por EIJL el Mar Ago 08, 2017 1:55 pm
4. You mean that a twin-engine aircraft could have a problem accomplishing this mission?
Not exactly. The concern to those who have flown challenging SAR missions throughout all of Canada’s vast and demanding SAR AOR will be the apparent absence of a suitable backup electric/hydraulic generator capability in the C295W; this is a critical capability should an engine fail during the mission. SAR missions in the Halifax SAR AOR are currently conducted by a four-engine aircraft, the CC-130H. Should the CC-130H lose an engine during a mission, the crew would likely stop the search and proceed directly to the closest suitable airfield to land and fix the problem. The loss of an engine is a much less significant event for a four-engine aircraft than one which has only two to begin with, for it has the required backup generator capability with the remaining three engines. This is not the same for a two-engine aircraft. This explanation by an aircraft engineer (see italicized text below) on the problems associated with an engine failure on a two-engine aircraft makes it clear why this issue is critical.
Grounds for APU Concern:
For electrical system reliability, specific rules are included in the applicable airworthiness standard. In particular, it is mandatory to demonstrate that at least three or more reliable and independent electrical power sources are available and that the failure of all three must be assessed as extremely improbable. If this requirement cannot be met by the existing design, a further fourth independent power source must be available to power the essential functions for continued safe flight and landing at an adequate alternate airport. In accordance with these standards, an electrical supply from aircraft batteries will not be pertinent to an Extended Twin-engine Operations (ETOPS) qualification because of their limited life (30 to 60 minutes).
When one engine fails, the aircraft propulsion system must be “protected” in order to reduce the likelihood of a second engine failure. On the failure of one engine, in the absence of a source of power that is independent of the propulsion engines, all of the aircraft power demands (cabin pressurization, engine and propeller anti-icing systems, aircraft hydraulics and all aircraft electrical loads) will fall on the remaining engine, the stress of which, even when the loads are reduced as far as possible, increases the probability of that engine experiencing a mechanical failure. Added to this is the stress of the crew when operating for an extended period with only one engine operative and the cabin environment degraded to reduce demands on the system. The combination of the remaining engine assuming all of the propulsion demands of keeping the aircraft in flight, while serving the entirety of the other system loads, will result in the remaining engine operating close to its mechanical and temperature limits for the required diversion time (approximately 330 minutes for C295W). These stresses will lead to a significant reduction in the reliability of the remaining engine and thus increase the probability of that engine’s failure and a consequent catastrophic event.
Equipment redundancy (back-up) concept and architecture is therefore a key design factor in the mitigation of extended range operation safety and airworthiness risks. If the non-propulsion loads listed above, such as de-icing and cabin pressurization, can be borne by another power source such as an auxiliary power unit (APU) the stress on the remaining engine and the crew is reduced. An APU is an engine that is mounted permanently on the aircraft that is independent of the aircraft propulsion engines. It is capable of generating electrical power and heated pressurized air. It can power equipment and instrumentation on the aircraft independently of, or in addition to, the electrical power and pressurized air that is produced by the propulsion engines, and thus can assist or replace those functions of the propulsion engines when desirable or necessary for continued flight. Normally, all transport aircraft designed with military specification and all the civil aircraft designed to operate medium-long range are equipped with an APU. The Airbus C295 is not; Team Spartan’s C-27J, yes.
Virtually all two-engine aircraft flying extended missions (those which take them far from suitable landing airfields) mitigate the en route loss of an engine by using an onboard APU. The APU can be started in flight and provide that much needed backup generator capability for the flight back to a suitable airport. The civil aviation world has developed a means of assessing the suitability of a two-engine aircraft to conduct these extended missions. This system is the above-mentioned ETOPS. The required ETOPS rating is based upon the time it would take for the aircraft on a single engine to reach the closest suitable airfield and is measured in minutes. The rating assigned is based upon the capability of the aircraft and the reliability of its systems. It is useful to note that the highest ETOPS rating achieved by an aircraft not equipped with an APU is ETOPS 120; this for the ATR series of aircraft. This is notable because the ATR series–including regional passenger and cargo aircraft in the range of 50 to 75 seats–is a close cousin of the C295W and utilizes the same system architecture. For an aircraft with the one-engine speed of the C295W, it would require a rating of approximately ETOPS 330 to be capable of safely fulfilling the required FWSAR mission. Without an APU or similar device, and with a safety record which features several incidents involving loss of an engine, the C295W would be rated only marginally better than the ATR, not anywhere close to what it needs to successfully conduct that FWSAR mission out into large portions of the Halifax SAR AOR. Those concerned will know that for extended twin-engine operations in Canada, Transport Canada mandates the use of an APU for ratings above ETOPS 120.
