Aircraft handling after dual engine shutdown in-flight / BFCM modelling

[Bestandskraft]

When shutting down both engines in flight, the aircraft is simultaneously in a dual hydraulic failure and in a total electrical failure situation.

Since the Backup Flight Control Module's (BFCM's) 2-speed electric motor is powered by the Right Main AC Bus, and this bus is not powered by the emergency generator, the BFCM will not operate. So far, this is correctly modelled in 2.7.9.18080.

What puzzles me, however, is that the severely degraded handling characteristics the flight manual warns about in a dual hydraulic failure situation even in case the BFCM is available do not actually materialize in the game.

According to my tests, with both engines shut down and without the BFCM I can maneuver around just fine and extend and retract the landing gear, flaps and airbrakes multiple times. This leads me to conclude that

a) the effect of losing hydraulic pressure on the control surfaces is not modelled at all, and/or

b) the BFCM incorrectly does operate in a dual-engine flameout situation even though its indicator shows "OFF".

One might object that the hydraulically operated flight controls continuing to function is normal behaviour if combined and flight hydraulic pressure is still available. In fact, in the above situation, neither system drops below approximately 2100 psi.

This however, does not seem to be correctly modelled by itself, since I consider it highly unlikely that the real F-14 could have maintained 2100 psi in both systems with both engines at 0% RPM (thus, not windmilling), the aircraft almost at stalling speed and after recycling the above mentioned control surfaces multiple times (probably indefinitely). Interestingly, both hydraulic system pressures rapidly drop to 0 after touchdown.

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On a related note, I am unsure about the RL AUTO(LOW) mode of BFCM operation, so what follows is not (yet) a bug report but probably a lack of system knowledge:

According to the BFCM diagram in NAVAIR 01-F14AAA-1, 15 May 1995, Change 1 - 1 February 1997, p. 2-72, in AUTO(LOW), the 2-speed electric motor comes on when

1) the Right Main AC Bus is energized,

2) both flight and combined pressure are/drop (if "are" or "drop" is correct remains to be seen) below 2100 psi.

If the "are" interpretation is true, shortly after electrical power is applied to the jet, the ON flag should be displayed in the LOW window as soon as the backup hydraulic pump has built up 500 psi, the EMERG HYD switch being set to AUTO(LOW).

However, in the game, when cranking the right engine prior to having cranked the left engine, only when the flight hydraulic pressure passes approximately 1500 psi upwards does the ON flag come on very shortly (I don't know how to explain this momentary switch-on), and permanently when the pressure drops back below 2100 psi upon returning the crank switch to OFF.

Still assuming the "are" interpretation is true, the only explanation I have for the ON flag not being displayed in the LOW window immediately after applying electrical power is that without the combined system having previously charged the backup hydraulic reservoir, the backup hydraulic pump is unable to achieve the 500 psi required to switch on the BFCM. This, however, would mean that the BFCM cannot operate at all prior to having cranked the left engine, since the flight hydraulic system lines are not connected to the backup system hydraulic lines according to the diagram. In this case, the ON flag should not come on at this point regardless of flight hydraulic pressure.

If we assume that the AUTO(LOW) mode does not come on when pressures ARE below 2100 psi, but only when they DROP below 2100 psi from above (this interpretation seems to be reinforced by p. 2-73), this still does not explain why the LOW window changes to ON during the flight hydraulic pressure DROP even though the combined hydraulic pressure remained at 0 psi all the time, thus never DROPPED below 2100 psi.

Moving along, I would assume that once electrical power is removed from the jet without engines running and with flight hydraulics back at 0 psi, hydraulic pressure in the BFCM should drop rapidly below 500 psi since the 2-speed electric motor no longer drives the backup hydraulic pump. However, once reconnecting external power with the EMERG HYD switch set to AUTO(LOW) or LOW, the BFCM immediately reactivates (ON flag). This would indicate that the BFCM pump switches back on with power available even though the flight hydraulics just ARE below 2100 psi and did not DROP below that level (let alone the combined hydraulic system, which I did not even touch if I incorrectly start by cranking the right engine).

A further bit of information that supports the "are" interpretation is p. 7-15, where upon returning the EMERG HYD switch to AUTO(LOW) during the EMERG HYD test, one should observe the OFF flags displayed in both the EMER FLT HI and LOW hydraulic pressure windows. In the game, the LOW ON flag remains active after either the flight or the combined hydraulic pressure has dropped back below 2100 during the preceding engine cranks, thus not conforming to the mentioned checklist step. The flight manual seems to contradict itself here, since the BFCM not switching OFF unless either hydraulic system has increased back above 2400 psi is normal behaviour IAW p. 2-73.

Edited January 7, 2022 by Bestandskraft

[HawkCZ6]

You indeed raise some very good points. Unfortunately the F-14's hydraulic modelling seems pretty basic, which can already be seen after performing a few quick tests. In a cold & dark jet, try windmilling the left engine using ground air and moving the wings without right engine. Even at 20% RPM (and only on one engine), the wings move at their normal rate. Wiping the controls, changing flap settings, nothing has any effect on the hydraulic pressure, even after you stop cranking the engine and the RPM drops. Pressure in the combined system drops at the exact same rate as if you didn't touch anything. It is the same with BFCM, which produces the same control surfaces movement rate as it is with both engines operating regardless of whether it's running on Low or High.

 

Quote

..since I consider it highly unlikely that the real F-14 could have maintained 2100 psi in both systems with both engines at 0% RPM (thus, not windmilling)

In fact they are (at least in my experience) windmilling at roughly 3-6% N2 (F110). I don't think that is sufficient to provide any hydraulic pressure, but in Heatblur's F-14, it is enough to provide normal control surface movement rates. During my tests at lower speeds, COMB hydraulic pressure fluctuated between 1200 PSI and 2100 PSI regardless of hydraulic load (after reaching 2100, it suddenly dropped back to 1200 and slowly recharged). During the peaks at 2100 PSI, it was possible to change wing sweep, flap setting and extend/retract gear. This hydraulic load in itself doesn't affect COMB pressure, as it simply continues to do its thing of going up and down. And as you mentioned - even at 3% N2, FLT pressure doesn't go below 2000 PSI. Obviously, yanking the stick doesn't affect the pressure AT ALL and the control surface movement rates are identical with normal operations. That doesn't really seem right.

 

Regarding the BFCM activation, I agree that it does seem a little weird. From my understanding, one of the two should be true:

A) When spawning a cold & dark jet, activating the ground power should automatically start the BFCM in AUTO(Low) mode, since both FLT & COMB are below 2100 PSI.

B) When shutting down engines, waiting for hydraulic pressures to drop and then applying ground power, the BFCM should not activate. (and as it is now, it does)

I guess it would make sense for the system not to run on ground power in Auto, thus making B correct. Maybe there's some other variable that HB knows about, but if there is, it doesn't seem to be mentioned in NATOPS.

 

Also, little off-topic, but while we're discussing hydraulics, the wing sweep performance is not modelled correctly. As per F-14B NATOPS, page 2-81:

"Failure of either the combined or flight hydraulic systems permits the wings to move at a reduced rate."

Currently, it does not. The wings move at the same rate with both or only COMB system operating. They don't move at all when only FLT is operating.

 

tl;dr on what should in my opinion be investigated / fixed:

- Different hydraulic system pressures resulting in same primary / secondary flight control movement rates

- 3-6% N2 windmilling RPM providing enough pressure for both FLT & COMB hydraulic systems

- BFCM activating in Auto(Low) when on ground power

- Wing sweep performance with only one hydraulic system operating