Royal Dutch Navy NH 90 accident and helicopter aerodynamics
Updated: Dec 18, 2020
On July 19, 2020 a NH90 Dutch Navy helicopter ditched in the sea near her base vessel located near the island of Aruba. The accident tragically took the lives of two young naval aviators.
The (civil) Dutch Safety Investigation Board (OVV) conducted an abbreviated investigation which mainly indicates factual information about the accident. In addition it poses a number of questions, apparently in order to provide a starting point for the more in depth investigation to be conducted by the Military Safety Board (IVD). Consequently the OVV report did not provide specific safety recommendations as per ICAO Annex 13 format. These are supposedly to be developed by the IVD and most likely will not be made public.
Below is the OVV report for the readers review. It is published in Dutch only and also available on the OVV website.
The report provides a harrowing account of the loss of the lives of two of the four crew members. There are many lessons to be learned from this accident.
Several elements can be addressed, among which; survivability aspects of the aircraft design, effective use of survival equipment, training of crew in conjunction with the used survival equipment, efficacy of used survival equipment itself, efficacy of organisational structure on board the vessel and efficacy of resources available to the ships crew. And also helicopter performance management.
Please note that the OVV report is just a factual summary of the accident itself. Naval and offshore helicopter operations (both civil and military) are a massive global industry from which many safety lessons can be learned. In the follow-up of this accident, a thorough documentary research can potentially provide very useful information to support mitigation and repetition of similar accidents.
In the Civil World of Helicopter Operations in Europe, EHEST addresses helicopter operational safety issues and actively promotes and publishes safety leaflets.
An interesting study, published by the Dutch NLR (Dutch National Aeronautical Research Institute) was found on the EHEST website and placed below with added bookmarks for searcheability. The Study is focussed on technological solutions for helicopter safety issues. Of course there is no replacement for solid training and organisational efficacy but the study is very interesting nonetheless.
Sumburgh Super Puma Accident 2013
Another tragic fatal accident took place in 2013 with a commercial off shore helicopter.
Although the accident took place in very different circumstances with a different helicopter type, there are some similarities with the NH90 accident.
The differences are:
The Super Puma operated in IMC and on instruments until just before impact.
The helicopter was conducting a final approach and was not on downwind leg, like the NH90 was.
The similarities are:
The helicopter was low and slow and below the minimum power speed of 80 knots for the type.
Control was lost when the increasing descent rate was attempted to be arrested just before impact by the pilot flying.
Airspeed was gradually decreasing preceding the accident.
The AAIB report is very comprehensive and provides a detailed account of the cascade of events and addresses a number of survivability and infrastructural aspects as well as helicopter performance elements.
AAIB did corroborate the performance aspects with the helicopter manufacturer's performance models and concluded that the phenomenon of Vortex Ring State was attributable to the accident. See report page 42 section 1.12.4.
In addition AAIB published a comprehensive set of safety recommendations.
Both AAIB accident report and Safety Recommendation report are below for download and are also downloadable from the AAIB website
This blog will not make any comments or any judgement about any issue potentially contributing to the accident. Being an engineering web site, it will however highlight an element that was an important contributing factor to many helicopter accidents in the past and may have played a role in this accident.
It's called Vortex Ring State.
Vortex Ring State (VRS) is terminology generally used and in the USA often called "Settling with Power"
Back to the NH90 accident; The objective of the ill fated flight was to train deck officers in guiding a flight in to a safe tanding on the vessel. Seven safe approaches and landings were conducted but on the eighth pattern, on the downwind leg the helicopter lost height from very low altitude leaving no time for a powered arrest and the helicopter hit the sea, the emergency flotation inflated but the helicopter turned over and remained afloat upside down with the structure submerged. It was determined that the last downwind leg was flown low and slow; in fact so slow that the airspeed would be close to zero knots with a ground speed of around 25 knots. Although not mentioned anywhere in the report (neither confirmed nor excluded), the pilot may have been confronted with the onset of Vortex Ring State (VRS).
A typical helicopter has a power curve similar of a fixed wing aircraft but the power required curve touches the y axis as a helicopter is obviously able to fly (under conditions) with zero airspeed.
