What do you do when air traffic control "asks" you to perform a high speed descent? They may not use those words, but you know that their request for you to "hustle on down" is the same as saying, "if you don't, expect some very long vectors." So you do it. Now, depending on how you execute your orders, you might be on a razor's edge with the aircraft's limitations. It isn't a good place to be.
— James Albright
This crew found themselves in this situation and didn't understand the correct way to correct from a small dip into "VMO-land" without hurting anyone or the aircraft. It provides a good lesson for any of us flying in the real world.
- Date: 13 September 2017
- Time: 1610
- Type: Boeing 737
- Operator: Virgin Australia
- Registration: VH-VUE
- Fatalities: 2 of 5 crew, 0 of 151 passengers
- Aircraft Fate: No damage
- Phase: Descent
- Airport: (Departure) Melbourne, Victoria
- Airport: (Destination) Adelaide, South Australia
On the afternoon of 13 September 2017, the crew of a Boeing 737-800 aircraft, registration VH-VUE (VUE) and operated by Virgin Australia (VA), were operating a scheduled passenger service from Melbourne, Victoria to Adelaide, South Australia. The scheduled departure time for this flight was 1605 CST.
The flight crew comprised the captain and the first officer (FO), and these pilots had flown VUE together on the preceding flight. The cabin crew comprised the cabin supervisor (CS) and three other cabin crew. There were 151 passengers on-board.
In preparation for the flight to Adelaide, the flight crew reviewed various information relating to the en route and destination weather conditions. The captain briefed the CS, advising the expected flight time was 65 minutes and that the forecast weather was the same as local conditions in Melbourne, which the CS perceived as cloudy.
For the flight to Adelaide, the FO was pilot flying (PF) and the captain was pilot monitoring (PM). At 1610, VUE departed from Melbourne and climbed to flight level (FL) 360. The departure, climb and cruise were uneventful.
At about 1642, the aircraft was south-east of Adelaide, maintaining FL 360 and approaching top of descent. Air traffic control (ATC) issued the crew clearance to conduct a DRINA NINE ALPHA standard instrument arrival route (STAR) for an approach to runway 23, and when ready descend to FL 250. This STAR provided tracking information including a series of waypoints, altitude and speed restrictions, positioning the aircraft to the north-east of Adelaide to commence an approach to runway 23.
Source: ATSB Safety Report, p. 1
ATSB Safety Report, fig. 1
Figure 1 shows the flight path of VUE, with the DRINA NINE ALPHA STAR waypoints overlaid. [...] This procedure required inbound aircraft track to the DRINA waypoint, then to pass overhead COMLY at or below 13,000 ft.
A few minutes after issuing the STAR, when VUE was 136 NM (252 km) south-east from Adelaide and 93 NM (172 km) from the DRINA waypoint, ATC canceled all speed restrictions and instructed the crew to maintain maximum speed on descent, which the pilots understood as an instruction to make a high-speed descent. The FO said to the captain that they would ‘see how (the aircraft would) go’ with an airspeed of 320 kt, but that this might be too fast. The captain responded by saying ‘she’ll be right, don’t overspeed’.
Source: ATSB Safety Report, p. 1
VMO for this aircraft at this point was 340 kt. (From the ATSB Safety Report, p. 13) Selecting 320 kts seems about right.
Prior to commencing the descent, the crew set up the aircraft’s flight management system (FMS) based on the ATC clearance, and then commenced the approach briefing. The crew programmed a selected airspeed of 310 kt into the FMS.
At about 1652 the captain briefed the CS for the descent, advising that the aircraft would commence its descent to Adelaide in around 30 seconds, and that the expected arrival time was 15 minutes earlier than planned.
Soon after VUE began descending, airspeed started to increase. The FO made a number of changes to the autopilot mode and settings in order to prevent airspeed from increasing too much. However, airspeed continued to increase. The crew remarked that the changes to the autopilot mode and settings did not help much, with the FO saying that the autopilot ‘doesn’t (manage airspeed) very well . . . it just doesn’t like to hold her steady’. The captain reminded the FO ‘don’t overspeed’.
