Narrative:

My aircraft is a 1995 model beech king air C90B with PT6A-21 powerplants and factory installed mccauley 4-BLADE propellers. The propellers were overhauled in 2001 per the manufacturer's 6-YEAR recommendation. I am the second owner. The aircraft was in the maintenance shop this month for a routine phase inspection which is required after each 200 hours of operation. During the pre-inspection engine run-up checks; the mechanic noticed that both propellers were operating at a speed just below the bottom of the green arc on the RPM gauge (approximately 1010 RPM). It is important to realize that the 90 RPM difference between the bottom of the green arc and where the propeller RPM needles actually indicated in this case is seen on the gauge as a distance of less than the width of a toothpick. The gauge numbers get very crowded at the low end of the RPM range (the spacing between graduations is not linear). In fact; if one looks at the gauge from above the horizontal; as you would in a normally seated position; the needle parallax illusion can place it in the green arc. Based upon this condition (and following a call to the mccauley tech rep) the propellers were declared condemned and unairworthy. No discussion about how long this condition has existed; or how it got that way; or in what phase of ground operation this occurs; or whether the propellers have been operated in the green arc by the pilot; just condemned. Furthermore; according to mccauley; there is no non-destructive test available to prove whether the propellers are serviceable. The reason given by mccauley is that 'operating the propellers under 1100 RPM causes the propeller to operate in reactionless mode. This causes very high stresses in the blades and the hubs' (the preceding is a direct written quote from mccauley). However; the time the propellers spend below 1100 RPM during every startup and shutdown is apparently exempt from this reactionless mode high stress? I have talked with mr X; from an engineering co; and others about this issue. My understanding of his explanation is that what we are trying to avoid is a 'reactionless vibration' mode in which two opposing blades can flex one way while their 90-degree-off counterparts are flexing the other way. If the propeller is viewed in a slow motion video; the edge of the disk appears like a ruffled garter; with the blade tips tracing a path that can vary as much as four inches. According to mr X; this can be quite destructive if allowed to exist for a significant length of time. But; he said that it is near impossible to get the propeller to exhibit this odd behavior. In fact; during testing of his own 4-BLADE propeller stc; mr X had to have a baron's high-power propeller wash hitting the back-side of the king air's propeller while they threw buckets full of water at the propeller to initiate the vibration. Furthermore; the 'reactionless vibration' only occurs in a very narrow range of speed (less than 10 RPM wide) down at around 900 RPM. In other words there is a sharp spike in the range of speeds around 900 RPM where the phenomenon can occur if conditions are just right. In summary; there is an extremely narrow speed range; strong external stimulus; and significant time-in-range required to excite the dangerous 'reactionless' mode of vibration; and it is quite far below the conservatively established 1100 RPM minimum speed. In other words; the chances of propellers incurring cracks; when set at just over 1000 RPM as mine are; is essentially zero. He has offered to talk with my maintenance shop and help them understand this primarily because this same shop just condemned a set of hartzell propellers on a B200 for the exact same reason (raisbeck/hartzell propellers are common on ka-200's). The pilots operating handbook in my king air 90 has a table in the limitations section; engine operating limits. Starting: --. Low idle: 1100 (min). High idle --. Takeoff and maximum cont: 2200. There are many other engine operating parameters in the actual table; most of which have footnotes to provide clarification and additional guidance on what to do if limits are exceeded. There is no such footnote associated with the propeller low-idle (min) speed. There is a second table in the pilots operating handbook limitations section which looks like the following: powerplant instrument marking. Red (radial) line minimum limit: --. Yellow arc caution range: --. Green arc normal operating: 1100 to 2200 RPM. Red (radial) line maximum limit: 2200 RPM. Notice that there is no minimum limit red line and no yellow arc called for. There is no redline on the actual propeller gauge in the plane. There is no discussion of why this is. Just prior to this table there is a paragraph with the heading 'propeller rotational speed limits.' one would expect that here is just the place to point out this critical and potentially dangerous 'reactionless mode' of propeller vibration. This is what it says: 'transients not exceeding 5 seconds: 2420 RPM. Reverse: 2100 RPM. All other conditions: 2200 RPM.' there is no discussion or mention of minimum rotational speed limitations or high stress propeller conditions contained in the FAA approved pilots operating handbook/aircraft flight manual in my aircraft. Any reasonable person; upon reviewing the limitations