The Good, the Bad, and the Ugly - Picture

Last night I told you about an incident involving a Cessna 172. Today I have a picture of one propeller tip so you can see what happens when a spinning prop strikes pavement. This is one of the "best" curls I've seen in a prop, though I'll have to admit to not seeing all that many.

When running, in this position the blade would have been moving downward. The forward movement of the airplane tends to cause the tips to bend aft. Successive strikes change the initial bend into a curl, the rotation of the prop gives it a characteristic spiral. It is a very interesting, if expensive and disturbing, shape.

The Good, the Bad, and the Ugly

The weather is good. It looks like the daytime surface temperature will be above the magic 50 degrees Fahrenheit at which point we are obliged to remove the oil cooler cover plate. Most airplane engines have an oil cooler. Some are mounted directly to the engine, some like mine are mounted some distance away. In either case it is like a tiny radiator, engine oil circulates through and is cooled by air flowing through the engine compartment. In cold weather the cooler is too efficient so it is desirable to reduce the amount of airflow. On my Cherokee this is accomplished by removing the air duct that delivers cooling air, inserting a specially constructed aluminum plate and reconnecting the duct. There is no low temperature at which I must install the plate, but it must be removed above 50F.

This is also a good time to re-install the wheel pants. Wheel pants are fiberglass covers that reduce the drag caused by air flowing around the wheels. In the winter time, snow slush and ice can get jammed in the pants. This is as bad for the plane as it sounds because during a flight this can freeze the wheels solid making landings difficult or unsafe.

While getting this intimate with the airplane it is also a good time to look at every thing in and around the engine and landing gear to see how things are wearing, aging etc. Everything looked good. The test flight went well, everything stayed attached so I went for a long enjoyable tour around the local area.

So what could be bad, or ugly? It seems last night a Cessna 172 had a hard landing, collapsing the nose wheel. No one was injured. I don't know any of the details, and it would be unfair to speculate, but some things are know to all pilots. Tricycle gear equipped airplanes have relatively delicate nose wheels. They are designed essentially to keep the propeller out of the mud and steer on the ground. They are not designed to take the forces of landing so pilots are trained to land on the main gear first. When the nose gear breaks on landing, the propeller no longer has adequate clearance. With the engine turning the prop strikes the runway repeatedly getting curled back along its length. The engine is shock loaded to the point where an inspection or over hall is required. An accident like this can easily result in the insurance company writing off the airplane.

No pilot likes to see the result of an accident sitting alone and forlorn where is was dragged off the runway. To me the bent, twisted and crumpled aluminum that used to be an airplane is ugly.

Safety Audit or Safety Management?

The Edmonton Journal is reporting that "Transport Canada fears ending regular airline safety audits could be risky" in an article based on a March 2006 departmental risk assessment obtained through an Access to Information Act request. The Transport Minister Lawrence Cannon counters that the Safety Management System (SMS) actually increases oversight.

The only exposure I've had to the SMS was presented at Transport Canada safety seminar. Necessarily it could not be an in depth or exhaustive treatment of the subject, but none of it struck me as suitable to enforcing safety without cooperation from aviation companies. The safety audit system likely has problems as well, but I guess like the authors of the risk assessment I'm yet to be sold on SMS.

TSB Report: Difficulty to Control Cessna 150G

TSB Reports - Air 2000 - A00O0210

I came across this report a few months ago. It has all the hall marks of any aviation accident except in this case the PIC, a flight instructor, is able to regain control and bring about a safe recovery. There is an important lesson here for light piston owners and renters. This post is based on the Transportation Safety Board report, Environment Canada weather archives, and my opinions. Let's start with the summary.

Summary

The student pilot and the flight instructor took off from the Kingston, Ontario, airport to practice stalls in the Cessna 150 aircraft. The instructor first demonstrated the stall and recovery, then had the student attempt the same procedure. On his first stall recovery attempt, the student was slow to apply back pressure on the control column to bring the nose of the aircraft up. The instructor took control with the aircraft in a nose-low attitude. When the instructor applied back pressure, he found that the elevator control was restricted from full movement. Although he exerted considerable force on the control column, he could not get the elevator control back beyond neutral.

The aircraft reached a speed of approximately 190 miles per hour before the instructor was able to slowly pull out of the dive. The instructor was able to maintain altitude and fly back to Kingston Airport for an emergency landing by using a combination of back pressure on the elevators, full nose-up trim, and an engine power setting of 2500 revolutions per minute. During final approach to the runway, as the instructor applied flap to slow the aircraft, the elevator controls became free, and he was able to carry out a normal landing. The aircraft sustained substantial damage to the wings, flaps, and ailerons as a result of the overspeed situation.

The class 4 instructor had accumulated some 300 hours (60 hours instructional), the student 7.1 hours in nine calendar days. I can imagine them completing the ground briefing covering the lesson to be conducted, inspecting the airplane, starting and departing for the practice area. The report does not specify the weather but presumably any ceiling would have been high enough to allow the required altitude for recovery. Environment Canada archive data indicates that many of the preceding days were dominated by cloudy skies, fog and rain. The 13th is reported as mainly clear all day, certainly a good day for upper air work.