Recorded data indicates the crew began increasing the selected airspeed on the mode control panel (MCP) incrementally during the early stages of the descent when approaching the start of the STAR. (Figure 11 in Appendix B – Flight data recorder data shows changes to the selected airspeed, actual airspeed, autopilot modes and other recorded parameters during the descent.)
Source: ATSB Safety Report, p. 2
ATSB Safety Report, fig. 11
At about 1654, when VUE was descending through FL 335, the flight crew made the ‘cabin crew prepare for landing’ announcement. Shortly after, the flight crew continued the approach briefing, including briefly reviewing threats, then performed the descent checklist.
The crew selected the briefed descent airspeed of 320 kt at about 1656, when the aircraft was descending through FL 250, and the aircraft reached that speed soon after. VUE remained at 320 kt for the following few minutes.
About two minutes later, the aircraft was approaching waypoint DRINA descending through FL 220, when the FO saw the tops of stratocumulus cloud, which the FO thought were about at FL 150.
The FO suggested that it may be appropriate to activate the fasten seat belt sign. The captain responded that the conditions at the time were good, and the FO agreed with that assessment. The captain asked the FO if there were any reports or concerns of turbulence. The FO responded that there had been no reports and was not worried. The fasten seat belt sign remained off.
According to information provided in ATSB interviews after the accident, the captain also wanted to leave enough time for the cabin crew to complete their duties. The FO thought the clouds beneath VUE might be associated with turbulence later in the descent, but at that time the FO was comfortable with the seatbelts sign remaining off for ‘a bit longer’.
At around this time, the FO reduced the selected airspeed to 310 kt. Soon after, VUE commenced the turn towards COMLY.
At about 1659, VUE was 15 NM (28 km) from COMLY and descending through FL 170. Airspeed reduced by around 10 kt to the selected 310 kt.
Airspeed then started to increase, first gradually, then more rapidly. The flight crew observed the indicated airspeed approaching VMO (maximum operating speed), and the FO observed the airspeed trend indicator move past the lower band of the red bars on the primary flight display airspeed indicator. Both pilots expressed statements of concern and alarm.
Source: ATSB Safety Report, pp. 2 - 3
I am surprised by two things here. First, the pilot flying was reacting as if a passenger. If the speed is getting too high and the throttles are already at idle, there are two options: reduce the pitch or add speedbrakes. Second, I am surprised that both pilots "expressed statements of concern and alarm." Having the speed move past the lower band of the red bars doesn't seem like the time for alarm. It is, rather, a time to do something calmly and methodically.
The captain called ‘pull-up’ while also making two abrupt nose-up inputs on the left (captain-side) control column. The first was 49 lb (about 22 kg), which caused the autopilot to disconnect. The captain abruptly released the controls and then made a second control input of 28 lb (about 13 kg) about 4 seconds later.
Source: ATSB Safety Report, p. 3
These are two very large control inputs, which in my opinion are indicative of panic.
The FO heard the autopilot disconnect audible alert and saw the captain pulling on the control column, and acknowledged that the captain had control of the aircraft.
About 8 seconds after the captain’s initial nose-up input and autopilot disconnect, the captain prompted the FO to resume duties as pilot flying, and a normal transfer of control was performed. The flight crew perceived they had encountered severe atmospheric turbulence. Shortly after the FO resumed pilot flying duties, the captain said ‘better put the belts on’, to which the FO responded ‘check’.
The FO continued to fly the aircraft for around 30 seconds before re-engaging the autopilot. During this period, the FO made several nose-down inputs, peaking at 32 lb (about 15 kg) 10 seconds after the initial nose-up input. Shortly after the autopilot was re-engaged, cabin crew contacted the flight crew and reported that the cabin was not secure, and that one of the cabin crew members at the rear of the aircraft had broken their leg.
