Air Safety Link—EMB-120 Icing AD Doesn't Go Far Enough
Air Line Pilot, September 2001, p. 6
By Jan W. Steenblik, Technical Editor
The FAA on June 27 issued a new airworthiness directive (AD), applicable to all EMB-120 Brasilia turboprops, that prohibits using the autopilot when the airplane is in icing conditions, and requires removing the "light-heavy" inflation switch for the leading edge deicing boots so that the boots may be used only in the "heavy" mode. The AD further requires installing, on the main instrument panel, placards to warn pilots that they must turn the autopilot off and the deicing boots on whenever ice is detected on the airplane by visual cues or the ice detector illuminates.
The Departmento de Aviaco Civil (DAC), the airworthiness authority for Brazil, earlier this year issued two ADs mandating compliance with two service bulletins that are similar to, but differ somewhat from, the FAA’s AD.
The DAC said it had received reports of loss-of-control events involving EMB-120s in icing conditions. According to the FAA, "the DAC advises that such events indicate that the flight crews may not have correctly determined both the severity of the ice accretion and the need to take immediate action to prevent excessive loss of airspeed, especially when using the autopilot."
The FAA, although it issued the AD as a final rule, asked for comments.
ALPA responded, recommending changing language in the AD so that the FAA-approved airplane flight manual (AFM) revision and the cockpit placards would be consistent and include a broader set of conditions.; i.e., that pilots be directed to disengage the autopilot "whenever operating in atmospheric icing conditions, at the first signs of icing accretion anywhere on the airplane, or the illumination of the ice condition light, whichever occurs first."
The Association said it has numerous concerns "beyond the scope of this AD regarding the EMB-120 and its operation in icing conditions. These issues include (1) the adequacy of the aircraft’s basic design, particularly the adequacy of the deicing boots and the stall warning system, and (2) the operational issues of autopilot functionality throughout the normal flight envelope and maintaining crew situational awareness of the system status for aircraft trim settings."
ALPA said the wing icing boot coverage on the EMB-120 may not be able to remove enough ice from the wing to maintain adequate performance (lift and drag) and handling. The Association warned, "the handling aspect may be the most critical, since residual ice and ice remaining after boot activation can…result in loss of lateral control.
"The FAA should ensure that the [EMB-120’s] performance and handling are adequate for the full envelope of icing conditions specified in FAR Part 25 Appendix C. The scenario [of] extreme trim and an autopilot disconnect at its limit of elevator servo strain should be particularly evaluated for its adequacy in handling and performance."
ALPA warned that the design of the EMB-120’s pitch trim system "needs to be reviewed…to ensure that trim changes are communicated effectively to the crew." The airplane has no trim-in-motion aural warning to alert the pilots of trim inputs by the autopilot. ALPA noted, "the elevator trim tab can lose effectiveness in icing conditions [and] the trim tab may be run all the way to the stop by the autopilot. The autopilot elevator servo may be put under load up to its maximum force, at which point the autopilot logic disconnects the autopilot."
Current regulations require the manufacturer to show that the airplane can be flown with full trim applied, but numerous accidents associated with trim being at the limit without the pilots knowing it have shown that a trim-in-motion alarm would be helpful.
ALPA also said the angle of attack (AOA) that triggers the stall protection system (SPS) in the EMB-120’s flight control computer should be decreased in icing conditions. The EMB-120 SPS uses AOA to actuate a stickpusher and stickshaker. Embraer put a stickpusher in the EMB-120 because the airplane’s stall characteristics were not benign enough.
In icing conditions, ALPA said, the AOA at which the wing stalls "will likely decrease significantly. Therefore, a stall can occur before the SPS system activates when the airplane is accreting ice." Thus, ALPA reasoned, the flight control computer should be programmed to reduce the threshold AOA to activate the stickshaker and stickpusher when the ice protection systems are activated.
The Association, in raising operational issues, argued first that prohibiting use of the autopilot in icing conditions, while necessary, should be only an interim condition until Embraer redesigns the EMB-120 autopilot. ALPA suggested adding a pre-disconnect warning function to the autopilot.
ALPA also questioned how the current minimum speed to fly in icing conditions was derived. At least one airline, the Association noted, has determined that 160 knots, per the AFM, should be raised to 170 knots. ALPA urged the FAA to investigate this and establish adequate speed margins in icing, declaring, "The speed must be based on quantitative flight tests and analytical data…."
The Association reiterated past ALPA and NTSB recommendations to provide pilots with a cockpit alert that the airplane has entered atmospheric conditions more extreme than those for which the airplane has been certificated.
ALPA also called for improved flight training on icing issues, saying that the EMB-120 electronic attitude display indicator can be unusable at extreme bank and pitch angles and that pilots should be advised about this and given specific action to take, such as using the standby attitude indicator.