section of the pilots operating handbook/aircraft flight manual (specifically with propeller operating limits in mind); would conclude that 1100 (min) means nothing more than the lower setting of where the propellers normally operate. There is absolutely no hint of a danger zone associated with low propeller speeds. Finally; there is no discussion of dangerous propeller minimum speeds in the initial or annual ground school or simulator training material that all king air pilots receive at the insurance required training each year. Neither flight school X nor flight school Y have included a single word about this phenomena during the 20+ times that my chief pilot and I have attended over the years. I have an entire bookshelf of training binders/books/handouts from these various schools. We have consistently operated our propellers in accordance with the pilots operating handbook/aircraft flight manual and within the green arc (1100-2200) on the propeller RPM gauges by simply adjusting the condition levers as required or setting the throttle levers into 'ground fine.' after starting; the condition lever routinely goes to hi-idle for battery charging per the published 'normal procedures.' taxi operation always requires throttles in 'ground fine' because otherwise the C90 will taxi at 50 KTS. Both conditions bring the propellers solidly into the green arc (no need to account for gauge needle parallax). So; the bottom line here is that I find myself in a situation where I am told that I can have no control over the outcome because it does not matter how I have operated my aircraft or what my pilots operating handbook says. It only matters where the maintenance shop has set the low-idle propeller speed (accounting for gauge parallax critical in this case). After discussions with FBO X and FBO Y; both certified beechcraft repair facilities which have maintained my plane; I am told that the beechcraft maintenance instructions; used to set up the low-idle engine speeds; are also inadequate. The instructions provide only that the low-idle speed adjustment be made based upon turbine gas-generator speed (known as N1 or ng) and engine torque. The instructions do not mention setting or checking propeller speed (known as N2); nor do the instructions mention anything about high propeller stress as a consequence of getting it set wrong; or more specifically getting it set too low. So the shop also finds itself in a situation over which; granted it may have some control; but for which it has been given woefully inadequate guidance from the propeller and/or airframe manufacturer. In my research on this issue I have found that the only guidance that seems to be clear about propeller vibration concerns appears in far part 23.1549. Here is what it says: 23.1549 powerplant and auxiliary power unit instruments. For each required powerplant and auxiliary power unit instrument; as appropriate to the type of instruments: a) each maximum and; if applicable; minimum safe operation limit must be marked with a red radial or a red line; B) each normal operating range must be marked with a green arc or green line; not extending beyond the maximum and minimum safe limits; C) each takeoff and precautionary range must be marked with a yellow arc or a yellow line; and D) each engine; auxiliary power unit; or propeller range that is restricted because of excessive vibration stresses must be marked with red arcs or red lines. What is it about far PAR 23.1549(a) or (D) that is difficult for the manufacturers (who are bound by this) to understand? Or was it just missed? Or was it just not applicable? I cannot believe that such incompetence and obvious failure to communicate on the part of an equipment manufacturer has gone unnoticed or un-addressed; apparently for years; given what I am now expected to recognize is a very serious operational limitation. This issue needs to be fixed and fixed soon because if it really is the dangerous situation that mccauley insists that it is (which I am now beginning to doubt); there are going to be lots of propellers coming apart in flight. At the very least; they had better come up with operating and maintenance instructions and a non-destructive test to look for cracks. Callback conversation with reporter revealed the following information: the reporter reviewed the important points of the propeller situation; starting with the RPM indicators and the lower RPM area spacing of the non liner graduations at 1100 RPM and below. Most noteworthy is the lack of RPM indicator minimum limits markings red arc or radial as required in far part 23.1549 (a) or (D). The reporter stressed the publication's engine operating limits and propeller operating limits had no minimum radial red line limit. The reporter believes this situation; if indeed it is as dangerous as mccauley insists; then some information must be given to operators of be C90B equipment.

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Original NASA ASRS Text

Title: A BE C90B OWNER WAS ADVISED BY A MAINT FACILITY THAT BOTH PROPELLERS WERE NON-AIRWORTHY AND REQUIRED REPLACEMENT DUE TO HAVING BEEN OPERATED BELOW 1100 RPM. IT WAS NOTED THAT THERE IS NO RED ARC OR RADIAL ON RPM INDICATOR AT PROHIBITED RPM AND NO CORRESPONDING WARNINGS IN THE RESPECTIVE ACFT MANUALS OR MAINT INSTRUCTIONS.

