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Reliability Series - 2 of 5

Updated: Sep 7, 2022


In our last blog entry “Causes of UAV Loss” we explained what are the main causes of UAV losses and when they happen.

The US Department of Defense document “Unmanned Aircraft Systems Roadmap 2005-2030” uses the following definitions to categorize areas of a system failure leading to mission aborts or cancellations.

Power/Propulsion (P&P).

Flight Control. Includes all systems contributing to the aircraft stability and control such as avionics, air data system, servo-actuators, control surfaces/servos, onboard software, navigation, and other related subsystems. Aerodynamic factors are also included in this grouping

Human Factors/Ground Control.



Now, let’s talk about cause #2, Flight Control

Cause #2: Flight Control

As with any aircraft, all flight control systems and subsystems are subject to high stress all the time.

Servo malfunction, pitot tube blockage, navigation errors, rudder or elevator malfunction, software, avionics, and even aerodynamic design issues are very common in unmanned aviation.

Although the term “Flight Control” might make you think refers to airborne incidents, it also refers to taxiing and the ability to safely maintain runway heading during takeoff or landing.

Military and commercial aviation started a long way back and dealt with flight control problems from the beginning.

The first plane incident happened on July 14th, 1905. “Orville (Wright) smashed into the ground at more than 30 miles per hour (48 kilometers per hour), crumpling the front elevator. The Flyer III bounced three times, throwing Orville out through the top wing. Wilbur found him dazed and confused, lying on what was left of the elevator. Orville was still in one piece, but this potentially fatal crash forced the brothers to take a long, hard look at their aircraft design”. Looks like a Flight Control issue to us. Source:

According to the list of accidents and incidents involving commercial aircraft, the first recorded accident was on July 21st, 1919, “The Goodyear dirigible Wingfoot Air Express catches fire and crashes into the Illinois Trust and Savings Building in Chicago, Illinois, while carrying passengers to a local amusement park, killing thirteen people: three out of the five on board and ten others on the ground, with 27 others on the ground being injured.”

The list does not give a clear clue on the causes of the accidents or incidents, what we found is that the first accident that clearly states a flight control issue was on May 14th, 1923, “An Air Union Farman F.60 Goliath crashes near Monsures, Somme, France, due to the structural failure of a wing, killing all six on board “. The list can be found at:

How can we minimize this risk? Including redundancy features and/or using military grade and/or certified components. The downside is that manufacturing cost are higher but, when you have a few million airborne, considering the plane and payload, it could provide a real advantage.

Here are some examples of crashes due to flight control issues

Nov 8 2015 US Air Force MQ-1 Predator Mid-flight Near Ali Al Salem air base, Kuwait




MQ-1B Predator, T/N 98-3040


8 November 2015

On 8 November 2015, at approximately 1653 Zulu time (Z), the mishap remotely piloted aircraft (MRPA), an MQ-1B Predator, tail number (T/N) 98-3040, forward deployed from the 432d Wing, Creech Air Force Base (AFB), Nevada, conducted a combat support mission in the United States Central Command (USCENTCOM) area of responsibility (AOR). At the time of the mishap, the mishap launch and recovery element (MLRE) from the 46th Expeditionary Reconnaissance Squadron operated the MRPA from a deployed location in the USCENTCOM AOR. The MRPA experienced a left tail control surface failure, departed controlled flight, and crashed en route to the intended base of landing. The estimated cost of aircraft and munitions is $5.3 million. There were no reported injuries, deaths, or personal property damages.

At approximately 1638Z, the MLRE took handover of the MRPA from the mission control element. The aircraft was returning early due to an Outside Air Temperature sensor failure. The handover was uneventful. Over the next 15 minutes, the MLRE began a descent for landing and started working through appropriate checklist procedures for descent and arrival.

Evidence showed that the left tail and tail insert fell off the MRPA. Consequently, the MRPA became uncontrollable. The aircraft entered an unrecoverable spin and the impact destroyed the aircraft. All indications are that the maintenance personnel correctly complied with all maintenance actions and were not a factor to this mishap.

The Abbreviated Accident Investigation Board (AAIB) President found by a preponderance of the evidence that the cause of the mishap was the failure of the left tail clamp and/or left tail clamp bolts. The failure resulted in the airborne loss of the left tail insert and attached left tail. The loss of the left tail insert and the attached left tail surface resulted in an unrecoverable departure from controlled flight.

Full report at:

May 5 2015 US Air Force MQ-9 Reaper Classified



MQ-9A, T/N 11-4138


05 May 2015

On 5 May 2015, at approximately 0134 Local (4 May 2016 at 2234 Zulu (Z)), the mishap aircraft (MA), an MQ-9A, tail number 11-4138, assigned to the 432d Wing, Creech Air Force Base (AFB), Nevada (NV) and operated by the 33rd Expeditionary Special Operations Squadron, 435th Air Ground Operations Wing, veered off the runway after landing and crashed after conducting an intelligence, surveillance, and reconnaissance mission in the United States Africa Command Area of Responsibility. The MA sustained extensive damage and the wreckage was recovered. The damage to U.S. government property totaled approximately $6,703,852.06. There were no fatalities, injuries or damage to civilian property.

