Functional Check Flights 4/4 – Executing a Functional Check Flight

Some guidelines on the briefing are useful here.

1. All involved parties need to be present and listening. Let us remember that this is a pre-flight briefing, not a long maintenance diatribe on what has been done item by item to the aircraft. Such data should already have been reviewed and frankly anyone can only remember a certain amount of detailed information at a time. The briefing is run by the Captain or the Check Engineer and needs to stay relevant to the flight. By all means have background technical people there to answer any questions that may arise.
2. Everyone must understand the task, their role in that task, the planned check sequence and the way in which the flight will be conducted. Any limits and key words should be agreed.
3. The weather needs to be specifically briefed with regards to any impact on any of the abovementioned aspect of the task.
4. Likewise the airfield and ATC and airspace situation must be reviewed.
5. A brief flight risk assessment should be made. This deals with the practical “what will we do if this or that happens?” question. It is not a deep engineering risk assessment but rather a review of the sequence assuming that things may not always go exactly as planned. It should include the things most likely to cause a problem and the fall back plan should they happen.

The pre-flight preparation should consider any need for FMS programming regarding fuel transfer and also back up flight plans in case the maneuvers flown early in the plan erase waypoints. Also electrical checks can sometimes cause some interesting computer responses on modern aircraft.

As stated, it is expected that most airlines would use the standard checklists in the run up to getting airborne in their normal way. The difference is that a third crew member will probably be present in the jump seat. His or her role is to record data and to monitor the work of the pilots in a non-intrusive way but with a “right of intervention” should something occur that he doesn’t understand or that he thinks may be incorrect.

Irrespective of the good use of checklists, along with most of the test fraternity, always carrying out a quiet final configuration check just before take-off and also just before landing seems a useful safety habit to develop.

Really good crew communication throughout the check flight is required. Key words are sometimes useful.
These can include commands such as STOP or GO AROUND but there are also some unwritten but absolutely clear rules for events such as one crew member not being comfortable with the test progression.

If any test / check crew member says “I am not happy” the active pilot recovers immediately and the crew reviews the situation. Likewise if someone declares themselves as being “out of it” through workload or whatever, again, a recovery is carried out and then a re-brief to ensure all crew members are mentally on the same test point with the same level of understanding of the plan. Even silences need to be “listened to” as they can tell you that another crew member may be concerned about something. After a while it is possible to develop a “nose” for when it is not going according to plan and that is the time to slow it down and think about what is happening and whether the plan still makes sense. The pacing has to be led by the slowest crew member but of course there are situations where ATC has no choice but to dictate the check pace such as when you are in the pattern or on the approach. Often the aircraft may be carrying a snag or two by this stage and the impact has to be continually re-assessed against the “remain to do” checks. This is where good check crews work together to continually formulate a new and safe plan of action.

As regards external communication, if there are significant radio problems, then the safe continuation of a check flight quickly becomes very challenging and it may well be wiser to concentrate on getting on the ground safely to get the radios fixed before continuing with other checks.

Workload also has to be continually assessed on an individual and group basis. One person may become overloaded for a short while but if two out of the three reach this state then the situation can become very critical very quickly. The whole crew must never be allowed to reach this state, so if the test crew is only a two person crew the increased threat is obvious. A third, check qualified, crew member for this type of work is therefore strongly recommended.

One of the problems with workload is that it can rise very quickly and in such a way that the individual concerned, although aware that he or she is working too hard, is unable to take the decisions that will reduce that potentially dangerous situation. The person involved may even be unable to “see” the problem, never mind the solution.

As the workload increases the crew has to prioritize the tasks. The first priority is always securing the safety of the aircraft. Easy to say, but often this requires some tough decisions to be made and sometimes ones that local management may not be too happy with. Someone, normally the Captain, has to make it clear that until the technical systems issues are resolved or their implications fully understood, no more check points will be carried out. Enter a hold or ask for a vector away from the airfield, to give thinking time, are useful workload reducing techniques. If the crew has a problem that they do not understand they should put the aircraft back on the ground while they think about it. There is no room for “pressing on” when a situation is not understood and may be potentially dangerous or worse, catastrophic.

Secondly, the objective is to secure good quality check or test data. There is no point in being there to gather poor data that the engineers cannot use. And finally the whole process should be carried out as expeditiously as possible. It is not a pleasure flight, although when done well, it’s extremely enjoyable. The objective is to re-clear the aircraft so that it can get back into the air quickly and re-start earning revenue with passengers on board.

The whole purpose of a check flight is be able to give an aircraft a clean bill of health, so it is not surprising that if a snag is found there is a desire to find out as much as possible about that snag to help the mechanics.