Narrative: MY ACFT IS A 1995 MODEL BEECH KING AIR C90B WITH PT6A-21 POWERPLANTS AND FACTORY INSTALLED MCCAULEY 4-BLADE PROPELLERS. THE PROPELLERS WERE OVERHAULED IN 2001 PER THE MANUFACTURER'S 6-YEAR RECOMMENDATION. I AM THE SECOND OWNER. THE ACFT WAS IN THE MAINT SHOP THIS MONTH FOR A ROUTINE PHASE INSPECTION WHICH IS REQUIRED AFTER EACH 200 HRS OF OP. DURING THE PRE-INSPECTION ENGINE RUN-UP CHKS; THE MECHANIC NOTICED THAT BOTH PROPELLERS WERE OPERATING AT A SPEED JUST BELOW THE BOTTOM OF THE GREEN ARC ON THE RPM GAUGE (APPROX 1010 RPM). IT IS IMPORTANT TO REALIZE THAT THE 90 RPM DIFFERENCE BETWEEN THE BOTTOM OF THE GREEN ARC AND WHERE THE PROP RPM NEEDLES ACTUALLY INDICATED IN THIS CASE IS SEEN ON THE GAUGE AS A DISTANCE OF LESS THAN THE WIDTH OF A TOOTHPICK. THE GAUGE NUMBERS GET VERY CROWDED AT THE LOW END OF THE RPM RANGE (THE SPACING BETWEEN GRADUATIONS IS NOT LINEAR). IN FACT; IF ONE LOOKS AT THE GAUGE FROM ABOVE THE HORIZONTAL; AS YOU WOULD IN A NORMALLY SEATED POS; THE NEEDLE PARALLAX ILLUSION CAN PLACE IT IN THE GREEN ARC. BASED UPON THIS CONDITION (AND FOLLOWING A CALL TO THE MCCAULEY TECH REP) THE PROPELLERS WERE DECLARED CONDEMNED AND UNAIRWORTHY. NO DISCUSSION ABOUT HOW LONG THIS CONDITION HAS EXISTED; OR HOW IT GOT THAT WAY; OR IN WHAT PHASE OF GND OP THIS OCCURS; OR WHETHER THE PROPS HAVE BEEN OPERATED IN THE GREEN ARC BY THE PLT; JUST CONDEMNED. FURTHERMORE; ACCORDING TO MCCAULEY; THERE IS NO NON-DESTRUCTIVE TEST AVAILABLE TO PROVE WHETHER THE PROPS ARE SERVICEABLE. THE REASON GIVEN BY MCCAULEY IS THAT 'OPERATING THE PROPELLERS UNDER 1100 RPM CAUSES THE PROPELLER TO OPERATE IN REACTIONLESS MODE. THIS CAUSES VERY HIGH STRESSES IN THE BLADES AND THE HUBS' (THE PRECEDING IS A DIRECT WRITTEN QUOTE FROM MCCAULEY). HOWEVER; THE TIME THE PROPS SPEND BELOW 1100 RPM DURING EVERY STARTUP AND SHUTDOWN IS APPARENTLY EXEMPT FROM THIS REACTIONLESS MODE HIGH STRESS? I HAVE TALKED WITH MR X; FROM AN ENGINEERING CO; AND OTHERS ABOUT THIS ISSUE. MY UNDERSTANDING OF HIS EXPLANATION IS THAT WHAT WE ARE TRYING TO AVOID IS A 'REACTIONLESS VIBRATION' MODE IN WHICH TWO OPPOSING BLADES CAN FLEX ONE WAY WHILE THEIR 90-DEG-OFF COUNTERPARTS ARE FLEXING THE OTHER WAY. IF THE PROPELLER IS VIEWED IN A SLOW MOTION VIDEO; THE EDGE OF THE DISK APPEARS LIKE A RUFFLED GARTER; WITH THE BLADE TIPS TRACING A PATH THAT CAN VARY AS MUCH AS FOUR INCHES. ACCORDING TO MR X; THIS CAN BE QUITE DESTRUCTIVE IF ALLOWED TO EXIST FOR A SIGNIFICANT LENGTH OF TIME. BUT; HE SAID THAT IT IS NEAR