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Tue, Feb 15, 2011 — David Evans

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To achieve type certification, the new Gulfstream GVI will have to satisfy the special condition outlined below regarding reliability of its electrical system.


14 February 2011                    Federal Aviation Administration (FAA)

FR Doc 2011-3210                   Docket No. NM444

Special Conditions No. 25-11-03-SC

Notice of Proposed Special Conditions, Gulfstream Model GVI airplane, Operation Without Normal Electrical Power

The Gulfstream GVI (also referred to as the G650) will have numerous electrically operated systems whose functions are essential for continued safe flight and landing. The applicable airworthiness regulations do not contain adequate safety standards for these design features, hence these special conditions to establish a level of equivalency.

The Gulfstream GVI incorporates an electronic flight control system that requires a continuous source of electrical power in order to keep the system operable. The criticality of this system is such that its failure will either reduce the capability of the airplane or the flight crew’s ability to cope with adverse operating conditions.

The Mach 0.925 GVI will ostensibly be the fastest business jet flying, squeaking ahead of Cessna's Mach 0.92 Citation X

The current rule, Sec. 25.1351(d), Amendment 25-72, requires safe operation under visual flight rules (VFR) for at least 5 minutes after loss of all normal electrical power. This rule was based on mechanical control cables and linkages for flight control; they allowed the crew to maintain flight control of the airplane for an indefinite time following loss of all electrical power. This arrangement allowed the crew to fly the aircraft while trouble-shooting the cause of the electrical failure, re-start the engines (if necessary), and re-establish electrical generating capability, if possible.

To maintain the same level of safety, the Gulfstream GVI must be designed for operation with the normal sources of electrical power (engine generator and auxiliary power unit, APU) inoperative. For compliance purposes, a test of the loss of normal engine generator power must take into account that:

— The failure occurs during night instrument meteorological conditions (IMC) at the most critical phase of flight in terms of electrical loads.

— After engine generator power cannot be restored, the engine still has restart capability.

— The airplane can continue its flight safely to a divert airport.

— APU operation cannot be considered.

There are only four ways of adding electrical power without relying on an engine-mounted generator or an APU:

1. An hydraulically driven motor generator (HMG) of the type normally installed on ETOPS (extended range operation) aircraft. An HMG uses hydraulic pressure to produce AC current (and rectifiers to produce DC current). This HMG may not be a viable proposition for a business jet like the GVI due to bulk and weight.

2. A RAT (deployable air-stream driven generator, known as a ram air turbine, or RAT). Also known as an air-driven generator, or ADG, this system provides electrical power with the option of additionally operating an auxiliary hydraulic pump. For the GVI, if the hydraulic pump requirement was eliminated, it is probable that considerable generating capacity would provide enough electricity for flight essential DC power and an AC inverter for 115 volt AC.

A deployed RAT

3. Large battery banks (weight prohibitive).

4. Install an additional generator on one engine (but if that engine fails, so do both generators).

When thinking about certification issues regarding flight control, a “hybrid” APU (a small tail-mounted thrusting jet) might be considered. It could have its exhaust deflected to either (1) run air conditioning and electrics/hydraulics on the ground, per normal APUs, or (2) provide additional thrust for short or contaminated runway takeoffs.

These 250 lb to 1,500 lb-thrust turbojet engines are quite simple. For a Gulfstream GVI, such a small tail-mounted jet would provide a flight sustainer capability (loitering at 160-180 knots) from about 1,000 lbs of thrust. When not in use, clamshell doors would be closed and faired for minimal drag. Such a capability would not involve more weight than a fairly hefty APU. The hybrid APU/thrust engine could save fuel by taxiing for long distances in a queue. It would also save main engine hours (and foreign object damage exposure) by taxiing in on it and shutting down the main engines.

This approach could be taken for the GVI. More likely, however, Gulfstream engineers will opt for two independent RATs as the simplest, most facile answer to this special condition.

Comments to this proposed special condition are due 31 March 2011.

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