744 NAI

[Luftis]

Another question to which I fail to find a satisfying answer. Any thoughts would be greatly appreciated.

The 744 MEL allows you to dispatch with the NAI valve secured OPEN, provided the HP valve is secured closed. By keeping the bleed valve OFF for the entire flight (except during start), this procedure essentially has you using the Firewall Valve as your NAI valve by use of the NAI switches. (Selecting NAI ON with the Bleed valve OFF will automatically open the Firewall Valve, and since the NAI valve is already secured OPEN, provide IP bleed air to the Engine Nacelles. )

Now.. for the question;

The manual requires a enroute climb-penalty of almost 8 tonnes to be applied. Anyone care to guess why?

[NSEU]

The 3 fully serviceable engines have to provide more bleed than normal, the 4th engine less.

Could it be that the climb penalty is based on individual engine limits rather than averages?

[Luftis]

The 3 fully serviceable engines have to provide more bleed than normal, the 4th engine less.

Yes, that was my first thought.

But if you just dispatch with the Bleed Valve (PRSOV) secured closed, there is subsequently no penalty at all.. exact same situation where the 3 remaining engines must supply the entire bleed manifold. So that can't be it.

Any other ideas?

[Spanner Turner]

The manual requires a enroute climb-penalty of almost 8 tonnes to be applied. Anyone care to guess why?


As you've mentioned FWSOV I will assume a Rolls Royce installation - but same for other engine types.

The Nacelle Anti-Ice valve is a pressure-regulating type valve. In normal operation it only lets through enough air to maintain 20-28psi downstream of the valve. This only "robs" the minimum amount of air from the compressor.

However, when the valve is 'locked open' it discharges the maximum amount of compressor air available in the anti-ice duct. i.e its like having a 3.5" hole in your IP compressor. Makes it very inefficient, which in turn has a dramatic effect on HP compressor efficiency (and therefore thrust/EGT limit).

:ok:
.

[Luftis]

Thanks, Spanner.

That makes things a lot clearer. Yes, RR in this specific case but I notice the Pratts and GEs have a similar penalty as well.

best regards.