AH64-D Longbow of RSAF downed. what could have caused it?

[SURB]

Pardon me for the reincarnation of rather old discussion.

My question is regarding the design of tail rotors in helicopters.Some of the newer designs incorporate the tail rotor in a disc shaped space instead of the rotor being operating in the open.Also the enclosed disc shaped design usually has increased number of blades.

• What's the advantage of such a configuration?

• Does the arrangement is aimed at lowering the helicopter's acoustic signature and, so, making it stealthier?

• Or is it to save it from being damaged when under attack?

Also the Apache's are a state of the art,well advanced attack helicopter.If this configuration is much more effective than the previous designs.Then why the manufacturers are still opting to continue with the open configuration design on it?


I'll like to attach couple of images of the enclosed and the open configuration respectively for reference.

View attachment 5879

View attachment 5880


Thank you.

 

[Gremlin29]

That's called a fenestron tail. It's supposed to be quieter than a conventional tail rotor although my experience is that it makes quite a racket. In my opinion it's "big" advantage is that it is safer for personnel who may be near the tail when the helicopter is running. I would not say it is better than the traditional tail rotor, if it was Eurocopter wouldn't offer the same helicopter with a conventional tail rotor (compare the EC 130 to the EC 145).

 

[fretburner]

I don't really want to hijack this thread and I know this is a little off topic, but...

What's the benefit of having canted tails and tail rotors? I see that in the UH-60 and MH-53.

 

[Gremlin29]

The canted tail rotor creates lift that will raise the tail during hovering flight to improve pilot visibility and/or equalize control movement in the pitch axis (same amount of fore and aft cyclic pitch control).

 

[Kantervo]

That may be a bit off-topic, but I think that the more technologically advanced modern helicopters (it applies to any modern aircraft) become, the more operators rely on electronic equipment (especially, during maintenance and in-flight status sensors). Certainly, it's impossible to imagine how to check hi-tech equipment without hi-tech equipment, but some systems are yet to be examined in good old manner. Was a problem with the aircraft computer that doesn't show corrosion levels in blocks where it might occur solved? Is such an option possible at all? And it is also interesting to know, in case a fault code about the need to replace both AISBVs appeared during the flight, would it lead to the shut-down of both engines? Thanks.

 

[StobieWan]

Gremlin covered most of your points with the post:

"To give you an idea of what a non-emergency a failed AISBV is, there's not even an emergency procedure for it. It is a fairly simple device, installed on each engine and they operate independently. That's why I find the whole thing curious, because there's really no practical reason for them to both fail at the same time, the odds are just incredible. They can be turned on or off, or placed in manual or automatic mode. They can be turned on or off independent of each other. "

Basically as far as I can read it, a fault code mid-air would likely mean that the pilot would be advised not to operate the valve in question and probably be wise to get the code checked out as quickly as possible - but that there's no reason a valve failure would shut the engine down in and of itself.

If anyone can correct me if I've misunderstood this, please charge in.

 

[Gremlin29]

In flight, the anti ice valve is either open or closed. It's operation is checked prior to takeoff and confirmed by a rise in TGT. Each engine has it's own valve. They can be selected to turn on automatically, manually or to stay off. Automatic actuation is based on an ice detector that is not mounted on or near the engine. Even with just 1 engine operating, it is possible to continue level flight with the anti ice valve open, but don't ask me how I know. "If" both failed and the micro switches didnt' work and "if" their was no advisory anti ice was on, it still wouldn't shut down the engines. The pilots probably wouldn't even notice and most likely, it would be discovered the next time the aircraft flew and their operation was checked. Some emergencies require landing without delay. Others require landing as soon as possible. Then there are the land as soon as practicables. Anit ice valve failure doesn't require you to do any of these ie you may continue the mission.

 

[StobieWan]

One other thing that sprang to mind, I'm guessing that icing isn't so much of a concern for the RSAF - it's a pretty toasty warm climate, not like, say, flying out of an airbase into altitude in Afghanistan during the colder months ? It sounds like the valve checks are part of preflight - is it possible that check has been deprecated due to lack of use and the valves have ended up stuck due to lack of operation?

I understand (from your very clear explanation) that this won't cause a major issue but did wonder if both valves having apparently failed would be caused by lack of use? Or are they cycled automatically during pre-flight? This is just a pesky "I wonder how it works.." question

 

[Gremlin29]

The interesting part is that unless you pull the circuit breaker (sometimes done if the engine exhibits soft stall tendencies) during startup the valve will open and close during engine start. Some engines do soft stall so there is a procedure to pull the AISBV circuit breaker prior to engine start, it is then pushed back in after the engine starts. Some aircraft have a warm start switch that does the same thing from within the cockpit (the crew chief has to pop the breaker as it's outside the cockpit). An alternative approved method is to motor the engine for 60 seconds (air turbine starter turns the engine but the HMU does not supply fuel). The latter is popular because pilots are control freaks and they don't have to worry about a breaker they can't see. This is all just background info so far. The point is those valves should exercise during engine start but may not.

A major item, in fact it is it's own section of the check list is the engine HIT check (health indicator test). Every turbine helicopter I have flown (more than a few) performs HIT checks, it is not an Apache specific function. The HIT check measures the engines health by comparing TGT (at a specific power setting which is typically just a tad less than the power required to lift off the ground) to a base line TGT at a specific temperature and pressure altitude. Your TGT must fall within an upper and lower limit of the baseline. The tables are in a log book carried in the aircraft. Once the HIT check is performed the inlet anti ice is turned on and operation is confirmed by a slight rise in TGT. This check is performed seperately for each engine, because they are not sympathetic systems ie one engine does not affect the other. This is all done prior to before take off checks which are ..another major section in the checklist. Skipping steps in the check list is a HUGE no no.

I have worked very near the Singapore Apache dudes here in the states and they are extremely professional, I would be shocked to learn they were skipping a mandatory and huge section of the checklist.Contrary to what may be a popular opinion, the checklist is actually a small book that each pilot carries in the cockpit and normally will strap to one leg. It is a required item to have on your person when flying, flying without it would be like flying in gym shorts.

 

[StobieWan]

Excellent Thanks as ever - gives me a sense of what would usually be going on here,

Ian