With the captain communicating with the cabin crew, the FO (who was pilot flying) notified ATC that VUE would not meet the height requirement for COMLY due to encountering turbulence. ATC advised the crew that they could cancel all height requirements and reduce their speed. Around 90 seconds later, ATC contacted the crew of VUE to confirm their report of severe turbulence. The captain responded in the affirmative and added that the turbulence was associated with some cloud.
The cabin crew recalled that the flight had been smooth during the cruise and initial descent, with no turbulence experienced. At the time the flight crew made the ‘cabin crew prepare for landing’ announcement, the in-flight food and drink service had been completed and rubbish collected. Following this announcement, the cabin crew completed duties including securing the cabin for landing. The cabin crew then returned to the forward and rear of the aircraft, with the two cabin crew members in the rear of the aircraft standing in the galley eating a meal. Cabin crew members recalled an abrupt upset in the cabin, which they perceived as sudden and without warning. One of the cabin crew members in the rear galley was thrown up towards the ceiling, then fell to the floor. The cabin crew member felt their leg snap on landing, and was unable to move from the floor. The sudden changes in pitch attitude also injured the other crew member in the rear of the aircraft. This crew member struck their jaw on the galley bench and had other minor injuries to their body and face. The CS reported to the ATSB that they experienced muscular skeletal injuries from the accident and had sought out chiropractic care. After the aircraft had stabilised, the cabin crew member who suffered a leg injury was given oxygen using a portable on-board cylinder. At about 1704, the CS confirmed with the flight crew the requirement for an ambulance and that the injured cabin crew member was unable to move to a seat for landing and would stay on the galley floor. The CS then made positional changes among the cabin crew, moving the uninjured cabin crew member from the forward cabin to the rear, and the cabin crew member who sustained minor head injuries to the front. The CS also briefed able-bodied passengers in the last seating row about what the cabin crew might request, to assist with disembarkation if the situation escalated or in case of another emergency. The cabin crew members took their assigned seats for landing.
Source: ATSB Safety Report, pp. 3-4
The "severe turbulence" claim appears to be false.
The captain joined Virgin Australia (VA) as a Boeing 737 (737) first officer in 2003, and received command upgrade in 2012. The captain had a total flying experience of around 18,000 hours, including over 4,000 hours as captain and almost 6,000 hours as a first officer in the 737.
Source: ATSB Safety Report, p. 6
The FO joined VA as a Boeing 777 second officer in 2010, and qualified as a 737 FO in 2012. The FO had a total experience of around 8,500 hours, including around 3,500 hours in the 737.
Source: ATSB Safety Report, p. 6
VH-VUE was a Boeing 737-800. There were no indications that any mechanical issues with the aircraft were contributory to the accident. After the accident, maintenance personnel conducted a severe turbulence inspection which did not identify any damage or faults.
Source: ATSB Safety Report, p. 6
When the autothrottle is engaged in a mode to control airspeed, the airspeed is adjusted by increasing or decreasing the engine thrust. Alternatively, when the AFDS is in a mode that controls airspeed, airspeed is adjusted by changing the pitch attitude to increase or decrease the drag. To increase the airspeed, the pitch is reduced (nose down), and to decrease the airspeed the pitch is increased (nose up). Depending on the mode engaged, the AFS alters the pitch or thrust commands to attain and/or maintain either the airspeed selected in the flight management system or on the MCP.
Sudden changes in the magnitude of head/tailwind can have the effect of changing the airspeed. For example a decrease in a tailwind can result in an effective increase in airspeed, all other factors being equal. In order to maintain the target airspeed, the AFS will either pitch the nose of the aircraft up or down, or increase/decrease thrust (depending on the selected mode).
