Don’t throw that away
New External Power Monitors have built in memory

By Jim Jenkins
Aging Aircraft Public Affairs

Most people know the old cliché “Don’t shoot the messenger,” and many try to heed its warning. However, fleet maintainers have been doing this for years at a cost of nearly one million dollars a year.

The messengers in this case are aircraft external power monitors (EPM), which are needlessly “deep-sixed” time and time again.
External power monitors are the gatekeepers on military aircraft that ensure that the power supplied to the aircraft from external sources is not outside the limits that are considered suitable for aircraft electronic equipment. These suitability limits are contained in MIL-STD-704, the bible for ensuring compatibility among aircraft electrical systems, external power, and airborne utilization equipment.

The problem has never been that the external power monitor wasn’t able to perform its function. In a great majority of cases, external power monitors do exactly what they are designed to do; prevent unacceptable power from reaching expensive, sensitive aircraft components. Rather, the problem might be one of poor communication skills. The external power monitor could not tell the maintainers what was wrong with the external power, only that the power was bad. It couldn’t even tell the maintainer if the monitor was working properly. Since the external power monitor was a messenger that often prevented the maintainer from performing post-flight repair functions by removing external power from the aircraft without telling the maintainer why, it was rarely a welcome one.

Soon a new external power monitor will be available as a result of a cooperative arrangement among NAVAIR’s Aging Aircraft Integrated Product Team, the Power and Energy Division of Naval Air Warfare Center Aircraft Division Patuxent River, and the Defense Logistics Agency.
What makes the new power monitors “new and improved?”

First, why do aircraft need external power monitors and what do EPMs do. Aircraft normally receive power from an internal generator, driven by the engines or an auxiliary power unit (APU). However, it’s not always convenient or possible to start the engine or APU when aircraft maintainers or aircrew need to check out or work on aircraft systems. Power from external sources is plugged into the aircraft to allow maintainers and aircrew to continue their groundwork. The power from these sources can sometimes be outside the parameters that are required for the electronic components to perform reliably. The EPM continuously looks at certain parameters (voltage, frequency, etc.) of the external power. If the power is within certain parameters, the EPM allows the external power to be supplied to the aircraft.

The current external power monitor was designed over 20 years ago and, for the most part, does an adequate job. However, the new EPM will take advantage of advances in microcircuit technology to monitor additional parameters that are critical to the health of sensitive aircraft electronics. For instance, harmonic distortion is not currently measured and, if other parameters are within the permitted range, the power is allowed into the aircraft. However, studies have shown that harmonic distortion can significantly affect the longevity of electronic components.

To maintainers this means that electronic equipment on aircraft will be more reliable and last longer, which translates to fewer repair hours, greater operational readiness and an estimated cost savings of $3 million per year. However, this new capability to monitor additional parameters doesn’t do anyone any good if the communications skills of the EPM are not improved. Nobody will know that the reason for the trip was harmonic distortion. That’s where the second major enhancement comes in.

The new external power monitors will have a diagnostic capability with data that can be accessed by the maintainer. With its diagnostic ability the new EPM will no longer be the silent messenger that conveys its message by just preventing power that’s out of limits from reaching the aircraft components. Maintainers will be able to download the exact parameters that resulted in the “trip.”

Armed with information on what caused the EPM to trip, the aircraft maintainers will be able to talk to those people who repair and maintain the external power systems. Since power fluctuations are often dependent upon so many variables and it is difficult to duplicate the situation that actually caused the fluctuation, the maintainers will be able to say, “See, it wasn’t my power monitor. It’s your power.” There’s even enough memory in the new EPM to save hundreds of events to show trends in the external power.

“The new EPM will enable Fleet maintainers to quickly determine whether the external power source or the aircraft is the cause for rejected power,” said Joshua Masters, electrical engineer with electrical power branch of the propulsion and power department. “The diagnostics data will alert the maintainer to the cause of the EPM trip. The cause of rejected power has been elusive in the past with the maintainers often playing the game 'It's not my aircraft, it's your power. An aircraft equipped with a new EPM will report the conditions and reason why it rejected external power. The EPM will supply a diagnostic fault record which will direct the maintainer to the appropriate method of repair. In the age of the cars telling us their oil needs changed; the newly designed EPM will tell the maintainers ‘The power was rejected because of…’ We are designing the ‘Smart EPM.’”

The new power monitors, the M24021-7 and M24021-8, update the M24021-3 and –5 series power monitors. The improved design features modern microprocessor and circuit technology, which will provide superior protective functionality with state-of-the-art diagnostics, said Jeff Manor, of the Aging Aircraft IPT.

The modernization efforts will affect a wide range of military aircraft from all the services, some of which include the F/A-18, V-22, A-10, H-52, F-15, F-16 and H-60. The new monitors are the same footprint as the older ones so that no airframe modifications are required, the weight and volume are relatively unchanged and there will be a new J2 receptacle available to access the new diagnostic features of the new monitors.

“We’re taking advantage of technology in a couple different ways,” Manor said. “We’re going to use a microprocessor to monitor more parameters. We are actually widening the allowable power in some aspects, but we’re also measuring some things that we couldn’t do because technology was such that you couldn’t do it before – like distortion.”

Distortion over a long period of time causes damage to sensitive electronic equipment. Diagnostically, if external power causes the new monitors to trip, the new investigative feature will record what caused the trip and allow maintainers to address the actual causes of bad power instead of throwing away good power monitors.

“When they have a problem with power getting on the airplane, their first step is to usually throw one of these away at a cost of about $1,200 to $1,500 a piece,” Manor said.
The frequency of bad power distribution depends on the system, Manor said.
“Some systems are better designed than others for providing ground power,” Manor said. “It’s a readiness degrader if you can’t use your shipboard power or your ground power because it keeps tripping the monitor.”

The new diagnostic feature will record the reasons why the monitor was tripped. Maintainers can download that information and they then can go fix the real causes of the power problems.
The installation of the new monitors will provide an annual savings of approximately $700,000, because maintainers are not throwing away good equipment, and also a cost avoidance for the O-level maintenance because maintainers will not spend their time trying to fix a problem on the aircraft that really isn’t there to begin with. Additionally, the military will realize a savings of $3 million per year in reduced avionics depot level repair costs resulting from the improved screening of external power.

(Jeff Manor of the Aging Aircraft Integrated Product Team contributed to this article)