IMPOSSIBLE TO GET THE PROPELLER TO EXHIBIT THIS ODD BEHAVIOR. IN FACT; DURING TESTING OF HIS OWN 4-BLADE PROP STC; MR X HAD TO HAVE A BARON'S HIGH-POWER PROP WASH HITTING THE BACK-SIDE OF THE KING AIR'S PROPELLER WHILE THEY THREW BUCKETS FULL OF WATER AT THE PROP TO INITIATE THE VIBRATION. FURTHERMORE; THE 'REACTIONLESS VIBRATION' ONLY OCCURS IN A VERY NARROW RANGE OF SPEED (LESS THAN 10 RPM WIDE) DOWN AT AROUND 900 RPM. IN OTHER WORDS THERE IS A SHARP SPIKE IN THE RANGE OF SPEEDS AROUND 900 RPM WHERE THE PHENOMENON CAN OCCUR IF CONDITIONS ARE JUST RIGHT. IN SUMMARY; THERE IS AN EXTREMELY NARROW SPEED RANGE; STRONG EXTERNAL STIMULUS; AND SIGNIFICANT TIME-IN-RANGE REQUIRED TO EXCITE THE DANGEROUS 'REACTIONLESS' MODE OF VIBRATION; AND IT IS QUITE FAR BELOW THE CONSERVATIVELY ESTABLISHED 1100 RPM MINIMUM SPEED. IN OTHER WORDS; THE CHANCES OF PROPS INCURRING CRACKS; WHEN SET AT JUST OVER 1000 RPM AS MINE ARE; IS ESSENTIALLY ZERO. HE HAS OFFERED TO TALK WITH MY MAINT SHOP AND HELP THEM UNDERSTAND THIS PRIMARILY BECAUSE THIS SAME SHOP JUST CONDEMNED A SET OF HARTZELL PROPS ON A B200 FOR THE EXACT SAME REASON (RAISBECK/HARTZELL PROPS ARE COMMON ON KA-200'S). THE PLTS OPERATING HANDBOOK IN MY KING AIR 90 HAS A TABLE IN THE LIMITATIONS SECTION; ENGINE OPERATING LIMITS. STARTING: --. LOW IDLE: 1100 (MIN). HIGH IDLE --. TAKEOFF AND MAX CONT: 2200. THERE ARE MANY OTHER ENGINE OPERATING PARAMETERS IN THE ACTUAL TABLE; MOST OF WHICH HAVE FOOTNOTES TO PROVIDE CLARIFICATION AND ADDITIONAL GUIDANCE ON WHAT TO DO IF LIMITS ARE EXCEEDED. THERE IS NO SUCH FOOTNOTE ASSOCIATED WITH THE PROPELLER LOW-IDLE (MIN) SPEED. THERE IS A SECOND TABLE IN THE PLTS OPERATING HANDBOOK LIMITATIONS SECTION WHICH LOOKS LIKE THE FOLLOWING: POWERPLANT INSTRUMENT MARKING. RED (RADIAL) LINE MINIMUM LIMIT: --. YELLOW ARC CAUTION RANGE: --. GREEN ARC NORMAL OPERATING: 1100 TO 2200 RPM. RED (RADIAL) LINE MAXIMUM LIMIT: 2200 RPM. NOTICE THAT THERE IS NO MINIMUM LIMIT RED LINE AND NO YELLOW ARC CALLED FOR. THERE IS NO REDLINE ON THE ACTUAL PROPELLER GAUGE IN THE PLANE. THERE IS NO DISCUSSION OF WHY THIS IS. JUST PRIOR TO THIS TABLE THERE IS A PARAGRAPH WITH THE HEADING 'PROPELLER ROTATIONAL SPEED LIMITS.' ONE WOULD EXPECT THAT HERE IS JUST THE PLACE TO POINT OUT THIS CRITICAL AND POTENTIALLY DANGEROUS 'REACTIONLESS MODE' OF PROPELLER VIBRATION. THIS IS WHAT IT SAYS: 'TRANSIENTS NOT EXCEEDING 5 SECONDS: 2420 RPM. REVERSE: 2100 RPM. ALL OTHER CONDITIONS: 2200 RPM.' THERE IS NO DISCUSSION OR MENTION OF MINIMUM ROTATIONAL