最近几年，普惠（P&W）公司在新型商用喷气式发动机的交付方面一向表现不佳。不过，凭借一直坚持的持续进步的研发战略，多款普惠新一代超高效发动机终于即将开始服役。虽然这些发动机采用了一些可以追溯回上世纪90年代的设计，但其中也应用了大量新型技术，未来还可推广至更多的喷气式发动机。

虽然，性能提升可能会给厂商带来更多利润，但这也是有高昂代价的。首先，航空发动机的主要设计和制造问题，大多到研发后期才会真正暴露，而这时哪怕一丁点改动都会产生巨大成本。其次，由于国际航空机组普遍进入老龄期，全球大部分航空业客户的购机需求急剧增加。这些航空公司越来越焦急，也越来越不耐烦，而这些压力也将进而转嫁到飞机制造商和航空发动机厂商身上。

如今，一些畅销机型的订单已经排到了2020年以后。无论是订购的新发动机迟迟不能交货，还是收到货后的使用环节出现问题，都会让一些航空公司付出血本，而寻求可靠的解决方案也并不像想象的那么简单。

为了应对更大的需求，一些飞机制造商开始增加飞机的产量。然而，由于发动机厂商未能按时交货，大量已经完成的机身只能堆在停机坪上苦苦等待。此外，如果新的发动机在接受初始认证或交付给航空公司后又需要进行修改，此时飞机制造商则需要承担二次认证和延迟交货的成本，已经完工的飞机越多，需要承担的成本就越高。因此，保证发动机从一开始就能实现设计性能，并保证高可靠性一直是发动机供应商的重中之重。

如今，普惠的最新涡轮扇（GTF）发动机（属于PW1000 Pure Power静洁发动机系列）的到来，又引出了一些不容忽视的问题。正如《航空航天》杂志早些时候所报道的，GTF发动机代表了一种完全不同的设计理念。这种发动机在各个冲程均可提供最优的推进效率，可谓是史上最清洁环保的超静音发动机。

GTF发动机在同一根轴上的涡轮扇和低压涡轮机（LPT）之间增加了一个变速器，从而保证涡扇和涡轮机均能以各自的最佳速率运行，即涡扇转速更慢一点，而LPT更快一点。这款新款普惠发动机能实习够大幅提升能效、并带来显著的节油成果以及超低排放表现，一定会在飞机厂商中大受欢迎。事实正是如此，目前已有至少5款新机型决定采用这款发动机，分别为空客（Airbus）A320neo、庞巴迪（Bombardier）CSeries、巴西航空工业公司（Embraer）E190-E2、俄罗斯联合航空制造公司（UAC）MC-21及三菱(Mitsubishi)MRJ。

过去一段时间，普惠公司一直高度关注GTF发动机的研发，未来还有可能继续扩大这款发动机的应用范围。总体来说，当选庞巴迪CSeries支线机型（130到140座）选定发动机，是GTF的一个重要突破，代表这款发动机开始真正有机会挑战“战无不胜”的CFM-56和LEAP发动机。CSeries商用喷气机是庞巴迪公司商用喷气机系列中的顶级产品，该机型凭借自己的出色表现，逐步跻身于长期由A320和737客机占据的全球客机市场。

凭借更加轻质的高规格航空机身和超高效的低油耗发动机，CSeries喷气机本应在竞争激烈的商用航空领域占据一定优势。然而，先是机身研发进度滞后，接着又是GTF发动机在早期测试与评估环节出现故障，CSeries只能眼睁睁的看着自己优势尽失。具体来说，GTF的故障包括一起地面测试中发生的因输油管密封泄漏导致PW1500G发动机起火的问题。最终的调查结果显示，CSeries需要在再次启动前充分“冷却”，否则可能导致问题的出现。据悉，本次事故发生在CS-100认证测试之前的紧急测试阶段，已经处于非常后期，庞巴迪公司头疼不已，但也只能眼睁睁的看着CS-100的研发成本不断飞涨。

