It is O dark thirty and the aircraft has been on the line for the past hour in anticipation of transporting the company president and several high-ranking executives to the annual stockholders’ meeting. The flight crew has already completed the initial power on checks when the passengers begin to board. Punctually, the limo arrives and the lone occupant embarks up the stairs into the aircraft cabin. Upon noting the arrival of the top banana the crew begins the sequence of starting engines. The main entry door closes, remaining engines are brought on line, and personnel are in place waiting to marshal the aircraft to the taxi way.
Standard company operating procedure calls for the use of the taxi light to signal ramp personnel that the aircraft is ready to taxi, after several minutes and still no taxi light, the unthinkable happens. The main engines are shut down, a message crackles across the communications radio in the hangar, “We have a problem and need the assistance of maintenance.” The lone on-call technician quickly approaches the aircraft as the main entry door opens. Upon entry the first words the technician hears are those uttered by the top executive, “Will this take long?” The response to this question has the potential to be career altering.
Although the above situation is dreaded by most there are several precautions that can make it less stressful. Safety, legality, and passenger comfort are some of the main considerations when deciding if an aircraft is fit to fly. Many aircraft passengers may already be plagued with a fear of flying and hearing that the machine that is about to carry them may not be safe to fly can cause a sudden and uncontrollable adrenaline rush prompting emotional outbursts. Thorough knowledge of systems and knowing who can answer specific questions are some essentials in minimizing departure delays.
In the United States, Federal Aviation Regulations (FARs) may provide some relief when it comes to component or system failures on various types of aircraft.
In FAR Part 91 references are made to Inoperative equipment:
91.213 Inoperative
instruments and equipment
(a) Except as provided in paragraph (d) of this section, no person may take off an aircraft with inoperative instruments or equipment installed unless the following conditions are met:
(1) An approved Minimum Equipment List exists for that aircraft.
(2) The aircraft has within it a letter of authorization, issued by the FAA Flight Standards district office having jurisdiction over the area in which the operator is located, authorizing operation of the aircraft under the Minimum Equipment List. The letter of authorization may be obtained by written request of the airworthiness certificate holder. The Minimum Equipment List and the letter of authorization constitute a supplemental type certificate
for the aircraft.
Minimum Equipment List
Existence of a minimum equipment list (MEL) can be confirmed by the aircraft manufacturer. However existence is only part of it. Per the regulation, the document has to be approved by the airworthiness authorities and then adapted to the specific aircraft. Once the FAA master MEL is customized to a specific aircraft, the FAA office having jurisdiction over the aircraft operator will review the document and based on satisfactory findings, will issue a Letter Of Authorization (LOA). Only then is it legal to operate under the exemptions listed. It is also important to note that if a system or component is not mentioned in the MEL then it does need to be operational for flight.
The format of the MEL will often address aircraft systems, showing how many like devices are installed along with how many need to function to allow a safe flight. In certain situations flight procedures may require modification or maintenance action may be necessary to disable, test, or placard various inoperative systems or components.
An electronic file of approved Master MEL’s can be found at www.opspecs.com.
Have a Plan
“Failure to plan is a plan to fail” is a phrase that has specific significance to the aviation industry and is particularly appropriate here. After all, it is the fact that aircraft break, that justifies many of our existences. In the case of the high stress situation described earlier, if a plan had been created in advance and implemented at the time of the problem, the crew could have briefed the passengers that a delay would be encountered and they would be advised as soon as possible about the flight disposition.
In most cases a decision whether or not the flight is a go, can be made within 30 minutes of a detected problem. This does not in any way imply the aircraft will be ready to go within one-half hour, only that a thorough evaluation of the circumstances can be made and the passengers accommodated.
Talk to the Crew
Extracting information from the flight crew is a logical starting point in the process. Finding out exactly what they observed and when it occurred are often a big part of problem resolution.
This is when either a strong knowledge of the aircraft or knowing whom to call can be the make or break for the mission. It is important to understand that the best possible results that can be obtained as a result of a mechanically induced delay is restoring the problem device or system back to a fully operational condition. Once the crew debrief is accomplished and the information is assessed then a reasonable conclusion can be made if the aircraft can be repaired or returned to service in accordance with the MEL. At this time passengers can be advised of the anticipated length of the delay along with any options.