In this case, the total power required to maintain level flight is lowest at around 80 knots. If the airspeed slows down from that point, the required power to maintain flight will be higher, not lower. So if during slow flight the helicopter inadvertently slows down, more power needs to be pulled by raising the collective lever. Unless more power is selected, the aircraft will start descending. It looks like this may have happened in the accident, however was not referred to in the report.
What is Vortex Ring State (VRS)
A rotary wing rotor "disk" consists of two or more blades that generate lift in various ways by accelerating air downward, just like on a fixed wing aircraft. This is called "downwash"
Any three dimensional lifting surface (be it a rotor blade or a wing) generates a certain mass or rotating air from its tip.
This is generally know as a tip vortex and is caused by air, driven by pressure difference between upper and lower surface to rotate around the tip. Every blade generates a tip vortex trailing behind the flight path of the tip..
This is even visible by the naked eye in below picture where the atmospheric conditions make the core of the tip vortices condensate
As is visible in the picture the tip vortices trail downward with the downwash of the air accelerated by the rotor blades.
Note that each subsequent blade passes above the tip vortex of the preceding blade.
In forward flight and in hover, the blades sweep over the tip vortices of the preceding blades.
VRS develops when the helicopter descends with a significant rate while in hover or very slow flight. It descends into its own downwash and the blade tips do not sweep over the tip vortices of preceding blades but into them.
Under those conditions, the subsequent blades amplify each others tip vortices which can develop into a very powerful "Vortex Ring" around the circumference of the rotor disk.
This situation is visualised in below picture with help of spray booms attached to an Alouette II helicopter
In a Vortex Ring State, the downward accelerated air is recirculated into the rotor disk with a downward velocity and consequently can not be accelerated any further, limited by the available power and capability of the rotor system. Effectively the actual lifting area of the rotor blades is severely reduced to a point where the still lifting span of the blades can no longer develop the required lift to keep the helicopter aloft and start to stall. This expresses itself in vibration and an accelerating rate of descent that is unable to arrest, even by pulling all the power available.
The only recovery (up to recently) is to somehow achieve forward flight and make the rotor turn in undisturbed air. This is normally trained with an abundance of caution or not even practiced because of the risk of an unrecoverable blade stall that will severely reduce the controllability of the helicopter and potentially be fatal. In helicopter basic training a rule of thumb is that one would never exceed a rate of descend of about 300 feet per minute is slow flight or hover in order to avoid VRS. In large helicopter training this would be limited to simulators only.
Below video shows an example of VRS (which is often called settling with power)
Below is an interesting article (and very readable) by the United States Helicopter Safety Team (USHST) addressing the VRS onset and characteristics. In the Article the hover descent rate safety threshold of 300 feet per minute descend rate is disputed by Sikorsky Senior Technical Fellow Nick Lappos, who states that the VRS safety threshold is 70% of the downwash velocity. This implies that for helicopters with a high disk loading (weight divided by lifting rotor disk surface) generating a high downwash velocity, the safe rate of descend is much higher than for a light single such as the Robinson types.
In the case of the NH90 accident, it would be interesting to calculate this value and determine whether or not VRS would have played a role in the accident by way of exclusion.
Vuichard recovery from VRS
The Vuichard recovery is a technique only recently promoted by Claude Vuichard who developed, used and demonstrated the technique successfully with minimal height loss and a full recovery to controlled flight. At that time he was working with the Swiss Aviation Authorities (FOCA) as a senior flight examiner.
In an Alouette II recovery within 20 - 30 feet is claimed to be feasible.
Remarkable is that this recovery technique was only promoted in 2017.
For many experienced helicopter pilots, this was an unknown recovery technique at the time.
Basically the recovery consist of "sidestepping" the helicopter with the assistance of tail rotor thrust and bringing the helicopter into the upwind part of the vortex ring.
Below linked video is a powerful and compelling demonstration of the technique.
There is an abundance of helicopter safety publications of a huge diversity. Helicopter operations take place in very diverse circumstances, be it in mountainous terrain or naval operations where totally different sets of specific risks are driving decisions.
The military Safety Board, that will be following up on the accident, will have an abundance of information to process and has the possibility to tap from both civil as well as military naval and offshore world to develop a comprehensive set of safety recommendations in order to prevent a similar accident. This is owed to the families of the naval aviators that sadly lost their lives in this tragic accident.