Category: Featured

Several specifications and brochures concerning 400 HZ Ground Power Units (GPU) mention protection for No Break Power Transfer (NBPT). Few explain why it is included or what this requirement actually means.

Originally when power was switched from internal aircraft sources to an external source, the internal source connection was broken, before the external connection was made. This was a “break before make” transfer. It meant that there was a time without power during the transfer. This is shown in Figures 1, 2 & 3.


In Figures 4, 5 & 6 the internal and external sources are tied together before the aircraft internal side breaks. This is a “make before break” transfer. Power to the aircraft electrical loads is never interrupted. This is referred to as a No Break Power Transfer (NBPT).


In an earlier blog I explained why aircraft and most military equipment was powered by 400 Hz (cycle) power. This included the statement that this standard was worldwide. Wherever you fly there will be a 400 Hz power source available with a standard plug that fits your aircraft. Well almost. There are a few exceptions. Smaller & older aircraft use a 28 VDC system and smaller airports may only have a 28 VDC power source. So actually, we have two standards although the 400 Hz system is the workhorse of commercial & military aviation where high power is required.

During the last 16 years there has been a growing usage of a third system using 270 VDC. The 270 VDC system for aviation was first defined in MIL-STD-704B 17 Nov. 1975. Battery systems in this voltage area had been utilized in submarines since 1900.

The old reason for the 270 VDC system was increased power density and capacity.
The new interest is due to stealth concerns and design changes in modern fighter aircraft

Alternating current systems such as 400 Hz are not as stealthy as Direct Current (DC).

Although the 270 VDC military specification introduced in 1975 and last updated in 2004 (MIL-STD-704F) is still in effect, it does not contain all the requirements needed for the modern 270 VDC fighter aircraft.

The new generation aircraft, specifically the F/A-22 Raptor and the JSF (Joint Strike Fighter) F-35 Lightning, require a system that is more precise, has faster response, and is tailored to the specific requirements of the aircraft manufacturer. Simply put, although this is a 270 VDC system, the requirements define a power supply that far exceeds the old 2004 MIL-STD-704F performance. This required the creation of a 270 VDC system specific to the new aircraft.

The previous generation of fighter aircraft used a dual hydraulic system to move the flight control surfaces. This presented a large vulnerability should the aircraft suffer damage and fluid loss.

Newer aircraft now utilize multiple electrically driven hydraulic actuators at each control surface. This now requires the electrical system to respond to rapid load changes while maintaining a constant voltage.

Radar systems require more pulse power while the voltage stays constant.

The aircraft 270 VDC power system is ideally suited to meet all these demands. The performance of the on board power is matched by the performance of the external power supply built by FCX Systems. This allows full maintenance including flight control response tests to be performed inside the hangar.

The FCX external power supplies are tested for voltage stability and recovery at various load configurations, transitions and pulse loading. The power supply performance is recorded and examined for voltage transient response during the loading criteria to insure that voltage is maintained within the limits that are safe for the aircraft.

FCX supplied the first modern 270 VDC power supply used on the first F/A-22 Raptor in 2000 and continues to support 270 VDC aircraft today both in the field and on the JSF assembly lines in the US, Italy and Japan.

Last time, we discussed why the utilities supply 50 or 60 cycle (Hz) power and the aviation industry operates on 400 Hz power.

Obviously, the two systems are not compatible. 400 Hz is 6-2/3 times faster than 60 Hz and 8 times faster than 50 Hz. The speed of motors (and clocks) would be multiplied by the same factor. Also, a few things would just catch fire. However, we need a source of 400 Hz at the airfield so that aircraft can shut down their engines. The engines and internal generators burn fuel, create noise and produce undesired emissions.

The simplest approach would be to build a 400 Hz generator driven by an engine on the ground next to an airplane. But that would consume fuel, create noise and produce undesired emissions just like the aircraft. It would be a solution if no other power was available. However, usually utility power is not far away. The boarding bridge, hangar or other building will be using 50 or 60 Hz utility power.

But, didn’t we just say the two power systems were not compatible?

Yes, we cannot connect the two systems directly but there are ways of converting utility power to 400 Hz aviation power.

One way is to use a motor connected to the utility as the engine to drive a generator producing 400 Hz. This creates what is called a Motor Generator frequency converter, or “MG Set.” Electrical power to the motor generates horsepower out. The horsepower into the generator creates electrical power out. The engineering of the system ensures that the utility 50 or 60 Hz is correct for the motor and that the generator produces the proper voltage and 400 Hz frequency for the aircraft.

Although the MG Set solution is rather simple, it has a couple of drawbacks. First it is mechanical. The rotating parts require continuous lubrication. The air-cooling of the motor and generator create high ambient noise. Finally, because the efficiency of the MG Set is not good for normal and low loads, (50-70%) the operating power expense can be high.

A preferred method of frequency conversion is a Solid-State Frequency Converter (SSFC).

The solid-state design also takes power directly from the utility and converts it to a form acceptable to aircraft and military 400 Hz power systems.

The front end of the SSFC rectifies the utility power and creates a direct current (DC) voltage. DC is a steady voltage that does not have a significant frequency component.

The DC is then switched by power transistors to create an alternating current (AC) waveform at the required frequency of 400 HZ. The efficiency of the SSFC is quite high at all loads; typically approaching 91-94%.

Although both types of frequency converter cost about the same, the solid-state frequency converter design and provide a significantly lower operating cost when compared to MG Sets. Fuel power engine generator sets may be the only choice if utility power is not available.

The idea of making electricity useful for commercial lighting and motors initiated an argument. One side wanted direct current (DC). One wire will always be positive (+) and the other wire is always negative (-).

The other side supported alternating current (AC). In this system, the voltage is constantly moving between positive and negative. The shape of this changing voltage is a sine wave.


The changing AC voltage provided the advantage of using transformers to change the voltage. High voltage at low amps could be sent long distances over small wire, and then transformed to a safe lower voltage for distribution inside a home or factory.

Once the decision was made to use AC, the next question was, how fast should the voltage change? How many cycles per second (Hertz, Hz) should be produced?

As the frequency of the voltage change was made slower, the size and weight of the transformers and generators increased and became more expensive. As the frequency was made faster, more power was lost in the transmission lines, which also increased cost.

The most economical frequency for the power company was around 60 cycles per second. Some countries standardized on 50 cycles per second or Hertz (Hz).

When aviation began using electricity, it was DC power. As AC became more prevalent in aircraft, the primary concern was the size and weight of transformers, motors and power supplies. The idea was proposed to use a higher frequency to make the components lighter, since the length of power transmission was small, the increased power loss would be negligent.

A special generator was designed to create an output of 400 Hz. This allowed a motor which was the size of a watermelon to be replaced by one the size of a one-pound coffee can which could do the same work.

The saving of weight allowed increased cargo capacity and decreased fuel consumption. Power at 400 Hz for aviation was a success and became the standard of modern AC-powered aircraft.

Airports all around the world standardized on the same power system. This included the physical plug and cable as well as the 400 Hz power so that aircraft from anywhere in the world could land and be serviced wherever they landed.
The aviation power system of 400 Hz became one of the first worldwide-adopted standards.