Source: ATSB Safety Report, p. 7
Pilots can extend the 737 speed brakes to increase drag, using the speed brake lever. The Boeing 737 Flight Crew Training Manual (FCTM) provided procedures for using the speed brakes in-flight. These procedures said that ‘the speedbrake may be used to correct the descent profile if arriving too high or too fast’. These procedures also noted that using speed brakes enabled the aircraft to decelerate up to 50 per cent more quickly.
For this event, the pilots did not use speed brakes to reduce airspeed prior to the overspeed or during the recovery.
Source: ATSB Safety Report, p.
ATSB Safety Report, fig. 8
Personnel from VA secured a copy of the quick access recorder (QAR) data for analysis and provided a copy of that data to the ATSB. The FDR and cockpit voice recorder (CVR) were also removed and sent to the ATSB laboratory in Canberra for analysis. The following describes the recorded sequence, changes to the aircraft autopilot modes and key flight data parameters during the descent into Adelaide:
- The flight crew commenced the descent with the autopilot engaged. Shortly after commencing the descent, the FO selected the level change AFS mode. There was a gradual increase in airspeed from around 1653:30, when VUE was descending through FL 340.
- At 1654:10, the FO briefly changed the AFDS command mode to vertical speed, with a setting of -650 ft/min, winding back to -450 ft/min. This was consistent with the pilots controlling the aircraft speed by reducing the descent rate. A few seconds later, the level change mode was re-engaged.
- The computed airspeed reached the selected target airspeed (320 kt) about 1656:30, as VUE was passing through FL250. For the next few minutes, airspeed was stable at around 320 kt.
- Figure 8 shows the changes to the autopilot modes and airspeed from 1658:00, when VUE was descending through around FL 200. Figure 9 shows changes to the flight controls and the aircraft vertical acceleration during that period.
- At 1658:18 the FO reduced the target airspeed to 310 kt. While the AFDS responded accordingly by adjusting the pitch angle, airspeed remained at about 320 kt.
- The FO selected the vertical speed mode again at 1658:38, with a vertical speed of -2,600 ft/min. The AFDS responded by adjusting the pitch angle to meet the new selected vertical speed. The FO then gradually wound back the selected vertical speed to -1,200 ft/min. The aircraft achieved the selected vertical speed, but airspeed remained at around 320 kt. At 1658:52, the FO re-engaged the level change mode.
- At about the same time, the autopilot commanded a turn towards the COMLY waypoint, as part of the STAR. The tailwind started to increase from around 20 kt to 38 kt, which contributed to the computed airspeed decreasing towards the target airspeed (310 kt). As the computed airspeed approached the selected airspeed, the autopilot decreased the pitch attitude.
- At 1659:05, the FO then selected the target airspeed to 300 kt. However, airspeed began to increase, from about 310 kt to 320 kt.
- The AFS and both pilots responded to the increase in airspeed. At 1659:10, the AFDS reduced the nose-down pitch angle. At about the same time, the FO engaged the vertical speed mode, quickly winding the selected vertical speed back from -1,200 ft/min to -400ft/min.
- At 1659:14, as the aircraft was descending through around 17,000 ft, the tailwind component quickly reduced from 23 to 12 kt and the wind direction changed (as described in Atmospheric data recorded by aircraft above). The airspeed quickly increased towards VMO (340 kt).
- At 1659:16 (vertical purple dotted line in Figure 8 and Figure 9), the captain suddenly pulled back on the control column, with a maximum 49 lb backwards control force. This caused the autopilot to disconnect and resulted in a large increase in the aircraft pitch attitude.
- The captain suddenly released the controls after the autopilot disconnect, then pulled back on the controls again a few seconds later. The vertical acceleration rapidly increased to 2.3 g, before rapidly decreasing to 0.9 g. This was immediately followed by another rapid increase to 1.95 g, consistent with the second nose-up control input made by the captain.
- At 1659:19, the airspeed peaked at 341 kt, before declining quickly to around 280 kt.
