Flying qualities: forms of degradation
The future of the helicopter is immense and later the craft will be a very familiar sight in the air to everyone. It will also be capable of rendering a great number of services which no other craft can render, and can be described as the greatest friend in need in the case of an emergency.
Igor Sikorsky at the end of his lecture ‘Sikorsky Helicopter Development’ presented to the Helicopter Association of Great Britain at Mansion House, London, on Saturday 8 September 1947
8.1 Introduction and Scope
As Sikorsky foretold with confidence and optimism 60 years ago in Ref. 8.1, the helicopter would indeed serve mankind as a ‘friend in need’, but as often happens, its unique capability would be usable only by pilots exercising very high levels of flying skills, and, in dangerous and emergency situations, by pushing both safety and performance to the limits. Sikorsky talked in his lecture about the significance of the helicopter in rescue service. He recounted a recent occurrence with ‘satisfaction and great encouragement’, to quote:
The police rang up the factory to say that an oil carrying barge with two men onboard was in distress and was starting to disintegrate, water sweeping over the surface of the barge. We immediately dispatched a helicopter with a hoisting sling and in spite of a wind of 60 m. p.h and gusty, the helicopter quickly reached the barge and was able to hover 20-25 ft. over it, lower the hoisting sling and take the men off, one after another. The rescue was made as the end of the day was approaching and the general consensus of opinion was that these two would certainly not have been able to stay on the damaged barge overnight.
Igor Sikorsky presented the lecture published as Ref. 8.1 just a few years after the birth of the practical helicopter. He talked about ‘. . . absolute accuracy of the control’ and ‘… control as perfect as any other system of control.’ Today, we can only try to imagine the motivation, the courage and the optimism of the early pioneers as they shaped the first vertical flight machines with four axes of control. A few months later, on 19 April 1948, as reported in Ref. 8.2 and discussed in the Introductory Tour to this book (Chapter 2), a Sikorsky S-51 during a test flight at the Royal Aircraft Establishment would almost crash as the pilot momentarily lost control during a high-speed (4 g) pullout and inadvertent rapid roll to 90° of bank. The other side of the coin, so to speak, was experienced with the consequences of degraded handling qualities. Helicopter control, while qualitatively precise, would always require close pilot attentiveness and relatively high workload.
Chapter 7 ended with a discussion on the impact of flying qualities on safety and mission effectiveness. The twin goals of safety and performance, with the consequent tension between them, have pervaded the whole business of aviation since the Wright brothers’ first flight in December 1903. In the helicopter world, the performance-safety tension is perhaps strongest when flying close to the surface with what is sometimes described as mission imperative, or at the edges of the operational envelope, in harsh environments, or when the pilot has to deal with flight system failures. When flying close to the surface, the first priority for the pilot is to maintain a sufficient margin of ‘spatial awareness’ to guarantee safe flight. This spatial awareness also has a temporal dimension; the pilot is actually trying to predict and control the future flight trajectory. We can imagine a pilot flying to maintain a safe time margin, avoiding obstacles and the ground, with a relaxed control strategy allowing plenty of time for navigation and monitoring aircraft systems. The pilot will want to maintain a sufficiently long ‘time to encounter’ between the aircraft and any potential hazard, so that there is ample time to manoeuvre around, climb over or even stop, if required. But external pressures can make things more difficult for the pilot, increasing the workload. Imagine that the task is to transit, within tight time constraints, to deliver an underslung load to a confined forest clearing at night, with the threat of enemy action. Under relentless time pressures, the pilot has some scope for trading off performance and workload, depending on the requirements of the moment. He or she will be forced to fly low to avoid detection by the enemy. Increasing the tempo at low level reduces the safety margin; more precision or more agility requires higher levels of concentration on flight path guidance and attitude stabilization. The more the pilot concentrates on flight management, the more the global situation awareness is compromised with increased risk of getting lost or becoming disconnected with the military situation. Flying qualities affect and are powerfully affected by these demands and nowadays can be sensibly discussed only in terms of mission-oriented requirements and criteria, hence the considerable emphasis on the development of handling qualities engineering and the standards, particularly Aeronautical Design Standard-33 (Ref. 8.3).
