Monday, December 18, 2017
Transport Canada - Aviation Safety Letter Flying the Wrong SID: Why Does It Happen?

Flying the Wrong SID: Why Does It Happen?

by Gerard W.H. van Es, Senior Consultant, NLR-Air Transport Safety Institute, Amsterdam, the Netherlands

On April 29, 2001, an MD-83 was on a flight from Vancouver, B.C., to Seattle, Wash., taking off from Runway 08R of Vancouver International Airport. When the clearance delivery controller issued the clearance, he incorrectly gave a RICHMOND 6 standard instrument departure (SID). However, he wrote down the correct SID, VANCOUVER 2, on both the digital and paper strip. The tower controller, seeing VANCOUVER 2 on his strip, assumed that the MD-83 would follow that SID. After takeoff, the MD-83 turned right to a heading of 140° as called for by the RICHMOND 6 SID. The MD-83 now conflicted with a DASH-8 that had taken off ahead, also on a RICHMOND 6 SID. The tower controller noticed the conflict and instructed the MD-83 to turn left. The separation had reduced to 2 NM, whereas 3 NM is required.
Source: NLR-ATSI Air Safety Database.

A standard instrument departure (SID) is an instrument flight rule (IFR) departure procedure that provides a transition from the runway end to the en-route airway structure. There are many operational advantages in using SIDs, both for the pilot and the air traffic controller. For the pilot, a relatively complicated route segment may be loaded from a database and flown using the flight management system (FMS), thus assuring him of proper clearance from obstacles, ground, or other traffic. Air traffic control (ATC) may clear the aircraft for the SID, thereby reducing the need for further instructions during the initial climb phase of the aircraft. This in turn greatly reduces the controller/pilot workload and frequency congestion.

SIDs are primarily designed to comply with obstacle clearance requirements, but are often optimized to satisfy ATC requirements; they may also serve as minimum noise routings. Small deviations from the assigned SID occur on almost every SID flown. Small deviations are quite normal and pose no immediate threat to flight safety. However, large deviations from the assigned SID or flying the wrong SID can be hazardous. Such deviations may lead to (and have led to):

  • Close proximity to terrain or obstacles;
  • Close proximity to other aircraft; and
  • Airspace violations.

There are many reasons for which an aircraft significantly deviates from an assigned SID. A recent study conducted by the NLR-Air Transport Safety Institute revealed 38 different causal factors associated with significant SID deviations. According to the study, the most important causal factor involved pilots using the wrong SID. (This factor accounted for 20 percent of the analyzed occurrences). Flying the wrong SID can be a very hazardous situation, especially when there are multiple take-off operations in place (e.g. parallel departures).

Let us consider SID blunders more closely. Why would a pilot use the wrong SID? Again, there is no single causal factor. However, some factors are more significant than others because they occur far more frequently. The NLR-Air Transport Safety Institute safety study showed that similar-sounding SID names were often a factor in cases where the pilots used the wrong SID. This should come as no surprise when there are other SIDs available with similar-sounding names. Often, the difference is only a single letter or number. For instance, ELBA 5B sounds and looks very much the same as ELBA 5C. The similarity can easily lead to mistakes when selecting either SID. When using the FMS NAV mode for flying the SID, the pilot selects the SID from the FMS database. Depending on the type of FMS, a list of runways is presented first. The pilot selects the runway, and a list of corresponding SIDs is given. Sometimes a list of SIDs—where the SIDs are automatically linked to a corresponding runway—is provided first. It is often impossible for the pilots to recognize that they are flying a wrong SID: in the cockpit, all instruments indicate that the aircraft is exactly on the pre-defined route! Usually, ATC notices such errors much earlier than pilots. The following example clearly illustrates the problem:

Before departure, the crew received ATC clearance from Runway 12, PEPOT 1F SID. It was read back to ATC as IPLOT 1F without any correction from the controller. After departure, ATC monitored the departure well and took corrective action without delay when the controller noticed that the aircraft was flying the wrong SID. The SID should have been PEPOT 1F. Because of their prompt action, ATC prevented conflict with other traffic. IPLOT and PEPOT sound very similar when heard by radio.

This example also illustrates another important factor identified in many occurrences where the wrong SID is flown: the readback/hearback error situation in which the pilot reads back the incorrect SID, and the controller fails to notice. This is a classic air-to-ground communication error. In the example above, the pilots were cleared for the PEPOT 1F SID, but read back the IPLOT 1F SID, which went unnoticed by the controller.

Another typical error related to flying the wrong SID is crew expectation, as shown in the next example:

The planned SID for the flight was a DAKE departure as had been used for years for this runway. After departure, ATC informed the crew that they were supposed to fly ELBA SID as this had been the cleared departure. The crew stated that their minds had been set for a DAKE departure, and that they did not change the SID in the FMS.

Clearly, the crew expected to fly a particular SID as they always had for this runway. When the controller instructs a completely different SID, the crew fails to notice and often reads back the correct SID. Only after they have taken off will the controller notice that the crew are flying the wrong SID.

Finally, another important factor is illustrated by the following example:

An ELBO 1A SID for Runway 25R was inserted into the flight management computer (FMC) according to the operational flight plan. This was also passed by the clearance delivery. However, when the aircraft was taxiing to Runway 25R, the departure runway was changed into 25L with a BEKO 1F SID. The pilot not flying forgot to change the ELBO 1A SID that was originally programmed into the FMC. The aircraft flew the SID of Runway 25R after takeoff.

Last-minute changes to the SID or departure runway are yet another important factor related to flying the wrong SID. In the example above, the pilot should not only change the runway/SID in the FMS, but should also conduct new take-off performance calculations for the new runway. Often, the SID is completely forgotten in this process, and the FMS uses the originally programmed SID.

As shown in this brief article, there are several reasons pilots use the wrong SID. In many cases, pilots play a crucial role. However, controllers can also be part of the chain of events that lead to flying the wrong SID.

(NOTE: In some examples, the names of the SIDs and runways have been changed due to the confidentiality of the original data. However, all examples are based on real cases.)

The complete study on SID deviations, An Investigation Into Standard Instrument Departure (SID) Deviations, (NLR TP-2008-068), can be downloaded from the following Web site: www.nlr.nl/smartsite.dws?id=8744.

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