SPEED LIMITATIONS OR HIGH STRESS PROPELLER CONDITIONS CONTAINED IN THE FAA APPROVED PLTS OPERATING HANDBOOK/ACFT FLT MANUAL IN MY ACFT. ANY REASONABLE PERSON; UPON REVIEWING THE LIMITATIONS SECTION OF THE PLTS OPERATING HANDBOOK/ACFT FLT MANUAL (SPECIFICALLY WITH PROPELLER OPERATING LIMITS IN MIND); WOULD CONCLUDE THAT 1100 (MIN) MEANS NOTHING MORE THAN THE LOWER SETTING OF WHERE THE PROPELLERS NORMALLY OPERATE. THERE IS ABSOLUTELY NO HINT OF A DANGER ZONE ASSOCIATED WITH LOW PROPELLER SPEEDS. FINALLY; THERE IS NO DISCUSSION OF DANGEROUS PROPELLER MINIMUM SPEEDS IN THE INITIAL OR ANNUAL GND SCHOOL OR SIMULATOR TRAINING MATERIAL THAT ALL KING AIR PLTS RECEIVE AT THE INSURANCE REQUIRED TRAINING EACH YEAR. NEITHER FLT SCHOOL X NOR FLT SCHOOL Y HAVE INCLUDED A SINGLE WORD ABOUT THIS PHENOMENA DURING THE 20+ TIMES THAT MY CHIEF PLT AND I HAVE ATTENDED OVER THE YEARS. I HAVE AN ENTIRE BOOKSHELF OF TRAINING BINDERS/BOOKS/HANDOUTS FROM THESE VARIOUS SCHOOLS. WE HAVE CONSISTENTLY OPERATED OUR PROPELLERS IN ACCORDANCE WITH THE PLTS OPERATING HANDBOOK/ACFT FLT MANUAL AND WITHIN THE GREEN ARC (1100-2200) ON THE PROP RPM GAUGES BY SIMPLY ADJUSTING THE CONDITION LEVERS AS REQUIRED OR SETTING THE THROTTLE LEVERS INTO 'GND FINE.' AFTER STARTING; THE CONDITION LEVER ROUTINELY GOES TO HI-IDLE FOR BATTERY CHARGING PER THE PUBLISHED 'NORMAL PROCS.' TAXI OP ALWAYS REQUIRES THROTTLES IN 'GND FINE' BECAUSE OTHERWISE THE C90 WILL TAXI AT 50 KTS. BOTH CONDITIONS BRING THE PROPS SOLIDLY INTO THE GREEN ARC (NO NEED TO ACCOUNT FOR GAUGE NEEDLE PARALLAX). SO; THE BOTTOM LINE HERE IS THAT I FIND MYSELF IN A SITUATION WHERE I AM TOLD THAT I CAN HAVE NO CONTROL OVER THE OUTCOME BECAUSE IT DOES NOT MATTER HOW I HAVE OPERATED MY ACFT OR WHAT MY PLTS OPERATING HANDBOOK SAYS. IT ONLY MATTERS WHERE THE MAINT SHOP HAS SET THE LOW-IDLE PROP SPEED (ACCOUNTING FOR GAUGE PARALLAX CRITICAL IN THIS CASE). AFTER DISCUSSIONS WITH FBO X AND FBO Y; BOTH CERTIFIED BEECHCRAFT REPAIR FACILITIES WHICH HAVE MAINTAINED MY PLANE; I AM TOLD THAT THE BEECHCRAFT MAINT INSTRUCTIONS; USED TO SET UP THE LOW-IDLE ENGINE SPEEDS; ARE ALSO INADEQUATE. THE INSTRUCTIONS PROVIDE ONLY THAT THE LOW-IDLE SPEED ADJUSTMENT BE MADE BASED UPON TURBINE GAS-GENERATOR SPEED (KNOWN AS N1 OR NG) AND ENGINE TORQUE. THE INSTRUCTIONS DO NOT MENTION SETTING OR CHKING PROPELLER SPEED (KNOWN AS N2); NOR DO THE INSTRUCTIONS MENTION ANYTHING ABOUT HIGH PROPELLER STRESS AS A CONSEQUENCE OF GETTING IT SET WRONG; OR MORE SPECIFICALLY GETTING IT SET