Military standards have wholly embraced the concept of handling qualities levels and pilot assessment through the Cooper-Harper handling qualities rating scale, discussed extensively in Chapter 7 of this book. For an aircraft to be fit for service (i. e., according to ADS-33 ‘… no limitations on flight safety or on the capability to perform intended missions will result from deficiencies in flying qualities’), it has to exhibit Level 1 handling qualities throughout the normal operational flight envelope (OFE). Degradation to Level 2 is ‘acceptable’ following the failure of some flight functions, in emergency situations or when the aircraft strays outside the OFE. Some operators may also allow Level 2 handling qualities in parts of the OFE, provided exposure is limited, e. g., deck landings in high sea states. Even though guided and constrained by their own experience and standard operational procedures, pilots need to make judgements all the time as to whether a particular manoeuvre is achievable or not. Sometimes they make the wrong judgement but the usual outcome is that the pilot gets a second chance at the landing or to position the load or pick up the survivor. Failing a mission task element (MTE) might push the aircraft into Level 3, but provided the degradation is not too severe the situation is recoverable. A more sudden or rapid degradation can push the aircraft towards the Level 4 condition however, where there is a high risk of loss of control. Chapter 7 closed with a statistical interpretation of the consequences on flight safety of an aircraft exhibiting different handling qualities (see Fig. 7.45). Acknowledging the assumptions of the analysis adopted, we drew the tentative conclusion that for an aircraft exhibiting a mean HQR at the Level 1/2 borderline, the probability of loss of control would be approximately 1 in 109 MTEs across the fleet. In comparison, an aircraft that exhibited a mean HQR in the middle of the Level 2 range would have a probability of loss of control across the fleet of about 1 in 105 MTEs, a massive increase in risk to safety.
These conclusions are borne out by the accident data. For example, in Ref. 8.4, Key pointed out that 54% of all accidents on the H-60 Blackhawk in the 10-year period up to 1996 involved deficiencies in handling qualities or situation awareness. The data also revealed that marginal handling was much more of a problem for low-time pilots. In a complementary study on US civil helicopter accidents, Ref. 8.5 reports that of the 547 accidents that occurred between 1993 and 2004, 23% could be ‘… attributed to loss of control by the pilot – caused or aggravated by inadequate or deficient handling qualities’. The relationship between handling and safety is an important link to make, even more so because in the drive to ‘weather-proof’ flight operations future rotorcraft will be required to perform roles in more degraded conditions than is currently possible with safety, hence an understanding of the ways degradation can occur, and some of the consequences, can assist in forming the requirements for day-night, all-weather augmentation systems. This chapter addresses these issues and material is drawn from the author’s own research over the 10 years since the publication of the first edition of this book, e. g., Refs 8.6-8.9. During the second half of this period, the author relocated to The University of Liverpool, creating and building a research group focused on all aspects of Flight Science and Technology, and with a strong emphasis on flight safety. Central to the research at Liverpool is the Bibby Flight Simulator and, within this chapter, research results using this facility are presented liberally; the simulation facility is described in some detail in Appendix 8A.
To create a framework for the chapter, handling qualities degradation is described in four categories:
(a) degradation resulting from flight in degraded visual conditions;
(b) degradation resulting from flight system failures, both transient and steady state;
(c) degradation resulting from flight in severe atmospheric disturbances;
(d) degradation resulting from loss of control effectiveness.
Strictly speaking, category (d) should not occur, almost by definition, within the OFE and usually results from pilots inadvertently straying outside this, as a result of degradations in categories (a)-(c). Discussion on category (d) situations, for example, loss of heave control following entry into vortex ring, loss of tail rotor effectiveness in quartering flight or loss of pitch/roll control power in high-speed stall, will not be included.