Troubleshooting
If after debriefing the crew and possibly conducting some basic checks results in confirming of a problem not addressed by the MEL, fixing the aircraft becomes the only realistic option. And creating a plan of attack is a good first step. When in doubt, read the book, get a clear idea of exactly how the system is supposed to work. Look at previous fault history as well as maintenance records to see if any system components may be approaching a time change or may have a high failure rate. Does all this take time? Yes, but defining the most logical part of the system to begin troubleshooting may ultimately result in a shorter delay.
A thorough review of the entire operating system may provide some unique insight to be able to test various pieces of the system without taking things apart. For example, in some aircraft the pilot’s Directional Gyro (DG) may drive the pilot’s Horizontal Situation Indicator (HSI) along with the copilot’s Radio Magnetic Indicator (RMI). So, if the report comes in that the pilot’s HSI has failed, then looking at the copilot’s RMI may provide an idea if the problem is the HSI or maybe a failed gyro.
Gain access to components that will provide you the maximum amount of information in the shortest amount of time. Perhaps a specific system contains a circuit breaker, switch, relay, and a black box in addition to the flight deck display. If it requires about an hour to gain access to the switch and circuit breaker but the relay is behind a panel with a Velcro attached close out it may be most beneficial to go to the relay. With appropriate test equipment the circuit breaker and relay can all be checked quickly and thoroughly resulting in a very high probability of proper fault isolation.
Many aircraft manufacturers utilize master warning systems (MWS) that have the ability to monitor flight critical systems or components. Understanding the logic that triggers the warning is essential to problem resolution or application of an MEL procedure. With many electronic flight decks, the classic fault panel has gone away and is replaced with messages. Making a determination of when the indication appeared or what action had been completed prior to fault recognition is paramount to timely return to service and systems incorporating self-diagnostics often provide valuable clues to problem resolution. In addition some MEL’s will list specific along with combinations of fault codes and provide direction as to what can be considered a dispatchable situation.
In some cases deviation from the manufacturer’s procedures can generate faults. One such case involved reported failure of a pre-departure autopilot test. In this case the crew had modified the checklist to “improve cockpit flow” and by doing so would try to accomplish the test using an electrically driven hydraulic pump rather than engine-driven hydraulic pumps to operate the flight control systems. At the time of the test the aircraft was powered by a single generator installed on the auxiliary power unit (APU) and when the electric pump would start the high current consumption would result in a voltage drop on the bus and cause the autopilot to disconnect. As in many cases like this, the “shotgun” approach to troubleshooting was implemented and all autopilot associated components were systematically replaced. When that did not prove fruitful, more conventional diagnostic methods were used. It was determined then, that the auto-flight system would work consistently in flight and the test would never fail when external hydraulic and electrical power were used. Finally when the crew was questioned they mentioned they amended the preflight checklist.
They commented that the procedure worked fine in the flight simulator during training and they saw no reason why not to make it part of their standard operating procedure (SOP).
When making the decision to use MEL relief the nature of the trip may impose additional restrictions. In the event of the previously mentioned autopilot discrepancy, the fact that the preflight test could not be successfully accomplished was justification to assume the autopilot was not operational. The master MEL for that particular type aircraft does contain provisions with some airframe limitations to facilitate departure without the autopilot.
Today however, reduced vertical separation minimums (RVSM) are in place across the Continental United States (CONUS) and one of the requirements for any aircraft occupying airspace between 29,000 and 41,000 feet is a functioning automatic altitude holding system. In this case, even though the aircraft could legally be dispatched, there are crippling conditions that impact the trip.
Other Sources of Information
The aircraft Type Certificate Data Sheets may also be valuable sources of information when it comes to deciphering all the variables involved in deciding if airworthiness is attainable. In the eyes of the FAA and per FAR 91.213, operations with inoperative equipment can be interpreted as having an STC for the situation providing the MEL is properly followed and a valid LOA is on hand.
As with almost all situations, plans are more effective when reviewed and practiced. Perhaps one activity to consider is to assemble all pertinent members of the flight department and spend some time reviewing MEL procedures that deal with known problem systems. This may also be a good time to discuss interpretations. After all having a “generator” warning light illuminated may have an entirely different procedure than having the light inoperative.
Some providers of maintenance as well as flight training have either produced programs for utilization of the MEL or at least will review procedures if a special request is made.
The MEL is a document that will allow for the temporary safe operation of the aircraft until repairs can be made and unfortunately this device is sometimes abused.
Although departure delays are a fact of life in our business, it is the handling of these delays that highlight the true professionals. Having a plan in place where the entire flight department will participate can reduce the stress for all involved.