TOO LOW. SO THE SHOP ALSO FINDS ITSELF IN A SIT OVER WHICH; GRANTED IT MAY HAVE SOME CTL; BUT FOR WHICH IT HAS BEEN GIVEN WOEFULLY INADEQUATE GUIDANCE FROM THE PROPELLER AND/OR AIRFRAME MANUFACTURER. IN MY RESEARCH ON THIS ISSUE I HAVE FOUND THAT THE ONLY GUIDANCE THAT SEEMS TO BE CLR ABOUT PROPELLER VIBRATION CONCERNS APPEARS IN FAR PART 23.1549. HERE IS WHAT IT SAYS: 23.1549 POWERPLANT AND AUXILIARY POWER UNIT INSTRUMENTS. FOR EACH REQUIRED POWERPLANT AND AUXILIARY POWER UNIT INSTRUMENT; AS APPROPRIATE TO THE TYPE OF INSTRUMENTS: A) EACH MAXIMUM AND; IF APPLICABLE; MINIMUM SAFE OP LIMIT MUST BE MARKED WITH A RED RADIAL OR A RED LINE; B) EACH NORMAL OPERATING RANGE MUST BE MARKED WITH A GREEN ARC OR GREEN LINE; NOT EXTENDING BEYOND THE MAXIMUM AND MINIMUM SAFE LIMITS; C) EACH TAKEOFF AND PRECAUTIONARY RANGE MUST BE MARKED WITH A YELLOW ARC OR A YELLOW LINE; AND D) EACH ENGINE; AUXILIARY POWER UNIT; OR PROPELLER RANGE THAT IS RESTRICTED BECAUSE OF EXCESSIVE VIBRATION STRESSES MUST BE MARKED WITH RED ARCS OR RED LINES. WHAT IS IT ABOUT FAR PAR 23.1549(A) OR (D) THAT IS DIFFICULT FOR THE MANUFACTURERS (WHO ARE BOUND BY THIS) TO UNDERSTAND? OR WAS IT JUST MISSED? OR WAS IT JUST NOT APPLICABLE? I CANNOT BELIEVE THAT SUCH INCOMPETENCE AND OBVIOUS FAILURE TO COMMUNICATE ON THE PART OF AN EQUIPMENT MANUFACTURER HAS GONE UNNOTICED OR UN-ADDRESSED; APPARENTLY FOR YEARS; GIVEN WHAT I AM NOW EXPECTED TO RECOGNIZE IS A VERY SERIOUS OPERATIONAL LIMITATION. THIS ISSUE NEEDS TO BE FIXED AND FIXED SOON BECAUSE IF IT REALLY IS THE DANGEROUS SITUATION THAT MCCAULEY INSISTS THAT IT IS (WHICH I AM NOW BEGINNING TO DOUBT); THERE ARE GOING TO BE LOTS OF PROPELLERS COMING APART IN FLT. AT THE VERY LEAST; THEY HAD BETTER COME UP WITH OPERATING AND MAINT INSTRUCTIONS AND A NON-DESTRUCTIVE TEST TO LOOK FOR CRACKS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR REVIEWED THE IMPORTANT POINTS OF THE PROPELLER SITUATION; STARTING WITH THE RPM INDICATORS AND THE LOWER RPM AREA SPACING OF THE NON LINER GRADUATIONS AT 1100 RPM AND BELOW. MOST NOTEWORTHY IS THE LACK OF RPM INDICATOR MINIMUM LIMITS MARKINGS RED ARC OR RADIAL AS REQUIRED IN FAR PART 23.1549 (A) OR (D). THE RPTR STRESSED THE PUBLICATION'S ENGINE OPERATING LIMITS AND PROPELLER OPERATING LIMITS HAD NO MINIMUM RADIAL RED LINE LIMIT. THE RPTR BELIEVES THIS SITUATION; IF INDEED IT IS AS DANGEROUS AS MCCAULEY INSISTS; THEN SOME INFO MUST BE GIVEN TO OPERATORS OF BE C90B EQUIPMENT.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.