Source: ATSB Safety Report, pp. 12 - 13
The United States’ Federal Aviation Regulations (FAR) 25.1505 - Maximum operating limit speed defined VMO as ‘a speed that may not be deliberately exceeded in any regime of flight (climb, cruise, or descent’. These regulations state that VMO/MMO must be sufficiently below the design dive speed of the aircraft, to make it highly improbable that the latter speeds will be inadvertently exceeded in operations.10 Other regulations provide further guidance on the calculation of the margins between VMO/MMO and these other limit speeds.
The FCTM described the concept of VMO and the causes of VMO overspeed:
VMO/MMO is the airplane maximum certified operating speed and should not be exceeded intentionally. However, crews can occasionally experience an inadvertent overspeed. Airplanes have been flight tested beyond VMO/MMO to ensure smooth pilot inputs will return the airplane safely to the normal flight envelope…Although autothrottle logic provides for more aggressive control of speed as the airplane approaches VMO or MMO, there are some conditions that are beyond the capability of the autothrottle system to prevent short term overspeeds
In a section related to procedures for rapid descent, the FCTM stated:
When descending at speeds near VMO / MMO with the autopilot engaged, short-term airspeed increases above VMO / MMO may occur. These are most often due to wind and temperature changes. These short-term increases are acceptable for this manoeuvre and the autopilot should adjust the pitch to correct the airspeed to below VMO / MMO.
The FCTM said that any time VMO is exceeded, the maximum airspeed should be noted in the flight log. A VA Flight Crew Information Bulletin (FCIB) issued on 4 October 2016 similarly noted that pilots should make maintenance log entries and safety reports for any overspeeds.
The 737 maintenance manual required a structural examination of the aircraft if a VMO exceedance was greater than 20 kt (that is, where airspeed exceeded 359 kt). There were no maintenance actions required for VMO overspeeds less than 20 kt.
Source: ATSB Safety Report, p. 17
There were no specific procedures for the management of high-speed descents. As such, there were no procedures that described the target speeds pilots should adopt during high-speed descents, or how pilots should use autopilot modes or speed brakes to manage speed in these situations.
Other VA and Boeing procedures described the methods pilots should use to manage speed around VMO, including recovering from overspeed. The 737 FCTM provided procedures for avoiding and recovering from overspeed, including in the descent stage of flight. In the section titled ‘Overspeed’ the FCTM stated that:
If autothrottle corrections are not satisfactory, deploy partial speed brakes slowly until a noticeable reduction in airspeed is achieved . . .
When encountering an inadvertent overspeed condition, crews should leave the autopilot engaged unless it is apparent that the autopilot is not correcting the overspeed. However, if manual inputs are required, disengage the autopilot. Be aware that disengaging the autopilot to avoid or reduce the severity of an inadvertent overspeed may result in an abrupt pitch change.
During climb or descent, if VNAV or LVL CHG pitch control is not correcting the overspeed satisfactorily, switching to the V/S mode temporarily may be helpful in controlling speed. In the V/S mode, the selected vertical speed can be adjusted slightly to increase the pitch attitude to help correct the overspeed. As soon as the speed is below VMO/MMO, VNAV or LVL CHG may be re-selected.
VA sent company pilots a Flight Safety Notice in 2014, titled Exceedance of VMO/MMO and Assigned Altitude. Although this notice primarily related to overspeed associated with entry to Jetstreams on climb on in cruise, it highlighted that
VMO/MMO is the maximum operating speed and should not be exceeded intentionally, however small excursions of a short-term or transient in nature are within the design envelope of a jet transport aircraft. That is, there is minimal operational impact. Notwithstanding this, anytime VMO/MMO is exceeded the maximum speed and time above
VMO/MMO should be noted. VMO/MMO exceedance poses less risk generally than an excursion beyond a cleared altitude or Flight Level. A VMO/MMO exceedance is preferable to an altitude bust, or large abrupt control inputs.
Any VMO/MMO exceedance must be entered in the Maintenance log and a safety report must be submitted.