普惠在发动机故障的修复方面一直不遗余力。虽然现阶段各家航空公司须在再次启动飞机前充分冷却PW1100G发动机，但无论如何，CSeries历尽千辛万苦，终于开始服役了。然而，修复后PW1100G又出现了其他故障，严重影响了A320neo客机的交付计划。

这次PW1100G发动机在A320neo上发生的故障与CSeries并不完全一样，主要原因在于温度波动，但航空公司并不关心具体细节，因为结果都是一样的 -- 现在飞机不能按时飞了（与当下A320搭载的CFM-56或V2500发动机相比，PW1100G发动机启动所需时间更长。由于首批PW1100G已经于今年早些时候交付，普惠必须尽快拿出一个紧急修复解决方案）。这个技术隐患是在认证测试之后暴露的。如果发动机未能在熄火后充分冷却，再次启动时就会发生这个问题。具体来说，发动机涡轮轴的温度变化可能会导致特定组件失调，进而造成发动机振动。此时，由于发动机转子叶尖间隙变化和元件之间的相互摩擦，发动机可能因为磨损而产生进一步损坏。普惠公司一方面建议客户在再起启动发动机前一定要充分冷却，另一方面也在加紧准备一个更加完善的技术解决方案。

MTU航空发动机（MTU AeroEngines）也参与了GTF发动机的研发，这家德国公司负责的部分主要集中在高压压缩机。MTU表示，公司计划在今年晚些时候提供一个解决上述问题的软件升级措施。

显而易见，航空公司肯定希望在引进新机型的初期尽量不要出现问题，因为可靠性问题造成的拖延，会严重影响航空公司的形象，波音公司就曾因为787的延迟推出而名誉受损。如今，尽管购买首批新机型仍有一定好处，但需要面临的风险也正在增加。此时，飞机制造商一天不能修复问题，航空公司就一天无法放松心情。

A320neo版的GTF发动机情况也是一样的。当时原定的首批客户卡塔尔航空本来可以成为世界首家将这款机型引入客运领域的运营商，缺最终拒绝了普惠交付的产品。后来汉莎航空（Lufthanse）接手了这批飞机。尽管航空公司可能不得不接受现实在再次启动发动机时等待更长时间，但普惠已经研发出一种解决方案 -- 为发动机的转子叶尖增加一层涂层。现在，所有尚未交付的新发动机均采用了这种方案，而已交付发动机的修复可能最多需要2年时间。这项任务并不简单，工程人员必须拆开每一部需要进行修复的发动机进行操作，然后重新组装，大约每部发动机需要6周时间。不过，好在目前仅有不到60部已经交付的发动机需要修复。

可以想象，GTF发动机的问题曾引得各家航空航天和商业媒体的争相报道。不过，既然现在原因已经找到，普惠位于康乃狄克州和佛罗里达州的发动机装配线也已经进行了相关修复，那我们也应当着眼更高层面，认可这款发动机在销售方面取得的巨大成功。目前，这款发动机的年产量即将提升至800部，而普惠所面临的供货缺口超过7000部（包括所有发动机机型）。

普惠总裁Robert Leduc表示，PW1100G的初始服役可靠性非常出色，这款发动机在汉莎航空（Lufthansa）、靛蓝航空（IndiGo）和GoAir的出勤可靠率均稳定在99.7%以上。PW1100G还有很多其他优势：首先，作为这款发动机的最大卖点，GTF的“超低油耗”已经帮A320取得了超过15%的节油成果；其次，这款发动机的噪音水平很低，预计可以为本已相当安静的A320进一步降噪50%；此外，这款发动机的CO2/NOx排放也降低了60%。

未来，其他几款采用GTF系列发动机的商用机型也将在未来几年内陆续开始服役，这也会让GTF成为未来几十年中应用范围最广的航空发动机设计方案。尽管进一步的研发仍面临延期，但CSeries系列的销量仍然高居不下，目前还面临大约300驾的存货短缺，这对一款刚刚上市的机型来说是相当不错的表现。另外，巴西航空工业公司的190-E2系列看起来信心满满，似乎注定将重现公司现有机型的辉煌记录。但三菱MRJ和俄罗斯UAC MC-21的前景尚不明朗。如今的民用客机领域竞争激烈，其他成熟厂商在销量与设施支持方面均具有较大领先优势，这两款机型很难打入这个市场。不过，目前GTF发动机的整体市场规模非常庞大，这也证实了普惠的英明决定 -- 这家公司一直坚持新产品的创新研发，愿意承担风险，而且在出现初期问题时的反应十分迅速。