In other words, if you don’t have an APU, according to our civil air regulator, you can’t fly further from a suitable airfield than 120 minutes at single-engine speeds! By way of comparison, the C-27J aircraft requires an ETOPS rating of 240 for the worst-case scenario and with its APU and enviable safety record, it easily achieves that score. Certain categories of aircraft are exempt from ETOPS regulations. While military aircraft are exempt from the ETOPS regime, all professional militaries have their own way of assessing and dealing with the risk; the RCAF deals with this risk through its Airworthiness Program. The RCAF Airworthiness Program pays special attention to aircraft when they are being employed in new challenging missions for that aircraft type. The Canadian FWSAR mission is a particularly difficult mission for an aircraft such as the Airbus C295W, based as it is upon a commercial design for short range missions.
So, despite not being obliged to use the ETOPs rating system, lacking their own measure, the RCAF will often use a civil system such as ETOPs to assess the risk. All of Canada’s FWSAR aircraft over the past 50 years have been equipped with an APU so one would think that an aircraft being proposed which does not have an APU would be examined very closely indeed. Recognizing the importance of the APU, and using the tools in the RCAF Airworthiness Program, today’s CC-115 crews are currently reluctant/unwilling to fly any significant missions at all without a serviceable APU, let alone a mission which might take them far away from a suitable airport. Pretty much the only mission that they will fly without an APU is one which takes them directly back to the FWSAR main base of Comox, B.C.
5. So, do you think the APU must be a critical asset for any FWSAR platform?
Imagine, if you will, a case initiated by the Halifax Joint Rescue Coordination Centre on receiving information that a small freighter crossing the Atlantic from the Mediterranean Sea to Halifax is in considerable trouble. The ship has broadcast a Mayday saying they have had an engine fire and are taking on water. Two of the crew have been badly injured and need medical attention. Their last known position is just west of 30 W and well within the Halifax Area of Responsibility. It is winter and the forecast weather in the area is for low clouds and snow/ice pellets. 413 Squadron with its C295W aircraft is tasked to conduct the mission: to find the vessel, and to drop pumps/equipment and SAR techs to render a first responder capability. The mission’s first leg, the flight to St. John’s to take on fuel, is uneventful but one of the engines is not performing perfectly and is being monitored closely by the crew. After refuelling, the crew launches from St. John’s and proceeds towards the last known position, enjoying a brisk tail wind en route which helps reduce the time to respond, always an issue on longer legs due to the C295W’s relatively slow speed. As they approach the last known position, the crew contacts the vessel and prepares to initiate the deployment of kit and SAR techs. Without much notice the left engine, the one they had been monitoring, requires immediate shutdown. After quickly evaluating the situation, the crew contacts the vessel, passing on regrets that they have their own emergency to contend with and will be unable to render assistance. The crew calculates that it will take almost seven hours to reach the nearest suitable landing airfield and proceeds back towards St. John’s. The C295W single-engine altitude has them flying in cloud and the temperature is just below zero, so they begin to experience light icing conditions. Turning on the anti-icing adds to the load on the remaining engine. Knowing the greatly increased risk of overloading the single remaining engine, the crew takes steps to reduce the load as much as possible, including reducing the pressurization and temperature. It will be a long flight back on oxygen, bundled up to keep from becoming cold. Despite these measures the remaining engine is now operating at full capacity, putting it under extra strain. The crewmembers, with previous experience on the CC-130H and Buffalo, are now wishing that this aircraft had been equipped with an APU as they know well how effective it can be in reducing the load on the remaining engine. They also wish that the C295W single-engine speed was much greater as they fight into the wind that had been so friendly to them on the outbound leg. It will be a long flight back, and although they have sufficient fuel it will be a flight filled with considerable risk to the crew. Risk which grows with each passing hour as the remaining engine strains to keep the aircraft aloft, and the crew becomes more and more affected by the difficult conditions. Given Canada’s large SAR AOR, the difficult weather conditions during a good part of the year, the C295W’s less than perfect safety record, and its critical lack of a backup generator capability, this scenario has a high probability of occurring throughout the life of the aircraft. Similar situations have been recently experienced by the Mexican Navy and by the Portuguese Air Force with their Maritime Patrol Airbus C295s. Both cases ended with emergency landings and, in one case, damage to the aircraft.
All indications point to an unnecessarily elevated risk level for the C295W and crew to conduct FWSAR missions across much of the Halifax SAR AOR and across much of the North; therefore, from an airworthiness perspective the C295W should have been considered non-compliant for this mandatory requirement. For this not to be the case, Airbus would have had to perform substantial and risky aircraft design changes; at this time, there is little evidence of this. It is for this important reason that Leonardo has properly challenged the decision to award the contract to Airbus. Based upon this analysis, it is more than curious that Canada would award the contract to a marginally capable aircraft, with a serious operational deficiency and documented safety concerns, and which costs substantially over the stated budget, when there was a fully compliant bid which fit within the budget.
. 
En todo caso, el avión cumplió técnicamente también.