Laudable as this may sound, it can lead quickly to some very unhealthy situations. Great care needs to be taken when “snag chasing”. The implications of one failure needs to be understood across all the systems affected, as do the implications of selecting certain associated systems into a degraded mode so as to “isolate” a snag. Remember too, that there may be another dormant but un-reported snag in the system already, which when coupled with the original snag and the crew switching may put the aircraft into a serious risk area. We tend to think, with modern aircraft, that everything is captured by the BITE system or is presented to us through the Flight Warning Computers but this is not so. This brings us back to the issue of integrity and if the crew do not know all the ramifications of complex and multiple switching actions then they simply should not do it. Put the aircraft on the ground, examine the situation very carefully, call the manufacturer if in doubt and only then proceed after having tried to fix the problem.

No anomalous indication on an aircraft appears for no reason. Some are small, some have little operational significance, some are intermittent (the worst kind) but there is always a reason. It is no good just hoping a snag has somehow just “gone away”. It may indeed not be easy to reproduce the symptoms or it may be limited to certain very precise flight or meteorological conditions but it will still be there and if left, these types of snag have a habit of returning at the worst possible moment. Sometimes the smallest of apparent issues can lead to failure scenarios with some very serious consequences. Watch out particularly for snags associated with “enabling” functions like weight on wheels switches and sensors. Their impact can be seen over several systems. Pressure controllers are another area where a snag can turn from fairly benign to very serious pretty quickly.  

Note: In case the crew also needs to perform a Certificate of Airworthiness renewal which also includes checks, the crew must mentally separate these two demands and if possible clear the aircraft of snags first prior to completing the C of A renewal check points. If that is not possible (sometimes it is not) then crew awareness and good communication is essential.

Some checks are certainly more difficult to fly than others or some may have a more immediate impact if they go wrong. The failure of a generator to come back on line does not have the same immediately damaging effect as allowing the speed to exceed Vmo by too much. So it is sensible to treat the “tricky test points” with the care they demand and not to rush them. Think about them on the ground carefully and decide how they should be flown and what the “break off” point is. These tricky tests can include speed limit checks, envelope boundary checks, depressurizations, initial handling checks, low speed checks and of course some engine checks.

It is also important not to become tempted to “take a look at” some of the certification test points. There are many “interesting” experiences in this category that could have developed into sad stories. Taking one example like Vmca definition, fuel starvation on some types of aircraft has occurred in the past on this test (which is done at very low altitude) causing the remaining engine to stop. The job of the checker is not to try to re-define the basic certification criteria of the aircraft. Those criteria have been flown and examined by experts under strict weather conditions and rigorously controlled conditions. The checkers job is to check “this” line aircraft against a pre-defined and approved Airworthiness standard or to clear a reported squawk or snag.

With a modern aircraft check flight there are several “plans” being conducted at the same time. You have the desired planned check schedule. You have the approved Air Traffic Flight Plan which may involve some “on airways” flying and will often start with some sort of procedural departure. The FMS may have to be set up to a slightly different plan to ensure some functions work as they should like fuel transfer logics. There is also the Flight Warning Computer flight phase plan which may throw “stored” snags at you at pre-determined times and you also have an Air Traffic Control handover plan which drives the communication world and to some extent the workload. Finally, remember that you have no control over the most important “plan” and it’s called “the weather”.

The crews’ job is to safely carry out the check points whilst also conducting this “orchestra” of differing plans not all of which are in sequence and not all want to align conveniently. It’s not unusual to have a check point set up and ready, only to be asked to change frequency, squawk and then head straight towards a Cb.!! Or you may require an altitude or a block of altitudes only to run out of the ideal bit of airspace in which to do the next point.
Patience is required and it is this aspect that benefits most from pre-planning, a good weather examination and pre-consultation with the ATC guys. It may be that on some days it simply becomes impossible and the sensible conclusion is to keep it safe and call it a day. Such judgments are not easy as they often have a considerable cost implication.

Increasingly the area of cabin testing, as said before, is becoming more and more important. Complex seat systems and entertainment systems prevail and it is worth getting to know basically how they work. With the new larger aircraft, there is much closer integration between cabin systems and the flight deck, so cabin systems are no longer “something back there”. They are “passenger important” check areas that have to be thought about quite hard.

This whole paper could have dealt with pressurization issues that have occurred during testing but in order to be brief it is worth thinking about emergency oxygen if a depressurization is planned. Plan which oxygen sets the crew will use. The typical therapeutic oxygen bottles may be “a bridge too far” for someone working in the back of the aircraft to get to. Try getting one out of its stowage and in use in 20 secs and remember that if you are in the cabin checking something you may have to walk some distance to get to the bottle. A better idea is to select and allow a few well-placed oxygen masks to drop in the event of a full de-pressurization, so that the cabin checker can immediately take a seat and then breathe oxygen with the nearest passenger system.

Think also about communication with the guys in the back and ensure the ability to inform them of what is going on and when to be strapped in. Likewise, there are many tasks they can help with like wing inspections and they will need to be able to communicate with the flight deck.