今年下半年，随着积压的PW1100G订单开始陆续交付，相信空客停机坪上苦苦等待发动机的A320neo也将很快陆续交付至世界各地。对于普惠和空客而言，发动机小故障和飞机延迟交付的确都非常让人遗憾，不过这也同时反映了目前商用航空领域面临的最大问题：如何面对史无前例地巨大市场需求，并同时保证超高的质量控制标准。

Growing pains (and gains) of P&W's Pure Power engine

After many years of flat-lining in the supply of new commercial jet engines—with improved models, but with designs dating back to the late 1990s, relying on a continuous path of evolutionary development—a new generation of super-efficient powerplants is entering service, introducing many technologies that will also be applied to future engines.

But the step up in performance, with its potential for greater profitability, has come at a price, with major design and manufacturing issues emerging late in the program cycle, and the added challenge facing airplane manufacturers of having to meet the expectations of anxious and increasingly impatient customers against a backdrop of aging fleets and a surge in demand for new airplanes.

The top-selling jetliners are effectively sold-out for years ahead, stretching deliveries to well beyond 2020. Delays in the delivery of new engines, and further groundings due to in-service problems, are costing some airlines dear and achieving robust solutions is taking longer than envisaged.

As production flows are increased in assembly halls, completed airframes are starting to fill parking ramps as the delivery of engines is awaited. If new engines have to be modified after initial certification and airline delivery, then the delays and extra costs involved in rectification grow with the numbers of airplanes that have already been completed. So, getting the engines to deliver their promised performance with high reliability from the start has always been a goal that engine suppliers aim to achieve.

The highest profile problems, perhaps understandably, have come with the new engine that offers the most innovative technological design—the Pratt & Whitneygeared turbofan (GTF), which has developed into the PW1000 Pure Power series. As has been described earlier in Aerospace Engineering, this engine does offer a different design solution, with optimized propulsion efficiency delivered through the different engine stages, resulting in a quiet engine that is cleaner and greener than what has come before.

Using a gearbox to decouple the fan from the low-pressure turbine—the two seated on a common shaft—allows each element to be optimized, with the fan running at a slower rate and the LPT much faster. The promise of new levels of efficiency, significant fuel savings, and ultra-low emissions ensured that the new P&W engine would be popular with airframe manufacturers, and this is illustrated by the fact that different GTF versions have been chosen for no less than five new generation civil programs—the Airbus A320neo, Bombardier CSeries, Embraer E190-E2, UACMC-21, and Mitsubishi MRJ.

P&W has been highly focused on developing this new engine, which has great potential for scaling up later, and being selected to power the CSeries 130-140 seat regional airliner was an important breakthrough, providing a long-term opportunity to challenge the all-conquering CFM-56 and its replacement LEAP engine in the biggest aero-engine market sector. The CSeries has been a crucial program for Bombardier as it sits at the top end of its commercial jet family and nudges into the global market that has become dominated by the A320 and 737 families.

The combination of a lighter, high-specification airframe and highly efficient engines with low fuel burn should have given this program a head start in a very competitive slot within the commercial sector, but delays in airframe development and then the teething troubles that afflicted the early testing and evaluation of production GTF engines—including an uncontained PW1500G engine fire on a CS-100 during ground tests, caused by the failure of an oil feed tube seal—led to investigations that suggested a longer “cooling off” period was needed before attempting a re-start. The initial uncertainly over this incident was not helpful to Bombardier as CS development costs were soaring and this engine failure happened as Bombardier was into a catch-up testing phase prior to certification.

