Steve Ayres

Accident Analysis

With Steve Ayres

SAFETY

Pitot-static problems…

We tend to forget how much of what we do in the cockpit is derived from an accurate airspeed. So if that is lacking for whatever reason, then as Steve Ayres discovers, life can get very confusing

Many of us probably think identification of pitot-static problems begin and end during the take-off roll, but they can occur at any time, are usually insidious, difficult to analyse and have unpredictable outcomes. From the ‘human-induced’ failure to remove the pitot cover to leaks and blockages either from foreign matter or ice, can all manifest themselves in a variety of ways – and lead to serious confusion in the cockpit. When time is short for analysis, such as during take-off, premeditated action is essential, but given more time some diagnosis is possible although, as these incidents prove, don’t expect to find it easy!

Accident 1

A Mooney M20J impacted a car after aborting a take-off from Freeway Airport (W00), Bowie, Maryland. The private pilot and pilot-rated passenger sustained minor injuries.

The aeroplane sustained substantial damage. According to the pilot, he performed a pre-flight inspection of the aeroplane with no anomalies noted. Then, he taxied to the active runway and initiated the take-off roll. He noted that the airspeed rose to 40mph, but when he looked again a few moments later, the airspeed was still 40mph.

The aeroplane was about one-third to halfway down the 2,420ft runway when the pilot elected to abort the take-off, and subsequently overran the departure end of the runway.

The aeroplane broke through the airport perimeter fence and struck a car on a road just off the airport property. The aeroplane’s left wing sustained leading-edge impact damage and had separated from the airframe at the wing root. The right wing and fuselage were also substantially damaged.

A Federal Aviation Administration (FAA) inspector performed an examination of the aeroplane’s pitot-static system. The tubing that supplied air to the airspeed indicator was secure to the back of the instrument. Uncalibrated pressurised air was applied to the fractured tubing at the left-wing root, and the airspeed indicator moved and indicated up to 95kt. The pitot tube was undamaged and remained attached to the left wing. It was removed and no debris was noted in the ram-air intake or drainage holes.

An attempt was made to run pressurised air from the pitot tube to the left-wing root, however, due to impact damage on the wing, air was unable to pass through the tubing.

Due to the wing’s impact damage, investigators were unable to functionally test the portion of the pitot system between the pitot tube and the left-wing root. It is possible that the pitot static system was blocked during the accident flight, preventing the airspeed indicator from displaying airspeeds above 40mph and leading the pilot to decide to abort the take-off, then the blockage became dislodged during the accident sequence.

Accident 2

The pilot of a Cessna 421B departed on a short cross-country flight in the twin-engine aeroplane. Instrument meteorological conditions (IMC) were present at the time. While en route at an altitude of 3,000ft msl, the pilot reported that the aeroplane was ‘picking up icing’ and that he needed to ‘pick up speed’. The controller then cleared the pilot to descend, then to climb, in order to exit the icing conditions. Shortly thereafter, the controller issued a low altitude alert.

“The aircraft’s indicated airspeed began to decay while its ground speed, remained steady”

The pilot indicated that he was climbing, then radar and radio contact with the aeroplane was lost shortly thereafter. The aeroplane impacted a field about seven miles short of the destination airport.

Examination of the aeroplane was limited due to the fragmentation of the wreckage, however, no pre-impact anomalies were noted during the airframe and engine examinations. Extensive damage to the pitot-static and deicing systems precluded functional testing of the two systems.

A review of data recorded from onboard avionics units indicated that, about the time the pilot reported to the controller that the aeroplane was accumulating ice, the aeroplane’s indicated airspeed had begun to diverge from its ground speed as calculated by position data. About 17 minutes after take-off, the aeroplane’s indicated airspeed began to decay, while its ground speed, as calculated from position information, remained steady. About one minute later, fuel flow increased from 38 gallons per hour (gph) to 60 gph, consistent with an increase in engine power. At this time, the indicated airspeed had decayed to about 100kt, while the ground speed remained about 150kt.

For the final approx 2.5 minutes of the flight, the aeroplane was in a left turn. The aeroplane entered a climb to about 3,500ft msl, then began descending around 5,000ft per minute. The data recorded ‘SINK RATE’ and ‘PULL UP PULL UP’ annunciations, and the aeroplane’s rate of descent was arrested about 300ft agl. The aeroplane subsequently entered two additional sets of climbs and descents, receiving the same annunciations. However, the pilot did not recover from the third descent.

During this time, the aeroplane’s ground speed, and likely airspeed, exceeded the aeroplane’s manoeuvring speed (Va) of 150kt, and during both the first and final ‘SINK RATE’ annunciations, the aeroplane’s ground speed exceeded its maximum structural cruising speed (Vno) of 200kt. It is likely that the pilot became distracted by the erroneous airspeed indication due to icing of the pitot probe and subsequently lost control while manoeuvring.

