|University of Bielefeld - Faculty of technology|
Networks and distributed Systems
Research group of Prof. Peter B. Ladkin, Ph.D.
|Back to Abstracts of References and Incidents||Back to Root|
The first section describes the events of the accident, as reported in the technical press, and includes some observations.
The postcript, added nearly six months later, contains a brief description of the USAF reporting procedures for this accident, and the conclusions that were published in the report.
The following information comes from the press, civilian and military sources. All sources are referenced except for one. I attempted to include only technical information about the equipment and the (possible) progress of the flight and the accident. Any political information is omitted.
Throughout, I offer some personal observations written on 26 April, 1996, before the final report was published. These should be taken as my opinion, not fact - maybe a prediction in April of what could later be concluded - and as appropriate subject for agreement or disagreement under reasoned discussion. Such opinions are offered in the context: [ A personal observation. .......] These observations have not (yet) been checked against the USAF final report. I refer readers to this report for detailed information.
A US Air Force T-43A aircraft, flight IFOR 21, carrying US Secretary of Commerce Ron Brown and other US Government personnel crashed on the approach to Dubrovnik airport, Croatia, on 3 April 1996. It disappeared from radar at 14.52 local time, about 5 mins before expected landing (FI.1).
The accident airplane, a `military 737-200' (FI.1) crashed 98 feet (20 meters) below the summit (NYTS) of a hill to the left of the NDB Rwy 12 approach to Dubrovnik airport.
The aircraft was built in 1973, had accumulated 17,000h and 12,000 landings, and had undergone a major inspection in June 1995 (FI.1).
Minimum Descent Altitude (MDA) on the NDB approach for both straight-in Rwy 12 and circle-to-land is 2150ft. There are two NDBs: KLP at 11.8 miles (Final Approach Fix or FAF, 4000ft altitude) and CV at 1.9 miles (Missed-Approach Point or MAP, where one expects to be at MDA), both in line with the runway extended centerline.
The approach chart (JS-APP) indicates the accident hill clearly, close in to the CV NDB (the MAP). (IHT.1) reported that the aircraft was 1.8 miles off course on the approach; (FI.2) that the site is `1km (0.5nm) off the approach track'. The summit is in fact about 1.5 nautical miles off-course (JS-APP). Final approach course is 119 degrees magnetic. The crash site was reported to be at a bearing of 109 degrees from KLP (NYTS), but the chart shows the relative angle to be the inverse-tangent of 1/7, which is about 7.5 degrees.
[A personal observation. I have heard some lay speculation about potential aerodynamic problems, from people who are presumably thinking about the Colorado Springs and Pittsburgh B737 accidents, and are wondering about potential uncommanded rudder movements with B737s. Anybody who thinks the crash may have been caused by aerodynamic problems has to explain how to get this airplane to fly sideways 1.8 miles while at an altitude of somewhere between 4000ft and 2202ft. Doubtless with the right amount of fairy dust, everything will be possible to those who believe strongly enough!]
A brief look at a picture of the wreckage by an expert flight engineer elicited a comment that the planform of the wreckage was consistent with a CFIT accident, namely that the wreckage was not widely distributed and retained roughly the shape of the accident aircraft.
There's no timing given on the chart, thus flying the approach as a civilian, one must use CV as the MAP. Reception of both NDBs is thus required for the approach. In principle this may be accomplished with one ADF or RMI, but see the (personal) observation below. Anonymous USAF generals are quoted by (NYTS) as calling the navigation equipment of IFOR 21 `primitive' and `rudimentary'. (FI.2) denotes the navigation equipment aboard IFOR 21 as `primitive' and says:
[A personal observation. A think-through suggests to me that, flying the course with one RMI tuned to KLP, it could be nearly impossible to tell one's position relative to CV when near to CV. Visually, in the cockpit, passing an NDB 100m away and passing it 1.5 nautical miles away have similar effects - the needle swings and one swing is merely quicker than the other. Good civilian technique would suggest that the switch to CV should occur somewhere in the mid-approach, because it's generally safer to fly to an NDB than from one: the ADF/RMI becomes less sensitive to course deviations the further away one is from the NDB; correspondingly more sensitive the closer one is. Flying the course with two ADFs/RMIs seems to me to be a lot easier. One could more easily notice course deviations. A deviation of 7.5 degrees from course would yield a difference of 15 degrees between the two needles at the course midpoint.]
No report I've seen has said which beacon the ADF was set to at the time of the crash.
Being 7.5 degrees, 1.5nm off-course would probably be enough to fail the NDB approach check on a civilian instrument flight check ride with an NDB at the MAP.