P&W spared no effort in working to fix the problem created for airlines by having to wait longer between hot shutdowns and restarting. But while the CSeries has finally entered service, the delay in shipping modified production engines has seriously set back the schedule for the urgently awaited PW1100G powering the A320neo model.

In this version of the engine a different problem is connected to temperature fluctuation issues but for the airlines the outcome is the same—start-up times are delayed. (The PW1100G start-up time was longer than on the standard CFM-56- or V2500-powered A320s so clearly a temporary fix became urgent earlier this year as the first PW1100Gs were delivered for service.) This technical problem arose after certification and can occur when the engine is re-started before it has sufficiently cooled after shut down. The temperature variations along the engine’s turbine shaft can lead to misalignment of particular components and this can cause vibration and possible further damage due to rotor tip clearance difficulties and the possibility of components rubbing against each other, wearing away the seal. Customers were advised to allow more time before re-start ups, while the company perfected a technical solution.

German partner on the GTF program with P&W, MTU Aero Engines, is responsible for the high-speed, LPT and four of the high-pressure compressor stages, and is planning to provide a software upgrade later this year to reduce the risk of the earlier problems returning.

Operators like to introduce new aircraft with as few teething problems as possible for obvious reasons, as delays caused by reliability issues early on can lead to reputational damage, as was experienced by Boeing after the delayed introduction of the 787. But while there are incentives allowing for the increased risk of acting as the launch customer, the longer it takes to restore the promised performance, the more nervous key customer will be.

And so it has been with the GTF version of the A320 neo. Original launch customer Qatar Airways refused to accept its delivery, which would have given the airline the lead role of introducing the aircraft into regular passenger service. That task was subsequently taken up by Lufthansa. Although the airline has had to accept and allow for the extended re-start times, P&W has developed a modification based on the application of a new coating for the tips of the rotor. This is being applied to all new engines. Retro-fitting this to existing engines will take up to two years to work through, though thankfully less than 60 early-delivery engines are involved. This task is not simple as it involves the de- and re-assembly of each affected engine, taking around six weeks each time.

The GTF problems attracted the attention of the aerospace and business media as would be expected, but with the cause now established, and modifications being incorporated in all new engines coming off P&W assembly lines in Connecticut and Florida, it is important to see the issue within the wider context of a program that is enormously successful in sales terms, with an annual production output due to rise to 800 engines, and a total backlog of over 7000 GTF engines for all models.

Initial service reliability of the PW1100G has been exceptional, according to P&W’s President Robert Leduc, claiming dispatch reliability is over 99.7% on the first few aircraft in service with Lufthansa, IndiGo, and GoAir. On the subject of low fuel burn, perhaps the biggest selling-feature of the new GTF design has lived up to its game-changing claims, with an improvement over the existing A320 of over 15%. Other benefits include ultra-low noise levels that promise to halve the noise footprint of an already quiet A320 at many airports, and a 60% reduction in CO₂and NOx emissions.

The other commercial airplanes to be powered by the GTF family due to enter service over the next few years will ensure that the design becomes one of the most widely used in the decades to follow. The CSeries, despite extended development delays, has a sales backlog of little short of 300 airplanes, a respectable total at entry into service, and the Embraer 190-E2 family seems destined to repeat the success of the company’s current models. Prospects for the Mitsubishi MRJ and Russian UAC MC-21 are less clear, with both entering a highly competitive sector where rival established manufacturers have a clear lead in terms of overall sales and support infrastructure, but the combined global market for GTF engines is huge and confirms P&W’s wisdom of persisting with such an innovative new product, accepting the risks and then dealing swiftly with the resolution of early issues.

As the deliveries of PW1100Gs build throughout the rest of this year, the lines of new A320neos parked outside the Airbus assembly halls at Toulouse and Hamburg, awaiting engines to fit to their pylons, should soon start to disperse. For both P&W and Airbus, the glitch in engine and airplane deliveries has been most unfortunate, but it also reflects what has become the biggest issue in the commercial aerospace sector—coming to grips with unprecedented market demand while maintaining quality control standards.

Author: Richard Gardner
Source: SAE Aerospace Engineering Magazine