The instructor demonstrated the stall and recovery then had the student enter a stall and recovery. As the maneuver progressed the the recovery phase "the student pushed the control column forward aggressively, and the aircraft entered a dive". The instructor took over "when he judged that the student was not initiating an effective recovery." All is progressing as it should, unfortunately a very natural reaction by the student had reveled a hither to undetected flaw in the aircraft. As the instructor acted to recover from the stall he found "considerable resistance and was unable to pull the control column past the neutral position". The instructor was able to arrest the dive and maintain altitude with constant back pressure and high engine power. He set course for Kingston and declared an emergency. While maneuvering for landing he selected flaps in an attempt to reduce airspeed. In correcting for the pitch trim change caused by the flaps the instructor noticed he now had full elevator authority. A safe landing was made without further difficulty.

The aircraft was examined but no control anomalies were found. During the examination the cabin air knob was found pulled fully out. The aircraft had be retrofitted with an intercom using push to talk switches of the type that are held on the yoke with Velcro and connected to the intercom system by a self coiling cords. The cord on the right side was old and had lost much of its elasticity. The cord, now some four feet long was wrapped several times around the control yoke shaft in an attempt to keep it out of the way. It seems, however, that one of the wraps was loose enough to swing forward during the stall recovery, entangling with the cabin air knob. Once so engaged aft pressure would tighten the loop effectively jamming elevator control. In hind sight, if the instructor had released back pressure, as he did after deploying the flaps, he might have freed the controls sooner. In truth though, without identifying the fault first, he could not know if forward movements of the yoke might result in the elevator jamming in a more nose down position, resulting in a far worse situation than he was in.

In the end the only negative outcome was over speed damage to the airplane. Much less costly that it could have been. In return we get a very valuable lesson. Boxing the controls during preflight is not really enough. Are there any snares laying in wait in your airplane, or those you rent? I know I gave my airplane a close examination after reading this report.

Safe Flight.

Radar Loss of Separation Alerts, ELTs and Smoke Alarms

How are these things related? Well the simple answer is that they could save your life. An other answer is that they are all complex technologies which aim to solve pernicious problems by alerting humans to danger. They also share a property with all complex technologies, bugs. Bugs in an alerting system lead to two types of false alarms:

• False negatives, where a real problem goes undetected, or unreported by the technology.
• False positives, where an alert is issued when there is no problem.

False negatives lead to law suits and so get prompt (in the broadest sense of the word) attention from regulators and manufacturers. False positives are often seen as an inconvenience that can be most effectively dealt with by the human when alerted. Unfortunately humans usually have a direct response when face with a high rate of false positive alerts.

The tech that cried wolf

When smoke detectors first became available they had a very high false alarm rate. Often residents would solve the problem by disabling the smoke detector with predictable and unfortunate loss of life. However legally mandated installation in residential buildings created a huge market. Competing suppliers drove improvements to the technology that has brought the false positive rate down to a very low level.

Aircraft ELTs (Emergency Locator Transmitter, which Search and Rescue can use to find missing aircraft) come in three flavours by TSO designator (the Technical Service Order or TSO is one of the ways equipment can get approval for use in an aircraft, and is required for ELTs); C-91, C-91A and C-126. Due to a whole bunch of history that I won't go into, the C-91 and C-91A units were (in my personal opinion) never properly engineered to do the job. This has lead to a staggering number of false positives, and an appalling though not widely reported number of false negatives. While the aviation community was wrestling with the limitations of C-91/C-91A units, the maritime community was adopting the Category I and II EPIRB which would later be imported into aviation as the C-126 or 406MHz ELT. Through some creative and dogged pursuit of their mandate the aviation SAR community has never given in to the temptation to ignore this particular boy crying wolf. But the system has. Citing the better performance of the 406MHz technology, and the high cost of maintaining the 121.5MHz COSPAS/SARSAT packages processing of the 121.5MHz signal will cease 1 February 2009. If you read enough material on the subject you will also encounter references to the high false alert problem.

So why not just buy a C-126 ELT? This is a very complex issue. If you are involved in general aviation this is something you should educate yourself on and decide for yourself what to do.

Lastly, in the aftermath of a mid-air collision over La Mesa California Feb 8, 2006 it appears that controllers have displayed the same human frailty as early smoke detector owners. I am glad to see that at least some attention was given to fixing the false alerts. If the response is only to require controllers to forward alerts to pilots as the technology issues them, it would only move the problem from controllers (who are all professionals, current and experienced in assessing the traffic threat, of whom we can expect more) to pilots (who may only fly recreationally, be inexperienced in dealing with high traffic volume and the technology quirks, and finally just as able to fall into the trap of ignoring the boy who cried wolf).

If we want safe homes and safe airspace we need to deal as proactively and diligently with false positive alerts as we would with accident investigation reports. Replace the cranky smoke detector and give it fresh batteries, check ELTs after landing or servicing the airplane, be vocal when safety technology promotes more annoyance than safety.