Source: ATSB Safety Report, pp. 18 - 19
As a general principal, aircraft control systems are more sensitive in high energy states such as high-speed, high altitude flight. This means that control inputs have larger effect when the aircraft is at higher speed. The FCTM noted the potential for over-control due to increased control sensitivity at high-speed, stating:
There have been reports of passenger injuries due to over-controlling the airplane during high altitude, high airspeed flight when overriding the control column with the autopilot engaged or after disengaging the autopilot with the disconnect switch.
Pilots should understand that, in general, the airplane is significantly more sensitive in pitch response (load factor) to column movement at cruise than it is at lower speeds associated with takeoff and landing.
The captain reported not expecting to pull back very forcefully when taking manual control of the aircraft. However, reflecting on the development of the accident, the captain considered that their control inputs may have exacerbated what they perceived as the turbulence experienced by the aircraft.
Source: ATSB Safety Report, p. 20
The captain was highly concerned about overspeed, and this probably contributed to the captain’s assessment that the increase in airspeed towards VMO meant the aircraft was not in a controlled state, and that urgent action was necessary. The captain was mindful of avoiding overspeed during the descent, and made several comments to the FO along the lines of ‘don’t overspeed’.
The captain’s concerns about avoiding overspeed were influenced by a perception that Virgin Australia (VA) were also concerned about overspeed and wanted to reduce overspeed events. The captain reported hearing rumours that other VA crews had been subject to some form of management review after experiencing overspeeds. Although VA had changed their management of overspeed events prior to the occurrence, it is unknown how flight crew understood these changes. In this event, it is possible that the captain’s concerns about overspeed were a carry-over from the operator’s previous management of overspeed events.
The captain’s concern about the increase in airspeed towards VMO was also influenced by perceptions and beliefs about the airspeed limits of the aircraft. The captain indicated not being aware that there was a margin between VMO and the requirement for a maintenance inspection at 359 kt, or that there was a margin between VMO and the structural limitations of the aircraft.
Source: ATSB Safety Report, p. 30
- During a high-speed descent, a sudden decrease in tailwind associated with windshear caused airspeed to approach and exceed the aircraft maximum operating speed (VMO). The flight crew did not apply speed brakes to arrest the speed increase.
- In response to the airspeed rapidly increasing towards VMO, the captain (pilot monitoring) perceived a need to immediately intervene, and made pitch control inputs without following the normal take-over procedure and alerting the first officer (pilot flying).
- The magnitude of the captain's control input was probably greater than intended. This was influenced by a perception that the autopilot was not controlling the aircraft so an urgent intervention was required. The magnitude of the control input caused sudden pitch changes, resulting in the injuries to the cabin crew.
- Although the flight crew identified the risk of overspeed during the high-speed descent into Adelaide, they did not consider steps for mitigating that risk, or how they would manage an overspeed during the descent. This reduced the likelihood of the crew effectively responding to the unexpected increase in airspeed.
Source: ATSB Safety Report, p. 34
Will the aircraft come apart beyond VMO/MMO?
Is VMO/MMO a "brick wall" that you must never exceed for fear that the aircraft will come apart? No, not at all. The aircraft is tested at higher speeds and the aircraft must remain controllable so as to be returned to VMO/MMO without the need for exceptional pilot strength or skill. More importantly, there is no need to panic and react without thinking things through.
More about this: VMO.
14 CFR 1, Title 14: Aeronautics and Space, Federal Aviation Administration, Department of Transportation
14 CFR 25, Title 14: Aeronautics and Space, Federal Aviation Administration, Department of Transportation
Overspeed and pitch up resulting in cabin crew injury involving Boeing 737, VH-VUE, 42 NM ESE Adelaide Airport, South Australia, on 13 September, 2017, Australian Transport Safety Bureau (ATSB) Transport Safety Report, Aviation Occurrence Investigation, AO-2017-092, Final, 30 September 2020.