged) towing operations. Some aircraft are recommended to fly towing operations with one stage of flap setting extended which will stabilize the aircraft and lower the nose attitude – and stall speed.
Other than these considerations, there are no unusual precautions for towing operations. The weight of the banner is never a factor. It weighs less than a passenger. In the air, it causes drag. Lots of it. A typical full-size banner may cause more drag than a typical glider. As drag increases exponentially with increasing airspeed, this is the main reason why a banner is always towed at low speed. It also allows more time for the public to read the message. Typically, depending on the type of aircraft, it will cruise at speeds between 45 and 75 mph.
The presence of a passenger would have meant that there would have been restrictions on the size of the banner. From the TV footage I gather that the size of the banner would not have been the cause of the crash, nor would it have been a contributing factor. In many countries, carrying a passenger during towing operations is prohibited except if necessary for the purpose of checking out another pilot. The size of the banner would have been reduced in compensation.
The Wilga is a STOL aircraft: Short Take-Off and Landing. This means it can fly at very low airspeeds under full control without stalling. Cruise speed whilst towing would therefore be around 60 mph. Possibly even lower if necessary.
The aircraft can not take off with a banner attached. It will drag along the surface and will, if not preventing it from getting airborne certainly have the effect of an unacceptable prolongation of the take-off run. It also will more than likely cause damage due to tearing or friction to the banner. The aircraft does not land with the banner attached, either. Not normally. Before landing, the aircraft will make a low pass over a designated area on the airport and release the banner before it joins the traffic pattern for landing.
The banner is borne aloft by means of a pick-up manoeuvre. A grapnel on a line, usually about 30 – 40 feet in length, is attached to the quick-release hook at the tail of the aircraft. Attaching the cable requires opening and closing of the tail-hook on the aircraft, and this in itself is a check to ensure smooth operation. The banner itself is laid out in a pre-determined area on the airfield. A towing line is attached, ending in a loop. The loop is strung over two forked poles. A batsman or operator of a portable 2 way radio guides the pilot who will catch the loop with the grapnel during a low pass, followed by a fairly steep climb to allow the banner to peel off the surface, and minimise any dragging of the banner along the ground.
The banner pick-up procedure is the only time when the aircraft is routinely exposed to a higher than normal risk. If the climb is too steep (mainly a novice’s mistake) the aircraft may stall. Even if this happens, chances of survival are high as the banner will stabilize the aircraft which will often result in no more than a very hard landing. Another mistake, potentially lethal, is when the pilot thinks they have missed and puts the aircraft in a steep turning climb to quickly come around for another try. This can result in a spin from which, so close to the ground, recovery is not possible.
I have learned always to climb straight ahead to a safe altitude first which probably saved my life on one occasion when during an unassisted pick-up (without ground crew) I did not see the loop during my run-in. I came a bit lower to see what happened and noticed that the loop had fallen off the forked poles and was lying on the grass surface of the pick-up area. This meant I would have to release the grapnel, land, set up the loop over the forks and take off again. As I did not want to spend a lot of time searching for the grapnel I intended to come around again to drop it beside the banner. Even so, I climbed straight ahead and was just considering to start my turn when I felt a jolt. The grapnel had engaged and had picked up the banner from the grass – 1000 to 1 chance! The banner was hanging perfectly.
During all my years in aviation, I have only seen one pilot killed as a result of banner towing. It was a total freak accident. The aircraft was a Tiger Moth, now a priceless classic biplane, but then an ageing trainer and workhorse nearing the end of its useful life. Forward visibility from the rear cockpit – the normal position of the pilot during solo flight – was limited. The pilot had come in too low and had picked up the banner with the undercarriage. It could not be released so he had to land with it. The drag of the banner on the main wheels pulled the aircraft upside down. The pilot was not injured at all. Tragically, he panicked and released his safety harness before assistance arrived on the scene. He fell on his head and broke his neck.
According to the news, the recent crash took place when the pilot was coming in to land, at the end of the flight. I dispute the “expert’s” statement that towing a banner is putting the aircraft under any kind of risk or excessive strain except during the actual pick-up. And even then, the stresses imposed on the aircraft are well within safe structural limits, determined by the manufacturers and approved by the airworthiness authorities. According to the news reports, the aircraft had become safely airborne so the phase during which it had been exposed to a higher theoretical risk had been passed in safety. At the time of the accident, therefore, the aircraft should have been under relatively safe conditions of flight. A shot of the wreckage of the aircraft being pulled upright clearly showed petrol leaking from a ruptured tank. Therefore this indicates that the aircraft had not run out of fuel.
So what caused the crash? I have nothing to go on except projecting my (ample) experience with banner towing on the meagre news reports. So all I can come up with, awaiting results from the official investigation, is an educated guess.
In an aerial shot, it seemed that the banner was lying in the field directly behind the wreckage. This would seem to indicate that the banner may not have been released at the time of the accident. As I already mentioned, an aircraft when towing a banner will fly at very low but perfectly safe airspeed. The banner will be suspended behind it and trail approx. 15 feet below the aircraft’s flight path. It is not impossible that the pilot misjudged the altitude of the banner when coming in to drop it.
When there was a bit of a crosswind (according to the newsreader there was indeed a strong crosswind), we used to come in for the drop in a shallow dive. This would ensure that the banner would be at least at the same altitude or higher immediately prior to the drop. This in turn would allow us to come very close to the ground and at the same time preclude the banner drifting out of the allocated drop zone, perhaps too close to – or even over – the runway.
I suspect the pilot may either have been attempting to drop the banner, or may have intended to make a low pass with the banner for the benefit of the public and press, especially as he had been carrying a politician as his passenger.
Either way, my hypothetical explanation is that the aircraft came in too low and the banner, still attached, struck the ground. This would have caused a sudden and very abrupt deceleration resulting in a stall from low altitude. The aircraft would have struck the ground very hard. The tow line attached to the aircraft usually includes a weak link of calibrated maximum strength, designed to snap if the design load is exceeded. This may have detached the banner from the aircraft at the very last moment. Without the banner it might well have nosed-over.
Please be aware that this is no more than a guess!