Two separate flight crews operated the MA during the launch and recovery of the mishap flight. The recovery mishap crew (MC1) consisted of mishap pilot 1 (MP1) and mishap sensor operator 1 (MSO1). The launch mishap crew (MC2) consisted of mishap pilot 2 (MP2) and mishap sensor operator 2 (MSO2). All crews were current and qualified. All MA and Ground Control Station (GCS) maintenance records and inspections were completed and reviewed.

At approximately 0415Z on 4 May 2015, MC2 conducted an uneventful launch of the MA. At the end of the mission, the Mission Operations Supervisor completed a handover with MC1 to take control of the MA. At 2141Z, MC1 noticed they had a nosewheelsteering problem while conducting normal control checks during the descent to the deployed airfield. MP1 noticed after the control check that the nosewheel was frozen left at approximately 12 degrees from center. MC1 reviewed the emergency procedures section of their technical orders, noting that it did not cover a procedure for landing with the nosewheel frozen off-center. After troubleshooting, MC1 and maintenance concluded that the nosewheel servo had failed and was unfixable while airborne.

MP1 landed MA at 2234Z on the main landing gear while holding the nosewheel off the runway in order to reduce airspeed before the nosewheel touched down. Approximately 3000 feet down the runway, the nosewheel touched down and the MA veered left towards the runway edge. MP1 immediately applied full right rudder and right brake to maintain runway heading, but MA continued off veer off the runway. The MA departed the runway and came to rest approximately 20 yards from the runway edge, damaging the airframe and equipment.

The Abbreviated Accident Investigation Board President found by a preponderance of evidence that the cause of this mishap was a failed nosewheel servo driver that resulted in the inability of MC1 to safely maintain runway heading after landing.

Full report at:

Jun 27 2014 US Air Force MQ-1B Predator Mid-flight (mechanical failure) Nevada, US



MQ-1B, T/N 00-3068

Nevada Test and Training Range

27 June 2014

On 27 June 2014 at approximately 1301 hours local time (L), an MQ-1B, tail number 00-3068, crashed on the Nevada Test and Training Range (NTTR) approximately nine miles from Creech Air Force Base (AFB), Nevada, after it departed controlled flight following a missile launch from its left wing at approximately 1258L. The mishap remotely piloted aircraft (MRPA) belonged to the 11th Reconnaissance Squadron (11 RS). All members of the mishap crew (MC) were assigned to the 15 Reconnaissance Squadron (15 RS). The 11 RS and 15 RS are assigned to the 432nd Wing (432 WG), Creech AFB, Nevada. The MRPA, one missile, two missile electronic control units, and two missile rails were destroyed. Damage to United States government property totaled $4,624,512. There were no fatalities, injuries, or damage to other property.

The MRPA took off at 0633L, loaded with two live missiles, and flew for 5.5 hours prior to the MC taking control. Between takeoff and 1200L, the MRPA was used to conduct multiple training missions for crews other than the MC. The MC took control at approximately 1200L to conduct a training mission practicing missile launches at ground targets. The MC completed several simulated missile launches before attempting to actually launch one of the live missiles.

At 1259:35L the MC commanded the live missile on the left wing to launch. At 1259:37L three events occurred simultaneously. First, the MC received a warning the right wing control module (RWCM) had failed. Second, the RWCM lowered and locked the right wing aileron into a full down position. Lowering an aileron on only one wing of the MRPA causes the MRPA to roll and turn in the opposite direction. Third, the MRPA began a left roll and turn, un-commanded by the MC.

At 1259:38L the MRPA computer indicated the missile had fired. Shortly after that, the MRPA reached a roll angle of 60 degrees left, which angle was too steep to maintain controlled flight. At 1259:41L the MC’s data feed froze, eliminating the MCs ability to monitor the positioning and orientation of the MRPA. At 1259:46L, the MC received a warning indicating the MRPA lost satellite communication. The MRPA continued out of control until impacting the ground at approximately 1301L.

The Abbreviated Accident Investigation Board (AAIB) president found, by clear and convincing evidence, the cause of this mishap was the failure of the RWCM. There was no action that the MC could have performed which would have prevented the MRPA from going out of control.

Full report at:

Reference Library:

We have found very illustrative the following documents or web pages:

US Department of Defense: Unmanned Aircraft Systems Roadmap 2005-2030.

Although old, this document shows an in-depth understanding of how drones started to become a core element in military operations, the implications of UAV reliability, the regulatory framework, and the future of UAV development. The full document can be found at:

Drone Wars UK: Accidents Will Happen

Drone Wars published a dataset of just over 250 large military drone crashes that have taken place over the past decade (2009-2018). You can find the links and document here:

Dedrone: Worldwide Drone Incidents

This page keeps a log of all reported drone-related incidents worldwide, from a small drone invading airport airspace to a drone trying to deliver drugs and phones into a prison yard. Here is the info:

George Slensky: Analysis of UAV Military Aircraft Mishaps

Mr. Slensky analyses the main causes of US military aircraft both, manned and unmanned.

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