John,. .. .I'm note sure (have no idea) when the distinction "stall is partial breakdown and surge is total breakdown" came into use with the JAA, but it was a frequently used question in the old CAA engines exams for many years. . .. .Looking through the few references I have to hand:. .. .a. The 1955 edition of the RAF's AP3456 (they called it the AP 129 then ) . .does not discuss stall and surge as separate phenomena. It discusses only the situation where "excessive pressure at the rear of the compressor causes flow breakdown, such that the blades stall".. .. .b. The 1969 edition of the same book, amended up to 1972 standard (yes, I do have a strange library), distinguishes between stall and surge. It includes the statements "The aerofoil sections of compressor blades can be made to stall if the relative airflow is varied beyond the critical limits" and, "In an axial flow compressor, surging indicates a complete instability of flow through the compressor". It also includes a surge/stall map in which the significant elements are marked "Running line", "Surge line" and "Unstable area".. .. .c. The 1952 (revised 1974) edition of the Gas Turbine Engine by Pratt & Whitney Aircraft Group, does not provide any detailed distinction between stall and surge, but includes the vector diagrams I described in my previous post and includes the statement "When enough blades in enough rows are stalled, the compressor surges". This book includes a large number of surge maps, none of which include the word stall. . .. .d. The 1986 edition of the Jet Engine by Rolls Royce, does not provide a detailed distinction between stall and surge, but includes the statements, "…..the blades may stall because the angle of incidence of the air relative to the blade is too high (positive incidence stall) or too low (negative incidence stall" and " If the engine demands a pressure rise from the compressor that is more than the blades can sustain, surge occurs. In this case there is an instantaneous breakdown of flow through the machine, and the high pressure air in the combustion chambers is expelled forward through the compressor".. .. .Knowing the way the old CAA examiners (who are also some of the new JAA examiners) work, they probably looked at the AP 3456 and the RR book and came up with the distinction, "stall is partial breakdown and surge is total breakdown". Looking back to my earliest studies of this subject in the mid sixties (I can just about remember it), the JAA definitions were used by the RN.. .. .Having said all of that, I think that in most aviation-related crewrooms around the world, the sound of an engine coughing or grumbling (however briefly) is often met with the comment " somebody is surging his engines". . .. .Casual Observer,. .. .My suggestion that the locked-in rotating stall is unusual was based largely on circumstantial evidence (or rather lack of it). When the US military was having this problem with its (then new) turbofan back in the seventies or early eighties, the various journals (Flight and AW&ST) discussed it as if were a new phenomenon. This, coupled with the lack of discussion of this subject since then, led me to conclude that it was comparatively rare. On reflection however, I can see that the reported events might simply have reflected the initial awakening of knowledge of a common phenomenon, rather than an isolated incidence of a new one. . .. .The turbulent flow generated from a stalled aerofoil flows mainly from its upper surface, so in a compressor it will affect the blade above its convex surface (in the disc) more than the one below it. So in addition to cascading rearwards, the stall might well migrate around the disc. But the camber on the rotors faces away from the direction of engine rotation, whilst that of the stators faces in the direction of rotation. So the circumferential stall migration in the rotors should be in the opposite direction to that in the stator. Also, because the stators are not rotating, the absolute value of the rotational velocity of their stall is likely to be greater than that of the rotors. So if a stall rotates, the predominate direction of this rotation should be in the direction of engine rotation. In the case of the engine referred to above, the stall rotated in the opposite direction to engine rotation. You are of course correct (if I understand you correctly) in implying that if a single blade, or small group of blades stalls, it will be a rotating stall. But it seams to me that this will turn with the engine. Much of this is of course supposition on my part, and I accept the fact that you might well be correct. . .. .I have more difficulty with Captain Squelch's argument that all of the blades in a disc must be stalled simultaneously. To see what I mean, let's consider the case of front end (first row of rotors) stalling due to turbulent airflow entering the intake. Turbulent airflow is by definition different at all points, so every blade (and probably every point on every blade) will experience a different angle of attack and flow velocity. The probability of even any two blades stalling at exactly the same instant is therefore remote. If individual blades stall at different times then there must be times at which some, but not all of the blades in a disc are stalled. . .. .It seams to me that the idea that all blades in a disc must be stalled simultaneously only works if we apply the arbitrary definition "a blade in any given disc is considered to be stalled only when all of the other blades in that disc are stalled". But if we apply this definition, the concept of a stall rotating in a disc become meaningless. If all of the blades are stalled, what exactly does it mean when we say the stall is rotating in the opposite direction to engine rotation? If on the other hand only part of the disc is stalled, then stall rotation means that blades move into and out of the stalled condition as the pocket of stall passes over them. . .. .I don't think we need to be too concerned (or surprised) about our different terminology for stall and surge; There are precious few areas in which American English and English English coincide in everyday matters. So we shouldn't expect too much in such technical areas as this! And anyway, faced with the type of symptoms we are discussing here, some airlines would simply say "That is a perfectly serviceable engine, so get your chickens and goats onboard. We are already 2 days late and I cannot shut down the engines because that one hasn't got a starter fitted". (when considering this scenario readers should pick an airline to suit their personal prejudices).. .. .On a more serious note, some of the earlier posts appeared to suggest that stall/surge is entirely due to engine wear. The main purpose of my post was to illustrate that many of the causes of stall/surge are within the control of pilots. But to understand what should be done to avoid and recover from these problems, it is necessary to know something about their causes and behaviour. Precise definitions of terms often matter more in the examinations than in the day-to-day business.. .. .Lomapaseo,. .. .Your experiences in the 727 illustrates the fact that some engine locations are more problematic than others. The centre engine in a trijet will clearly experience more difficult airflow conditions than the outer ones, particularly at high angles of attack. Similarly, tail mounted engine are likely to be more prone to stall/surge than underwing ones.. .. .It's funny how a simple question so often turns into a complicated one. There must be a term for this……..PPRUNERISM perhaps?