Some further points made by the press:
Finally, a procedural consequence of the accident is that US Secretary of Defence Perry has ordered `the military' to equip all its `passenger planes' with `much more precise navigation gear' (NYTS).
A glossary of terms is contained below.
Runways are designated according to the nearest 10-degree heading. Dubrovnik's runway ies at a magnetic heading of 119 degrees and is thus designated Runway 12.
The US military uses airframe type designations which are disjoint from the civilian designations of the same or similar aircraft. The engineering specifications of such aircraft may be significantly different (as in the B720 and the military EC-12) or similar. The T-43A is equivalent to a civilian B737-200 airframe (FI.1).
An Approach Plate or Approach Chart is a map which aviators use to guide them in an instrument approach procedure. The approach itself is developed and certified by the appropriate aviation authority (in the case of Dubrovnik, the Croatian, formerly the Jugoslavian, government). The idea of the approach plate came from a certain Captain Jeppesen, whose tradition is continued in the Jeppesen-Sandersen Company.
When one flies an approach, one starts at the Initial Approach Fix (IAF). In the US, one may either pass over an IAF or receive radar vectors (heading commands from a controller) to the approach. There is a Final Approach Fix which one must pass over, and for non-precision approaches a further Missed Approach Point (MAP). The MAP can either be reached by a time-out (i.e., dead reckoning) from the FAF, or can be a designated fix. For the Dubrovnik RWY 12 Approach, it's passage over the CV NDB. One passes over the FAF at or above a designated altitude (4000ft in this case) and must maintain at or above a designated Minimum Descent Altitude (2150ft here) until MAP. If the runway or its threshold or its immediate environment (lights, landing zone, etc) cannot be seen at this point by the pilots, the planned landing must be aborted and a Missed Approach Procedure followed. This Missed Approach Procedure is designated on the approach plate.
I am indebted for the information in this paragraph to Lt.-Col. Thomas Farrier, Chief of Flight Safety Issues in the Office of the US Air Force Chief of Safety.
The USAF does not normally release results of its safety investigations to the public. In cases in which an aircraft accident results in a fatality, the destruction of an aircraft, or more than one million dollars in damage, a separate investigation is conducted, in the USAF governed by Air Force Instruction 51-503. This report is always publically available, based on the same facts, but protects the rights of individuals who may have been responsible for the loss.
The AFI 51-503 investigation was the only one conducted in this case, with the assistance of experts from Boeing, the NTSB and the FAA. The public's right to know all of the information was considered of overriding importance, because of the death of the US Secretary of Commerce. Lt.-Col. Farrier notes that USAF safety personnel acted on that report the moment it was completed. The report is 22 volumes, contained in a box measuring roughly 30 x 30 x 50 cm (!!).
The investigator's `Statement of Opinion, limited in its possible uses by regulation 10 U.S.C. Section 2254, attributes the accident to
Lt.-Col. Farrier points out that the investigator's `Statement of Opinion, limited in its possible uses by regulation 10 U.S.C. Section 2254, attributes the accident to
A report by Ramon Lopez in Flight International (FI.3) summarised the conclusions of the USAF's Accident Investigation Board report.
A general reference for acronyms and definitions is the
US Federal Aviation
Definitions and Abbreviations.
ADF Automatic Direction Finder, a navigation instrument which
indicates the bearing to the NDB generating the signal.
FAF Final Approach Fix
MAP Missed Approach Point
MDA Minimum Descent Altitude
NDB Non-Directional Beacon
RMI Radio Magnetic Indicator, a combined Directional Gyro and ADF
(IHT.1): Old technology Guided the Approach of Brown's Plane, by
Don Phillips and Bradley Graham of the Washington Post Service.
The International Herald Tribune, 6-7 April, 1996, p2.
Back to First Reference
(FI.1): Graham Warwick,
USAF 737 was `off course' before crash,
Flight International, 10-16 April, 1996, p8.
Back to First Reference
737 crashed using `primitive' navigation aid,
Flight International, 17-23 April, 1996, p6.
Back to First Reference
(JS-APP): NDB Rwy 12 Approach
(16-1), Dubrovnik ( http://www.jeppesen.com/lddu161.html), approach chart published by
Back to First Reference
(FI.3): Ramon Lopez,
USAF `broke orders' on CT-43 disaster flight,
Flight International , 19-25 June, 1996, p20.
Back to First Reference
Back to top
Back to `Computer-Related Incidents' Compendium.
|Copyright © 1999 Peter B. Ladkin, 1999-02-08|
by Michael Blume