Accident 3

A Royal Flying Doctor Service Pilatus PC-12/47E departed Jandakot Airport for Albany Airport WA.

About four minutes after commencing descent into Albany from flight level 210 and while in icing conditions, the pilot received an airspeed mis-compare indicated by an amber colouration on the airspeed tape on the primary flight displays (PFD). At this time the pilot reported that there was a light dusting of ice on the leading edge of the aircraft’s wings and on the radome. The pilot did not deem this level of icing to be a concern and did not observe any issues or receive any alerts from the aircraft’s anti-icing systems.

While continuing the descent the pilot compared the airspeeds displayed on the two PFDs with the airspeed indication on the electronic secondary instrument system (ESIS).

Based on the speed readings from the PFDs and the ESIS the pilot determined that the left PFD was likely displaying incorrect information. As the descent continued, the pilot observed the airspeed on the left PFD continuing to decrease. Having assessed that a blocked pitot tube was the likely cause of the issue, the pilot elected to climb the aircraft in an attempt to get clear of cloud. During this climb the pilot’s indicated airspeed increased and exceeded the aircraft’s maximum allowable speed accompanied by an overspeed alert. Consequently, he elected to discontinue the planned flight and return the aircraft to Jandakot. Unable to obtain visual conditions, the pilot again descended the aircraft at which point the left indicated airspeed reduced to zero – no stall warning was activated.

At 6,000ft visual conditions were obtained, however, the turbulence at this level was severe and the aircraft was climbed to 10,000ft. The pilot observed a heading mismatch on both the left and right PFDs, which continued to increase until there was reported 50-60° of indicated heading difference between them. Further, the pilot also reported that during this sequence the left PFD displayed an incorrect attitude, indicating that the aircraft was level when the nose was approximately 3° below the horizon.

Approaching Jandakot, the pilot reported all indications had returned to normal and remained that way until short final when an altitude mismatch and low airspeed warning was identified on the PFDs.

The ATSB determined that during the flight, water entered the aircraft’s pitot tube either as rain or an accumulation of moisture from flying through cloud. Due to a blockage in the pitot tube drain the water had been unable to escape. This in turn obstructed the flow of air to the aircraft’s air data attitude heading reference system, resulting in an incorrect airspeed being displayed on the left PFD and triggering mis-compare indications on both PFDs. Also, a heading mismatch was likely caused by the aircraft’s movement through an area of moderate to severe turbulence during the return to Jandakot.

Spurious instrument readings can create a more complex scenario for flight crew than an instrument failure. In this case the pilot’s recent training assisted in effectively assessing the situation, determining the likely failure mode and identifying the most accurate source of available data for a safe return to the departure airport.

Ayres’ Analysis

Having experienced a ‘pitot-static problem’ first hand, I learned how seriously confusing and potentially hazardous such events can be. I describe it as a ‘problem’ because, to this day, I am uncertain of the exact cause. It was a relatively cold VMC day with isolated towering cumulus. Everything seemed normal until levelling around 7,000ft when the ‘straight and level’ attitude required increasing amounts of power to maintain speed and the aircraft wanted to climb. Turns felt really ‘pitchy’ suggesting an elevated TAS. With no GPS derived ground speed it was only through setting specific engine powers that I had a sense of spurious instrument indications, which appeared to be under-reading airspeed. However, it took time and some serious head-scratching to work it all out and even then I wasn’t convinced I had it right. Being qualified in close formation helped, and it was only when comparing my aircraft’s performance with that of another, did I have confidence in which instruments could be trusted.

These incidents further illustrate clearly the problems we can all face. There are, of course, those who have failed to remove the pitot cover and those who have yet to do so. In such circumstances identifying the no-go point on the take-off roll is key to making that snap decision to continue or to abort. And what will you do if you do get airborne? A ‘pairs’ approach in close formation is perahps not an option for most of us but using GPS ground speed. However, experience shows focusing on a GPS speed rather than the big ’ole ASI can be challenging and certainly adds to the ‘pucker factor’.

You will want to be fast rather than slow on the approach, so an extended rollout on landing is almost inevitable. Diversion to a longer runway with a measured head wind component may also help to reduce some of the risk and make the cockpit workload less stressful. For me, though, the crew of the PC-12 made a really good point in that they felt it was an ‘Operation Proficiency Check’ (OPC) which helped them diagnose their problem. They state: “In the OPC, carried out about a month before the incident, the pilot, under the guidance of a check and training pilot, observed the aircraft’s performance at various engine power settings and aircraft attitude combinations”. That was mostly my experience, but expect the analysis to take time and to become discombobulated in the process! Having got the diagnosis sorted, you’ve just the landing to worry about!

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