For most jetliners, the inflight use of reverse thrust is prohibited (and often physically impossible), due to the potential for loss of control in the event of an inflight reverser deployment (a product of the large increase in drag - and [for aircraft with wing-mounted engines], even more importantly, large decrease in lift - produced by a reversing engine).
Some aircraft, however, are certified for inflight thrust reversal (typically to allow for steeper descents by using reverse thrust as an airbrake, or to shorten the landing roll by putting out the reversers over the threshold rather than waiting until after touchdown).
Now, jet engines do occasionally fail. As a result, one of the certification requirements for jetliners with non-centerline engines is that they have to remain fully-controllable even with one non-centerline engine shut down and the remaining engine(s) at maximum thrust.1 However, as most jetliners are not certified to use reverse thrust in flight (and most have mechanical interlocks that are supposed to physically prevent the reversers from deploying if the aircraft is airborne), they are not required to demonstrate controllability with one engine in full reverse thrust and the other(s) at full forward thrust (which, for most jetliners, would produce a thrust asymmetry around half again as great as that produced with one engine shut down and the other(s) at full forward thrust2, as well as, for aircraft with wing-mounted engines, a considerable lift asymmetry resulting from the severe airflow disruption inflicted by the reversing engine on its respective wing); indeed, these aircraft may not be controllable with one engine reversed in flight, especially at lower speeds where the aircraft's rudder and ailerons are less able to counter the large thrust asymmetry.
For aircraft that are certified for inflight thrust reversal, however, there seems to be no reason to exclude reverse thrust from consideration of inflight thrust asymmetries, as jet-engine thrust reversers, like the engines themselves, do fail from time to time.
In general, asymmetric-thrust situations involving reverse thrust would fall into two categories:
- One engine is in reverse, while its twin on the other side is nonoperational; this would occur if, while reversing engines inflight, one of the engines in reverse were to fail. This would produce a considerably-smaller thrust asymmetry than would occur with an engine in full forward thrust and its twin inoperative;2 however, for aircraft with wing-mounted engines, it would also produce a considerable lift asymmetry not present in asymmetric-thrust scenarios not involving reverse thrust, which would tend to aggravate the control difficulties posed by the thrust asymmetry. (Aircraft with tail-mounted engines, on the other hand, would have only to contend with the thrust asymmetry, and would be expected to have little difficulty with control in such a scenario.)
- One engine is in reverse, while its twin on the other side is in forward thrust; this could occur if an engine failed to go into reverse thrust when commanded but its twin reversed successfully, or if one reverser retracted without being commanded to do so, or (for aircraft with target-type reversers3) if the reverser on a reversing engine were to become physically detached in flight. This would produce a thrust asymmetry considerably greater than that from one engine in full forward thrust and its twin shut down; aircraft with wing-mounted engines would, on top of this, also have to contend with the large lift asymmetry from having one engine in reverse and the other not.
Do aircraft that are certified for the inflight use of reverse thrust have, as a certification requirement, to demonstrate that they can still be controlled even in asymmetric-thrust scenarios involving one or more engines in reverse?
1: Actually, all civil aircraft with non-centerline engines, no matter the type of engine, have to demonstrate controllability with one engine out and the other(s) firewalled, but this question focuses on jets.
2: For most jet engines, the amount of thrust produced at full reverse is only about half of that produced at full forward thrust. This is because:
- the exhaust jet from an engine at full forward thrust is directed more-or-less straight backwards, while that from an engine in reverse is directed at large angles above, below, and to the sides of straight forwards (the exhaust jet can't go straight forwards, because the engine is in the way), and
- for most high-bypass turbofans (the type used on all modern subsonic jetliners), only the bypass air (the air blown around the engine core by the fan, which accounts for most of the engine's thrust generation) is reversed; the core exhaust (the superheated air and combustion gasses coming off the turbine stages) goes straight out the back as usual, negating part (though far from all) of the reverse thrust generated by the redirected bypass air. (Turbojets and most low-bypass turbofans, commonly used by earlier-generation jetliners, do redirect the core exhaust forwards, as this is responsible for most [or, for turbojets, all] of these engines' thrust, but no subsonic airliners have been built with turbojet engines since the late 1960s, while large-scale production of airliners powered by low-bypass turbofans ended in the first few years of the 21st century [and most manufacturers had already switched away from low-bypass engines in the 1980s and early 1990s].)
3: Target-type reversers are the big obvious ones seen on most older jetliners with low-bypass turbofans (such as the DC-9 Classic and 737 Original) as well as a few newer designs (such as the DC-9-80 and Fokker 100/70), where the big bucket doors swing out behind the engine's tailpipe to redirect the exhaust forwards. In contrast, clamshell-type reversers (the type seen on the 727, the very first 737 Originals [before they were replaced by target-type reversers], and on most Airbus jetliners) feature doors that swing open from the sides of the engine to redirect the bypass air, and sometimes the core exhaust as well, forwards, while cascade-type reversers (seen on most high-bypass Boeing jetliners) slide the whole rear part of the cowling backwards and use cleverly-designed vanes to redirect the bypass air forwards through the gap opened up in the cowling.